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1. 2 4 The improved refrigerant sensor 111 includes a anode cathode assembly of similar construction to the prior art sensing device 11 a pair of bus bars 125 a pair of supply or drive contacts 128 and a pair of temperature sense or Kelvin contacts 129 The ends of the anode coil 26 may be attached to the centers of respective bus bars 125 One supply contact 128 is connected to an end of each bus bar 125 and one temperature sense contact 129 is connected to the opposite end of each bus bar 125 As with the prior art sensing device 11 the exposed end of the cathode wire 24 is connected to the cathode contact 30 to create a total of five contacts Each contact is mounted through the base 131 of a TO 5 transistor can which further includes a sample air exhaust hole 133 disposed adjacent the anode cathode assembly It should be obvious to one of ordinary skill that although as described and illustrated the bus bars 125 are separate elements from the drive contacts 128 and the temperature sense contacts 129 any of a variety of alternative construc tions may instead be used For example each bus bar 125 and its corresponding contacts 128 129 may be formed of a single piece of U shaped metal referred to herein as a U pin Each end of the anode coil 26 may be attached to the midsection of a respective U pin and the downwardly extending ends of the U would form the contacts 128 129 extending from the base 131 of the TO 5 can In anoth
2. and the sec ond rate of temperature increase is selected to be between 500 and 2000 degrees Celsius per second with an exemplary rate of approximately 1000 degrees Celsius per second In a preferred embodiment of a method of warming a detector 5 up the particular peak temperature to which the temperature of the sensing device 11 is raised is in excess of the desired operating temperature and is preferably chosen to be generally equivalent to the maximum safe operating temperature of the sensing device 11 for reasons which will become apparent below The sensing device 11 is held at that temperature for a selected period of time until the sensing device 11 is sufficiently warmed up The period of time for which the sensing device 11 is held above the desired operating temperature depends on a number of factors the most significant of which is the amount of time ranging from seconds to months which the detector 5 has been off It is believed that an unused sensing device 11 tends to depolarize in an amount which is proportional to the length of time since the last use Premature use of the detector 5 before the sensing device 11 is re polarized may exhibit unreliable behavior due to the instability of the temperature of the sensing device 11 or the bias current depending on whether the bias current control loop 52 or the temperature control loop 40 is currently in operation The sensing device 11 may be re polarized by heating it with the am
3. 14 provides a fixed current of much smaller magnitude than that which is generated through the anode coil 26 while the switch 15 is on In a suitable embodiment the current source may supply a current of 10 mA During that portion of each cycle when the switch 15 is off a current of approximately 10 mA is thus generated through the anode coil 26 The voltage drop across the anode coil 26 while the switch is off is directly proportional to the effective resistance of the anode coil 26 Because this resistance is a function of the temperature of the coil 26 which increases in approximately linear fashion and because the current through the coil 26 is constant while the switch 15 is off the magnitude of the voltage drop across US 6 644 098 B2 13 the anode coil 26 while the switch 15 is off thus provides a direct indication of the absolute temperature of the sensor Also while the switch 15 is off the anode voltage is very high with respect to the cathode voltage and thus the potential difference between the anode voltage and the cathode voltage is nearly equal to the voltage supplied by the batter power supply 12 Because the switch 15 is off over 90 of the time the average difference between the anode and cathode voltages is much larger than in prior art solu tions This results in a greater bias current and therefore greater sensitivity at lower sensing device 11 temperatures The resistor 36 and th
4. 16 is connected to the switch 15 in order to modulate the current generated through the anode coil 26 In operation the user turns the refrigerant detector 5 on and a desired temperature is provided to the refrigerant detector 5 in one of the manners described above using the temperature input means 46 The temperature control loop 40 supplies a pulse width modulation signal to the switch 15 at a set duty cycle During the off portion of the duty cycle the temperature control loop 40 converts and amplifies the voltage potential present at the anode contacts 28 and subtracts that signal from the desired set point signal pro vided from the temperature input means 46 The resulting error signal is amplified and phase compensated by the first processing means 50 in order to optimize settling time overshoot and ringing The output of the first processing means 50 is a duty cycle set signal which is provided as an input to the modulator 16 The modulator 16 then adjusts the duty cycle of the modulation to counteract against any rise or drop in the temperature of the sensing device When the US 6 644 098 B2 15 measured temperature of the sensing device 11 is lower than the desired temperature then the duty cycle set signal represents an instruction to the modulator 16 to increase the duty cycle thus leaving the switch 15 on for a greater proportion of the period of each cycle and allowing the anode coil 26 to be heated by the batter
5. 56 or the desired bias current may be input automatically by the microprocessor Once the switches 70 72 are so adjusted the voltage potential across the resistor 36 is amplified and subtracted from the desired set point signal provided from US 6 644 098 B2 23 the bias current input means 56 and the resulting error signal is amplified and phase compensated by the multi purpose processing means 60 and provided to the modulator 16 Thereafter the refrigerant detector 5 may be used as described with regard to the second preferred embodiment to indicate the presence of halogen molecules in an area of interest It should be noted that the multi purpose processing means 60 may utilize two separate sets of empirically determined coefficients as described previously the set in use being selected by the position of the switches 70 72 or may instead utilize only a single set of empirically deter mined coefficients which are valid for either loop thus simplifying the control by making the state of the switch irrelevant In a further feature of the present invention the refrigerant detector 5 may also utilize an improved sensing device 111 having a pair of bus bars and a pair of additional contacts to create a low noise low impedance device with a configu ration commonly known as a Kelvin Connection FIG 6 is a detailed diagrammatic view of an improved sensing device 111 suitable for use in the primary detection circuits 10 of FIGS
6. S Patent Nov 11 2003 Sheet 2 of 3 US 6 644 098 B2 10 GAIN 20 E E AMPLIFIER PS i 12 M 36 Y il 10 18 20 34 POSI E a o ge l2 y Mun 60 e 28 Ve a 3 a KDE MODULATOR NIS 96 J T FIC 5 U S Patent Nov 11 2003 Sheet 3 of 3 US 6 644 098 B2 V ws AMPLIFIER X ome SU oe ae Gee SSS oe oe A T E 4 18 ga POST OLTAGE Ou PROCESSOR mg 12 25 POTENTIAL ab 70 j ME E CONVERTER gg Mut bal b PURPOSE EUM Prot GS PRO r J6 1 Dat MODULATOR 15 4 Reese cesses wen auem emesm erae eee amot SIND UU HORNE que ape e am m a 128 US 6 644 098 B2 1 HEATED ELECTRODE REFRIGERANT DETECTOR UTILIZING ONE OR MORE CONTROL LOOP CROSS REFERENCE TO RELATED APPLICATION This application is entitled to the benefit of and claims priority to U S patent application Ser No 60 262 525 filed Jan 18 2001 and entitled HEATED ELECTRODE REFRIGERANT DETECTOR UTILIZING ONE OR MORE CONTROL LOOP BACKGROUND OF THE PRESENT INVENTION 1 Field of the Present Invention The present invention relates generally to the field of gas sensors and in particular to the art of detecting halogenated refrigerants by applying control theory to an improved heated electrode technology to control the operation of the detector using an advanced sensing device and one or more control loops 2 Background Art Gas detectors for sensing th
7. current to maintain the ion current at the level of that leakage current It has been discovered that the leakage current is due to the absorption of moisture while the detector is not in use and is generally many times larger than the bias current required for normal operation Therefore the variable gain amplifier described may never provide enough gain at power on to raise the temperature of the sensor to its desired operating point Significant limitations are also placed on the performance of the detector of the 260 patent by the means by which a refrigerant is detected More particularly not only is the ion current being controlled by the feedback loop but it is also the process variable which is monitored for a condition indicating the presence of halogen molecules Unfortunately such an approach mandates the use of inher ent high pass filtering artifacts that reduce a signal level change into a time varying peak which lasts only a certain period of time even though refrigerant may still be present at the sensor Further the detector of the 260 patent is designed to compensate only for relatively slow fluctuations of the ion current and no adjustment is made by the feedback loop for spikes in the magnitude of the ion current which disappear before the end of the period of the gain amplifier is reached The single process variable approach thus per mits short term high magnitude fluctuations in the ion current which signi
8. gas detector having a heated first electrode and a second electrode herein the method comprises the steps of heating the first electrode to a predetermined absolute temperature upon reaching the predetermined absolute temperature placing the electrodes in a test location upon being exposed to one of the predetermined gases generating an increased electrical current in the second electrode and US 6 644 098 B2 25 maintaining the first electrode at substantially the prede termined absolute temperature while placing the elec trodes in the test location and while generating the increased current 5 The method of claim 4 further including the steps of selecting the predetermined absolute temperature and pro viding an indication of the predetermined absolute tempera ture to the gas detector 6 The method of claim 5 wherein the step of providing an indication of the predetermined absolute temperature takes place while the detector is being operated 7 The method of claim 5 wherein the step of providing an indication of the predetermined absolute temperature includes the step of predefining the predetermined absolute temperature during manufacturing 8 The method of claim 5 wherein the step of providing an indication of the predetermined absolute temperature includes the step of entering the predetermined absolute temperature into the gas detector 9 The method of claim 4 wherein the amount of heat applied to the firs
9. of the second gain amplifier 54 which is a non inverting amplifier of well known construction and may have a gain of 16 is connected to the cathode contact 30 and carries a voltage level proportional to the bias current of the sensing device 11 The output of the second gain amplifier 54 is connected to one input of the second subtractor 58 while the other input of the second subtractor 58 is con nected to the bias current input means 56 The bias current input means 56 may be any suitable means for inputting a voltage level corresponding to a particular desired operating 10 15 20 25 30 35 40 45 50 55 60 65 16 bias current magnitude Empirical study has determined that increasing the bias current results in greater sensitivity but substantially reduces the life of the sensing device 11 In a preferred embodiment these factors are balanced by main taining the bias current in the range from 0 4 uA to 0 8 uA which corresponds to a voltage range of 40 mV to 80 mV when the resistor 36 has a resistance of 100 KOhm If a microprocessor is utilized then the preferred bias current input means 56 may be a pre programmed set point which could be programmed into the microprocessor in order to eliminate user interruption Moreover a plurality of pre programmed set points may be provided for different purposes and the bias current input means 56 may include a selection means for selecting the preferred setting w
10. operation condition includes monitoring the bias current the step of increasing at a second rate occurs on the basis of the magnitude of the bias current being substantially equal to zero the step of monitoring an operation condition includes monitoring absorbed moisture in the sensing device the step of increasing at a second rate occurs on the basis of the substantially all of the initial quantity of absorbed moisture being evaporated the step of monitoring absorbed moisture in the sensing device includes determining whether any absorbed moisture is present the first rate of increase may be between 50 and 100 degrees Celsius per second and the second rate of increase may be between 500 and 2000 degrees Celsius per second The present invention also includes a method of operating a heated electrode refrigerant detector the method including US 6 644 098 B2 11 the steps of defining a sequence of desired temperature values and adjusting the temperature of the detector accord ing to the defined sequence In features of this method the step of adjusting the temperature includes for each desired temperature value in the sequence the steps of determining the next desired temperature value in the sequence controlling the tempera ture of the detector to effect the desired temperature value monitoring the temperature of the detector to determine if the desired temperature value has been reached and repeat ing the controlling and m
11. the bias current may then be ramped down quickly until the desired bias current magnitude is reached and the bias current is then held steady at that level as described previously Significantly the transition of the detector 5 from the temperature control mode used to warm up the sensing device 11 to the bias current control mode used for normal operation usually results in a significant drop in the tem perature of the sensing device 11 as the sensing device 11 drops from the predetermined peak temperature used for re polarization to an operating temperature which is pre dominantly dependent upon the magnitude of the bias cur rent and the age of the sensing device 11 Over the life of the 10 15 20 25 30 35 40 45 50 55 65 22 sensing device 11 this desired operating temperature which in this embodiment is actually whatever temperature is required to maintain the desired bias current magnitude gradually creeps upward until it nearly equals the maximum safe operating temperature of the sensing device 11 at which point the sensing device 11 must be replaced In a variation of the third embodiment of the present invention shown in FIG 5 the temperature control loop 40 and the bias current control loop 52 are combined into a single loop 64 The single control loop 64 which once again may be implemented by either digital microprocessor and code or analog discrete components means includes a vol
12. the presence of halogenated gases may be determined by monitoring the current generated through the second electrode referred to as the bias current for a sudden increase in magnitude created by introducing the device to such gases These sensors are commonly utilized by tech nicians to determine whether a refrigerant leak exists and to pinpoint its source Advantageously heated electrode sensors have low elec trical power requirements and good sensitivity and such sensors exhibit excellent selectivity in that they tend to ignore most chemical vapors which may be present in a typical test environment as well as water vapor Unfortunately prior art heated electrode sensors also suffer a number of drawbacks First and most significantly the bias current is dependent not only upon the presence or absence of halogenated molecules at the electrodes but by the temperature of the device as well Thus sudden changes 10 15 20 25 30 35 40 45 50 55 60 65 2 in temperature are frequently misinterpreted as an indication of the presence of halogenated molecules because their respective effects are the same each causes an increase in the bias current of the sensor U S Pat No 4 305 724 to Micko the 724 patent discloses a combustible gas detection system including a sensor temperature control system The detection system includes a sensor element having active and reference sen sors for detecting co
13. the user This maximum safe operating temperature may preferably be determined empirically by the manufacturer and provided to the user either in written technical information or via the microprocessor if one is used The maximum safe operating temperature may then be utilized by the user to estimate the remaining useful life of the detector 5 as follows As the anode 26 is variably heated to maintain a constant bias current in the cathode 24 the actual operating temperature of the sensing device 11 may be sensed either directly or derived from the actual duty cycle frequency and compared to the maximum safe oper ating temperature The remaining useful life of the detector 5 may then be determined as a function of the difference between the actual operating temperature and the maximum safe operating temperature If a microprocessor is utilized then the remaining useful life may be automatically pro vided to the user in terms of time but it should be obvious that a simple function may be used to instead convert the temperature differential to a period of time manually It should also be obvious that a graduated series of indications of remaining useful life may be provided to the user such as through the use of a green LED being lit when a minimum useful life remains a yellow LED being lit when the useful life is almost depleted and a red LED being lit when the useful life has been reached It should also be obvious that the remaining useful
14. 0 55 60 65 10 perature of the sensing device and the third temperature is the ambient temperature of the sensing device before the sensing device is heated The present invention also includes a method of preparing a heated electrode refrigerant detector for use the detector including a sensing device wherein the method includes the steps of maintaining the actual temperature of the sensing device at a first temperature while maintaining the actual temperature of the sensing device at the first temperature generating a bias current the bias current decreasing in magnitude over time monitoring the bias current and on the basis of the monitored bias current reducing the actual temperature of the sensing device to a second temperature which is a desired sensing device operating temperature In features of this method the first temperature is gener ally equal to the maximum safe operating temperature of the sensing device the temperature reducing step is executed on the basis of the negative slope of the monitored bias current over time being less than a predetermined value and the temperature reduction is effected by reducing the magnitude of the bias current to a desired operating level The present invention also includes a method of re polarizing a heated electrode refrigerant detector having a sensing device operable at an operating temperature the method including the steps of elevating the temperature of the sensin
15. 1 Once the refrigerant detector 5 is operative the user may utilize it to detect the presence of halogen molecules and accordingly to identify a refrigerant leak To detect a leak the refrigerant detector 5 may first be reset in a location which is known to be free of halogen molecules The refrigerant detector 5 may then be moved to the desired test location If the sensing device 11 is moved into the presence of halogen molecules the bias current will momentarily start to increase resulting in a corresponding initial increase in the voltage across the resistor 36 In reaction the bias current control loop 52 will adjust the duty cycle to lower the temperature of the sensing device 11 thus effecting a corresponding decrease in the magnitude of the bias current in order to effectively keep the bias current constant In this embodiment the presence of halogen molecules may thus be indicated by a rapid reduction in the temperature of the sensing device 11 rather than by an increase in the bias current However in order to avoid having to monitor the temperatures of the sensing device 11 directly the post processor 18 may be adapted to receive information related to the duty cycle of the modulator 16 and to control the leak detection indicators and alarms 20 on the basis of that information rather than on the basis of the temperature of the sending device 11 This is because the temperature of the sending device 11 is related to the power a
16. 4 151 641 A 5 1979 Mitoff 29 611 4 157 311 A 6 1979 Orth et al 252 408 34 wherein the detection circuit includes a sensing device having first and second electrodes the first electrode being connected to the power source for heating the first electrode the temperature controller is operatively connectable to the detection circuit for maintaining a temperature of the first electrode at a predetermined magnitude and the current controller is operatively connectable to the detection circuit for maintaining a current in the second electrode at a predetermined magnitude 20 Claims 3 Drawing Sheets PROCESSING MEANS US 6 644 098 B2 Page 2 U S PATENT DOCUMENTS 5 400 015 A 3 1995 5 444 435 A 8 1995 5 448 905 A 9 1995 5 490 413 A 2 1996 5 608 384 A 3 1997 5 841 021 A 11 1998 5 858 739 A 1 1999 Williams II et al DENE 340 632 De Castro et al Your eee 436 151 cited by examiner T 73 31 05 PNE 73 40 73 23 2 5 897 836 A 5 932176 A EE 340 632 5 969 231 A 340 632 6 085 576 A 6 289 719 B1 6 336 354 B1 Martell et al 422 90 Yannopoulos et al 422 98 Qu et al eee 73 31 05 Sunshine et al 73 29 01 Bloemer et al 73 23 21 Suzuki et al 73 31 05 U S Patent Nov 11 2003 Sheet 1 of 3 US 6 644 098 B2 10 34 i 18 28 42 12 ox VOLTAGE E Hu d ag 36 m SM 7 W Nao U
17. S Related U S Application Data Yokogawa Corporation of America Operation Manual for 60 Provisional application No 60 262 525 filed on Jan 18 Top Gun Model No H10Xpro Refrigerant Leak Detector 2001 8 pages Leybold Inficon Inc User s Manual for D TEK Refriger 6 Int CE Goes HOSB 39 04 GOLN 27 04 ant Leak Detector 10 pages GOIN 27 407 GOIN 27 46 GOIN 33 22 Primary Examiner Helen Kwok Assistant Examiner David J Wiggins 52 US Cl one 73 25 01 73 25 05 73 31 05 74 Attorney Agent or Firm Baker amp Hostetler LLP 422 90 422 98 422 109 324 443 57 ABSTRACT 58 Field of Search 73 25 01 25 05 73 23 2 31 05 422 08 90 109 324 98 A gas detector for sensing the presence of at least one 109 610 464 443 444 predetermined gas is operative in conjunction with a elec Dip E trical power source and includes a detection circuit a 56 References Cited temperature controller and a electrical current controller U S PATENT DOCUMENTS 2 404 474 A 7 1946 Collins ce 73 25 01 3 347 635 A 10 1967 McKee 23 232 E 3 449 939 A 6 1969 Monomakhoff 73 25 01 3 607 084 A 9 1971 Mackey et al 23 232 E 3 616 678 A 11 1971 Batzies 73 25 01 3 739 260 A 6 1973 Schadler 324 33 3 912 967 A 10 1975 Longenecker 315 107 3 991 360 A 11 1976 Orth et al 324 33
18. United States Patent US006644098B2 12 10 Patent No US 6 644 098 B2 Cardinale et al 45 Date of Patent Nov 11 2003 54 HEATED ELECTRODE REFRIGERANT 4 171 341 A 10 1979 Morgan 422 98 DETECTOR UTILIZING ONE OR MORE 4 203 199 A 5 980 Morgan seess 29 612 CONTROL LOOP 4 237 721 A 12 1980 Dolan 73 23 2 4 244 918 A 1 1981 Yasuda et al 422 95 x n 4 298 573 A 11 1981 Fujishiro oo 422 94 75 Inventors URS EE boc US 4 305 724 A 12 981 Micko 23 232 obert Zubik Miami FL US 4 327 054 A 4 1982 Yasuda et al 0 422 95 a 4 520 653 A 6 1985 Kaiser 73 23 2 73 Assignee uS ES Inc 4 609 875 A 9 1986 Jeffers ose 324 455 amar 4 870 546 A 11 1989 Dunham et al 340 632 A illiams II et al 324 468 Notice Subject to any disclaimer the term of this 4 910 463 A 31990 Wi d 5 104 513 A 4 1992 Lee et al cm 204 425 patent is extended or adjusted under 35 5 198774 A 3 1993 Williams II et al 324 468 U S C 154 b by 159 days 5 226 309 A 7 1993 Stetter et al 73 31 06 5 284 569 A 2 994 Lee etal 204 425 21 Appl No 09 838 169 5 297 419 A 3 1994 Richardson we 73 25 03 22 Filed Apr 19 2001 5 351 037 A 9 1994 Martell et al 340 632 65 Prior Publication Data List continued on next page US 2002 0092341 A1 Jul 18 2002 OTHER PUBLICATION
19. absolute temperature of the heater anode based on its effective resis tance When the absolute temperature of the heater anode drops enough below a desired value a temperature regula tion circuit closes the switch and a greater amount of current is supplied to the heater anode When the temperature of the heater anode reaches the desired value again the tempera ture regulation circuit opens the switch and a lesser amount of current is supplied to the heater anode Thus as the US 6 644 098 B2 3 temperature of the sensing element fluctuates greater or lesser heating may be applied to the heater anode by the temperature regulation circuit Unfortunately although this circuit provides some control over the absolute temperature of a heated electrode refrigerant sensor the regulation is relatively crude effectively permitting control only by turn ing an auxiliary heat source on and off At best the tem perature of the sensor is thus roughly held in a general range with the upper approximate limit being the desired tempera ture and the lower approximate limit being the temperature at which the transistor of the switch is cool enough to allow the auxiliary power supply to be coupled in At worst however such a crude controller may allow the temperature of the sensor to oscillate wildly and even dangerously under certain conditions Further the circuit allows no adjustment to be made to the triggering temperatures at which the auxili
20. ary source is turned on or off Thus a need exists for a more sophisticated temperature control system suitable for use with a heated electrode refrigerant detection system which allows the temperature of the sensor to be rigidly maintained at a particular absolute value rather than within a wide range of temperatures and wherein that value is adjustable Another disadvantage of prior art heated electrode sensors is that their lifespans are frequently limited much more than is necessary It is well known that the operation and lifespan of heated electrode sensors are limited by the number of alkali ions in the sensor It has been found that the bias current and the rate of depletion of ions are directly related to each other Thus as the sensor is used the ions are depleted and when no ions are left at all the sensor is dead Unfortunately the sensitivity of the sensor is directly related to the bias current and so the greater the sensitivity of the sensor the more quickly the sensor is used up Prior art heated electrode sensors fail to take these characteristics into account and are thus used up more quickly than is necessary In addition the exposure of prior art sensors to high concentrations of refrigerant even for a relatively short period of time causes a correspondingly high bias current which results in an immediate reduction in sensor sensitivity and a considerable shortening of the sensor s lifespan This effect is kno
21. asymptotic value Because this asymptotic value may vary this state may be accurately derived automatically by measuring the negative slope of the bias signal while the temperature is held in excess of the desired operating temperature Once the slope has decreased to a predetermined value which may be determined empirically warm up of the sensing device 11 is complete and normal operation of the detector 5 may be initiated It has been found that the combined steps of the meth odology described hereinabove reduces the amount of time required for the safe warm up of the refrigerant sensor 5 from a minute or more to a range of less than two seconds for a sensor that has been recently used to approximately 15 seconds for a sensor that has been idle for many weeks It should be obvious that although this ramped technique for warming up the refrigerant detector 5 may be most effec tively implemented using a microprocessor an approxima tion may also be implemented manually If a microprocessor is utilized it may of course be used to implement the other functions of the respective temperature and bias current control loops 40 52 as well It should also be obvious that similar warm up procedures may also be utilized for a detector 5 using only a temperature control loop 40 Once the refrigerant detector 5 has been warmed up the user may choose to set either directly or via microcode the controllable switch 62 to route the output of the bias
22. ce is electrically connected to at least one of the supply contacts the gas detector has a bias current sensing circuit electrically con nected to the cathode contact the gas detector has a current source electrically connected to at least one of the supply contacts and the gas detector has a switch for bypassing the current source The present invention also includes a method of making a sensing device for a heated electrode gas detector the method including the steps of inserting a cathode wire into an uncoated anode coil to form an electrode assembly after inserting the cathode wire into the uncoated anode coil coating the electrode assembly with a ceramic material and firing the coated electrode assembly In features of this method the inserting step includes inserting an uncoated cathode wire into the uncoated anode coil to form the electrode assembly the firing step is accomplished by applying a heating current to the anode coil the method includes the step of biasing the coated electrode assembly by applying a biasing voltage to the electrode assembly and the steps of firing and biasing are carried out substantially entirely simultaneously The present invention also includes a method of making a sensing device for a heated electrode gas detector the method including the steps of inserting a cathode wire into an anode coil to form an electrode assembly coating at least part of the cathode wire and at least part of the anode
23. coil with a ceramic material to form an unfired electrode assem bly and biasing the unfired electrode assembly to form a depletion region In features of this method the biasing step includes biasing the unfired electrode assembly by applying a biasing voltage to the anode coil the method further includes the step of firing the unfired electrode assembly by applying a heating current to the anode coil and the steps of firing and biasing are carried out substantially entirely simultaneously the firing and biasing steps are completed within one hour The present invention also includes a method of effi ciently preparing a heated electrode refrigerant detector for use the detector including a sensing device wherein the method includes the steps of determining a first temperature the first temperature being a desired sensing device operating temperature determining a second temperature the second temperature being higher than the first temperature gradually raising the actual temperature of the sensing device from a third temperature until the second temperature is reached wherein the third temperature is substantially less than the first temperature and after reach ing the second temperature lowering the actual temperature of the sensing device until the first temperature is reached In features of this method the second temperature is generally equal to the maximum sustainable operating tem 10 15 20 25 40 45 5
24. current control loop 52 to the temperature error subtractor 48 A desired bias current may then be input into the refrigerant detector 5 using the bias current input means 56 Alternatively the controllable switch 62 may be adjusted automatically from one position to the other on the occur rence of some predetermined phenomenon such as the negative slope of bias signal dropping to a predetermined value Once the switch is adjusted to route the output of the bias current control loop 52 therethrough the bias current control loop 52 converts and amplifies the voltage potential across the resistor 36 and subtracts that signal from the desired set point signal provided from the bias current input means 56 The resulting error signal is amplified and phase compensated by the second processing means 60 and then provided as the reference temperature setting to the tem perature control loop 40 via the controllable switch 62 Significantly a separate set of empirically determined filter coefficients is required for the second processing means 60 from the ones required for the first processing means 50 If the preferred method of warming up the sensing device 11 is utilized then the initial entered bias current magnitude is the magnitude of the bias current when the bias control loop 52 is first switched in Typically the bias current magnitude at that time is considerably greater than the desired bias current magnitude described previously However
25. d to the detection circuit the position of the switch is determined on the basis of an operating condition of the gas detector the sensing device includes a cathode wire an anode wire at least partly surrounding the cathode wire and having oppos ing ends a pair of supply contacts electrically connected to respective ends of the anode wire a pair of temperature sense contacts electrically connected to respective ends of the anode wire and a cathode contact electrically connected to an end of the cathode wire and the temperature control loop is electrically connected to the temperature sense contacts and an output of the bias current control loop is electrically connected to an input of the temperature control loop The present invention also includes a method of control ling a gas detector for sensing the presence of at least one predetermined gas the gas detector having a heated first electrode and a second electrode wherein the method includes the steps of heating the first electrode to a prede termined absolute temperature upon reaching the predeter mined absolute temperature placing the electrodes in a test location upon being exposed to one of the predetermined gases generating an increased current in the second elec trode and maintaining the first electrode at substantially the predetermined absolute temperature while placing the elec trodes in the test location and while generating the increased current In features of
26. during all of its previous usage Thus after burning in the sensing device 11 subsequent exposure of the sensing device 11 to reactive gases like halogen while the device 11 is being heated causes ions to flow from the anode 26 to the cathode 24 causing an increase in the bias current This characteristic may therefore be used as an indicator of the presence or absence of halogenated mol ecules at the sensing device 11 The battery power supply 12 may be any readily available battery device which in a typical embodiment may supply an unregulated voltage in the range of 4 to 8 VDC The switch may be a transistor or other suitable device capable of propagating a current through the anode coil 26 of the sensing device 11 at a suitable input frequency and duty cycle which as described herein may be 20 kHz and less than 10 respectively At its typical operating temperature of 600 C to 1000 C the anode coil 26 has an effective resistance of approximately 1 ohm Thus during the brief portion of each cycle when the switch 15 is on a current is generated through the anode coil 26 of approximately 4A to 8A Because of the large magnitude of this current a first capacitor 32 and an inductor 34 are provided on the power supply side of the sensing device 11 to filter the current spikes of generally short duration typically 1 5 usec to 4 0 usec which would otherwise present significant noise on the power supply The current source
27. e or may be stored in advance in a lookup table or the like An algorithm suitable for this purpose utilizes as input a starting temperature value an ending temperature value and a total ramp time and repeat edly calculates as a function of the elapsed time relative to the total ramp time a series of output temperature values which gradually increase from the defined starting tempera ture value to the defined ending temperature value along a uniform slope This combined series of temperatures col lectively defines a preferred profile of temperature over time Functionally the temperature changes according to this preferred temperature time profile are implemented as fol lows As each temperature in the temperature time profile is entered into the temperature control loop 40 an error signal representing the difference between the actual temperature of the sensing device 11 and the entered temperature is continually generated by the temperature error subtractor 48 amplified and phase compensated by the first processing means 50 and provided to the modulator 16 which gradually adjusts the duty cycle of the modulation until the entered temperature is reached The amount of time required to ramp the temperature of the sensing device 11 up is dependent upon the amount of time since the detector 5 was last used When a heated electrode gas detector 5 goes unused for a period of time the sensing device 11 tends to absorb moisture through hy
28. e presence of halogenated gases and other gases are well known FIG 1 shows prior art gas detector type suitable for this purpose commonly referred to as a heated electrode sensor This sensor utilizes a cathode wire and an anode wire made of platinum palladium or an alloy thereof Typically the cathode is repeatedly coated with a ceramic material such as a mixture of an alkali metal silicate and oxides of aluminum or silicon with a drying period between each coat and then inserted into an anode coil formed by several turns of the anode wire The anode cathode assembly is then coated further with the same mixture except for the ends of the anode and the exposed end of the cathode and dried After the final drying the anode cathode assembly is fired in a kiln and then installed in a housing with the exposed ends of the anode and cathode connected to anode contacts and a cathode contact respectively The final assembly is then energized and biased over many hours by applying a electrical current through the anode coil and a voltage across the anode coil to the cathode wire The ceramic forms an electrically resistive layer between the electrodes When heated by an electrical current passing through a first of the electrodes an outer layer depleted of ions develops along the electrodes When this layer is exposed to reactive gases like halogen ions flow across the depletion zone and the conductivity of the device is increased Thus
29. e second capacitor 38 are connected to the cathode contact 30 on the sensing device 11 Thus when a bias current is generated at the cathode contact 30 a voltage which is proportional to the bias current is gen erated across the resistor 36 and filtered by the second capacitor 38 In a typical embodiment the resistor 36 may have a value of 100 KOhm and the second capacitor 38 may have a value of 0 1 uF Thus when the temperature of the sensing device 11 remains relatively constant bringing the sensing device 11 into the presence of halogen molecules would cause a noticeable change in the voltage level across the resistor 36 As described hereinbelow a signal corresponding to the bias current voltage level is one of the one or more signals which may be provided to the post processor 18 in order to provide information about the presence or absence of halo gen molecules at the sensing device 11 to the user Another signal which may be provided to the post processor 18 is a signal corresponding to the temperature of the sensing device 11 during the off periods of the switch 15 Yet another signal which may be provided to the post processor 18 is the duty cycle set point signal which is used to set the duty cycle at which the modulator 16 is operating The post processor 18 is capable of detecting or recognizing certain predetermined conditions at the sensing device 11 and controlling one or more leak detection indicators or alarms 20 to
30. e tempera ture signal for an indication of the presence of at least one predetermined gas In features of this method the presence of a predeter mined gas is indicated by a decrease in temperature the bias current is a first signal and the temperature signal is a second signal the first electrode includes at least two ends and the generating step includes generating the temperature signal at one or more of the ends of the first electrode the step of generating the bias current includes the step of generating the bias current according to a duty cycle and the step of maintaining the magnitude of the bias current at a generally constant level includes maintaining the magnitude of the bias current at a generally constant level according to the value of the duty cycle The present invention also includes a method for sensing the presence of at least one predetermined gas at a sensing device having first and second electrodes wherein the method includes the steps of heating the first electrode 10 15 20 25 30 35 40 45 50 55 60 65 8 generating at the second electrode a bias current generat ing a first signal at least partially representative of the magnitude of the bias current the magnitude of the bias current being a first operating condition generating a second signal at least partially representative of a second operating condition maintaining the magnitude of the bias current at a generally c
31. ected preferred absolute temperature at least the monitoring comparing and varying steps are repeated substantially continuously during operation of the gas detec tor the selected preferred absolute temperature is a first preferred absolute temperature and the method further includes the steps of selecting a second preferred absolute temperature providing an indication of the second selected preferred absolute temperature to the gas detector adjust ably heating the first electrode generating an increased current in the second electrode upon being exposed to any of the predetermined gases monitoring the temperature of the first electrode while the increased current is being generated comparing the monitored temperature to the second selected preferred absolute temperature and varying the heating of the first electrode on the basis of the outcome of the comparing step The present invention also includes a method for sensing the presence of at least one predetermined gas at a sensing device having first and second electrodes wherein the method includes the steps of heating the first electrode generating at the second electrode a bias current moving the sensing device into the presence of one of the predeter mined gases maintaining the magnitude of the bias current at a generally constant level during the moving step gen erating a signal at least partially representative of the tem perature of the sensing device and monitoring th
32. ed and Sealed this Twenty seventh Day of September 2005 en WE aa JON W DUDAS Director of the United States Patent and Trademark Office
33. efore operating the new 10 15 20 25 30 35 40 45 50 55 60 65 4 sensor at a highly elevated temperature and seriously reduc ing the life of the new sensor An improved sensor which continually and automatically adjusts the operation of the electrode to provide sufficient sensitivity over an extended lifetime of the sensor is needed U S Pat No 3 739 260 to Schadler the 260 patent discloses a method of operating a halogen detector of the heated electrode type A current supply unit supplies current through a current setting means to the electrode to heat the anode thus creating a fundamental ion current flow between the anode and the cathode The presence of halogenous gas at the electrode causes an increase in the ion current which is amplified and its magnitude indicated by an indicator and or an alarm In addition another amplifier is connected in a feedback loop between the output of the electrode and the current setting means When the magnitude of the ion current varies by a predetermined amount the variable gain amplifier supplies a signal to the current setting means to adjust the heating supply current to the anode in a direction to counteract the variation Unfortunately the detector of the 2260 patent suffers from some serious drawbacks First because at power on there is typically a leakage current which flows through the electrode the feedback loop will operate to adjust the supply
34. ent means for adjusting the voltage level relative to its current value rather than entering the desired voltage level directly Regardless of the method or apparatus utilized to input the desired temperature set point the first subtractor 48 deter mines the difference between the signal from the output of the first gain amplifier 44 and the signal from temperature input means 46 and makes the difference available at its output The output of the first subtractor 48 is connected to the first processing means 50 both of which may easily be constructed by one of ordinary skill in the art of signal processing methods and apparatuses The first processing means 50 which is an analog or digital filter whose coef ficients may be determined empirically by one of ordinary skill may be utilized to amplify and phase compensate the signal from the first subtractor 48 The output of the first processing means 50 is connected to the input of the modulator 16 to provide a duty cycle set signal to the modulator 16 The modulator 16 is a pulse width modulator which utilizes an oscillator to provide a reliable output signal at a uniform frequency with a controllable duty cycle The value of the duty cycle is dependent upon the output from the first processing means 50 In an exemplary embodiment the output signal from the modulator 16 has a frequency of approximately 20 kHz and a duty cycle ranging from approximately 3 to 8 The output from the modu lator
35. er variation an off the shelf TO 5 assembly having five vertical but separate pins may be utilized by bending the upper ends of two pairs of the pins toward each other so that they touch underneath the can Each end of the anode coil 26 may then be attached to a respective pair of pins at the junction formed by the ends of the pins Additional varia tions for the bus bar arrangement will also be readily apparent to one of ordinary skill in the art The supply contacts 128 are utilized to supply the heating current from the battery power supply 12 to the anode coil 26 The temperature sense contacts 129 are utilized to measure the voltage potential across the anode coil 26 during the off periods of the switch 15 during the off periods of the switch 15 As described previously the voltage potential across the anode coil 26 is proportional to the resistance of the anode coil 26 which is approximately linearly related to the temperature of the sensing device 111 and thus provides a direct indicator of the absolute tempera ture of the sensing device 111 The use of these additional contacts helps to optimize the temperature sensing of the 10 15 20 25 30 35 40 45 50 60 65 24 anode coil 26 while eliminating non linearities due to lead resistance and noise due to dirty or high impedance con tacts It should be clear to one of ordinary skill in the art that this improved sensing device 111 may be used b
36. etermined magnitude the magni tude of the current being at least partly dependent upon the temperature of the first electrode while heating the first electrode determining information at least partly represen tative of the operating temperature of the gas detector comparing the operating temperature information to infor mation representative of a maximum operating temperature and determining the remaining useful life of the gas detector on the basis of the comparison In features of this method the information at least partly representative of the operating temperature of the gas detec tor and the information representative of the maximum operating temperature are both particular values the deter mining information step includes sensing the actual operat ing temperature of the gas detector the information at least partly representative of the operating temperature of the gas detector and the information representative of the maximum operating temperature are both particular temperature val ues the information at least partly representative of the operating temperature of the gas detector is a particular duty cycle value which corresponds to the operating temperature of the gas detector the step of comparing the temperatures includes subtracting the operating temperature value from the maximum operating temperature value the step of determining the remaining useful life includes determining the remaining useful life of the gas detec
37. ficantly shorten the lifespan of the sensor Thus a more sensitive and longerlasting heated electrode leak detector is needed which uses a control loop and a plurality of process variables to more reliably detect the presence of a refrigerant Finally another drawback of prior art sensing devices is the length of time required to assemble and burn in a anode cathode assembly Existing methods require both the anode and the cathode to be coated with the ceramic material before assembly and then further coated thereafter and require considerable periods of time for drying between the various coatings Further prior art methods require an assembled anode cathode assembly to first be fired in order to sinter the ceramic material before biasing and the assem bly to create a depletion region A need exists for a manu facturing method which may be completed in a much shorter period of time than is possible using known methods SUMMARY OF THE INVENTION Briefly summarized the present invention relates to a gas detector having a heated electrode sensing device for sens US 6 644 098 B2 5 ing the presence of one or more predetermined gas and one or more control loops for controlling the operation of the sensing device Broadly defined the gas detector according to one aspect of the present invention is operative in con junction with a power source and includes a detection circuit the detection circuit including a sensing device havi
38. g device above the operating temperature until the sensing device is substantially re polarized and decreasing the temperature of the sensing device to the operating temperature In features of this method the method further includes the step of monitoring the magnitude of a bias current generated by the sensing device and the initiation of the step of decreasing the temperature of the sensing device is depen dent at least partly upon the magnitude of the bias current and the method further includes the step of monitoring the amount of time for which the temperature of the sensing device is elevated above operating temperature and the initiation of the step of decreasing the temperature of the sensing device is dependent at least partly upon the amount of time The present invention also includes a method of effi ciently preparing a heated electrode refrigerant detector having a sensing device for use the method including the steps of turning the detector on increasing the actual temperature of the sensing device at a first rate of increase monitoring at least one operating condition of the sensing device and on the basis of an operating condition of the sensing device increasing the actual temperature of the sensing device at a second rate of increase until a desired sensing device operating temperature is reached In features of this method the sensing device is capable of generating a bias current and the step of monitoring an
39. gro scopic action particularly when the detector goes unused for more than a day The moisture can be evaporated quickly by energizing the coil 26 thereby raising the temperature Unfortunately a rapid rise in temperature such as a rate of 10 15 20 25 30 35 40 45 50 55 60 65 20 hundreds of degrees Celsius per second may cause the ceramic portion of the sensing device 11 to crack Thus the rate of temperature increase must be limited until the mois ture is substantially removed from the sensing device 11 at which time the rate of temperature increase may be raised substantially to minimize the overall warm up time The presence of moisture in the sensing device 11 is indicated by the existence of a bias or leakage current caused by the conductive effect of the moisture which may be detected before or after power or heat is applied to the sensing device 11 The evaporation of substantially all of the moisture from the sensing device 11 is indicated by the magnitude of the bias current dropping to zero Thus by monitoring the bias current the temperature control loop 40 may detect the proper time at which to switch from the first rate of temperature increase to the second rate of tempera ture increase In an exemplary embodiment the first rate of temperature increase is selected to be between 50 and 100 degrees Celsius per second with an exemplary rate of approximately 75 degrees Celsius per second
40. he gas detector having first and second electrodes wherein the method includes the US 6 644 098 B2 7 steps of selecting a preferred absolute temperature provid ing an indication of the selected preferred absolute tempera ture to the gas detector adjustably heating the first electrode upon being exposed to one of the predetermined gases generating an increased current in the second electrode monitoring the temperature of the first electrode while the increased current is being generated comparing the moni tored temperature to the selected preferred absolute tem perature and varying the heating of the first electrode on the basis of the outcome of the comparing step In features of this method the step of providing an indication of the selected preferred absolute temperature includes the step of entering a value corresponding to the selected preferred absolute temperature into the gas detector the step of providing an indication of the selected preferred absolute temperature includes the step of predefining the selected predetermined absolute temperature to the gas detector during manufacturing the step of varying the heating of the first electrode includes the steps of reducing the temperature of the first electrode upon determining that the monitored temperature exceeds the selected preferred absolute temperature and raising the temperature of the first electrode upon determining that the monitored temperature is below the sel
41. herein the plurality of set points may include a first set point by which sensitivity is maximized a second set point by which sensing device life is maximized and a third set point by which the above described compromise between sensitivity and sensing device life is reached Additionally in the microprocessor controlled system it should be clear that the input from the bias current input means 56 and the output of the second gain amplifier 54 may both be digitized and so the values processed by the second subtractor 58 may be digitized values rather than actual voltages If the bias current control loop 52 is instead implemented in discrete components the bias current input means 56 preferably includes a keypad for numerically inputting a particular bias current which may then be automatically converted to a corresponding voltage level but it should be clear that other devices may be used to input a particular number and that alternatively a user could input the voltage level correspond ing to a particular bias current magnitude directly without any need for conversion Alternatively the bias current input means 56 could include an adjustment means for adjusting the voltage level relative to its current value rather than entering the desired voltage level directly Regardless of the method or apparatus utilized to input the desired bias current set point the second subtractor 58 determines the difference between the signal from the outpu
42. ice 11 shown in FIG 1 or the improved sensing device 111 shown in FIG 6 an uncoated cathode wire 24 may be inserted into the uncoated anode coil 26 with the combination then being coated with one or two coatings of the ceramic material described previously The unfired anode cathode assembly may then be mounted within the housing which may be a standard TO 5 can The sensing device 11 is then energized thus firing and biasing the sensing device 11 simultaneously in a relatively short period of time It has been found that satisfactory performance in terms of sensitivity and repeat ability may be achieved in as little as thirty minutes thus reducing assembly time dramatically As shown the sensing device 11 may be electrically connected to the rest of the primary detection circuit 10 via its anode contacts 28 and its cathode contact 30 As is well known in the art when thus installed in a suitable circuit such as the primary detection circuit 10 of the present invention a bias current is generated at the cathode contact 30 The magnitude of the bias current is dependent on the average potential difference between the voltage drop across the anode coil and the cathode voltage the temperature of the sensing device 11 the length of time the sensing device 11 has been operating the ambient concentration of halo genated molecules surrounding the sensing device 11 and the history of the sensing device s exposure to halogenated molecules
43. inform the user of the presence of a refrigerant leak In the first embodiment of the present invention shown in FIG 2 the control loops 22 include only a temperature control loop 40 The temperature control loop 40 which is preferably implemented digitally using a microprocessor and appropriate code but may also be implemented using discrete components includes a voltage potential converter 42 first gain amplifier 44 a temperature input means 46 a first subtractor 48 and a first processing means 50 The voltage potential converter 42 is a switched capacitor syn chronous differential to single ended converter which con verts the differential temperature signal present at the anode contacts 28 into a single ended signal The voltage potential converter 42 also receives a synchronized input from the modulator 16 so that only the voltage present at the anode contacts 28 during the off time of the switch 15 is converted The output of the voltage potential converter 42 is connected to the first gain amplifier 44 which is a non inverting amplifier of well known construction and may have a gain of 150 The output of the first gain amplifier 44 is connected to one input of the first subtractor 48 while the other input of the first subtractor 48 is connected to the temperature input means 46 The temperature input means 46 may be any suitable means for inputting voltage level data correspond ing to a particular desired operating tempe
44. life may be determined as the differ ence between the actual operating temperature and a buff ered maximum effective operating temperature wherein the maximum effective operating temperature is lower than the maximum safe operating temperature and is established to allow the temperature of the sensing device 11 to be tem porarily increased during operation in order to compensate for significant changes in ambient conditions without exceeding the maximum safe operating temperature The maximum effective operating temperature may then be interpreted as the maximum temperature above which safe operation of the detector 5 may not be guaranteed under all operating conditions In a third embodiment of the present invention shown in FIG 4 both a temperature control loop 40 and a bias current control loop 52 are provided The control loops 40 52 which may be implemented by either digital microprocessor and code or analog discrete components means include a voltage potential converter 42 first and second gain amplifiers 44 54 temperature and bias current input means 46 56 first and second subtractors 48 58 first and second processing means 50 60 and a controllable switch 62 As described with regard to the first preferred embodiment of the present invention the voltage potential converter 42 is connected to the anode contacts 28 and the modulator 16 and converts the differential temperature sig nal present at the anode contacts 28 int
45. mbustible gases a controlled current source for providing electrical power to the sensor element a voltage to duty cycle converter for providing a square wave control signal of variable duty cycle and a bypass switch for bypassing the active sensor element in response to the control signal By increasing or decreasing the duty cycle the amount of electrical energy flowing to the active element is likewise affected and the temperature of the active sensor may correspondingly be either upwardly or down wardly biased When the presence of combustible gas begins to cause the temperature of the active sensor to increase the increase is detected by the temperature control system and the duty cycle is adjusted to counteract the increase and maintain the temperature constant Unfortunately the detection system of the 724 patent suffers from some drawbacks First the detection system of the 724 patent requires the use of a reference sensor Perhaps more importantly the temperature control system is used only to equalize the temperature of one sensor with respect to the other sensor In particular it includes no means for measuring the absolute temperature of either sensor or for independently setting the absolute temperature of either sensor to a particular chosen value This is sufficient in the active sensor type of the 724 patent because the presence of the gas sought may generally be indicated merely by the heat given off by the oxidation p
46. mperature sensing circuit electrically connected to at least one of the temperature sense contacts for moni toring the temperature of the anode cathode assembly and 26 a bias current sensing circuit electrically connected to the cathode contact 12 Asystem for controlling a gas detector for sensing the presence of at least one predetermined gas the gas detector having a heated first electrode and a second heated electrode comprising means for heating the first electrode to a predetermined absolute temperature means for placing the electrodes in a test location means for generating an increased electrical current in the second electrode and means for maintaining the first electrode at substantially the predetermined absolute temperature while placing the electrodes in the test location and while generating the increased current 13 The system as in claim 12 further comprising means for selecting the predetermined absolute temperature and means for indicating the predetermined absolute tempera ture of the gas detector 14 The system as in claim 13 wherein the means for providing the indication of the predetermined absolute tem perature while the detector is being operated 15 The system as in claim 13 wherein the means for providing the indication of the predetermined absolute tem perature includes a predefined absolute temperature 16 The system as in claim 15 wherein the predefined absolute temperature is de
47. nerated by the heated gas sensing device occur sequen tially and the transition from one of the controlling steps to the other occurs on the basis of at least one operating condition of the sensing device In another aspect of the present invention a controller for controlling the operation of a gas detector the gas detector for indicating the presence of a gas of a predetermined type and having a heated gas sensing device generating a bias current includes a temperature control loop for controlling the temperature of the heated gas sensing device on the basis 10 15 20 25 30 35 40 45 50 55 60 65 6 of at least one operating condition of the sensing device and a bias current control loop for controlling the bias current generated by the heated gas sensing device on the basis of at least one operating condition of the sensing device In features of this aspect the temperature control loop is operatively connected to a detection circuit during a first mode of operation which may be a warm up phase and the bias current control loop is operatively connected to the detection circuit during a second mode of operation which may be a normal operation phase the controller has a switch adjustable between at least a first switch position in which the temperature control loop is operatively connected to a detection circuit and a second switch position in which the bias current control loop is operatively connecte
48. ng first and second electrodes wherein the first elec trode is connected to the power source for heating the first electrode a temperature controller operatively connectable to the detection circuit for maintaining a temperature of the first electrode at a predetermined magnitude and a current controller operatively connectable to the detection circuit for maintaining a current in the second electrode at a predeter mined magnitude In features of this gas detector the temperature controller is operatively connected to the detection circuit during a first mode of operation and the current controller is operatively connected to the detection circuit during a second mode of operation the first mode of operation is a warm up phase and the second mode of operation is a normal operation phase the gas detector has a switch adjustable between at least two positions wherein in a first switch position the temperature controller is operatively connected to the detec tion circuit and in a second switch position the current controller is operatively connected to the detection circuit the position of the switch is determined on the basis of an operating condition of the gas detector and the sensing device includes a cathode wire an anode wire at least partly surrounding the cathode wire and having opposing ends a pair of supply contacts electrically connected to respective ends of the anode wire a pair of temperature sense contacts electrically co
49. nnected to respective ends of the anode wire and a cathode contact electrically connected to an end of the cathode wire The present invention also includes a method of control ling the operation of a gas sensing device the gas sensing device for indicating the presence of a gas of a predeter mined type wherein the method includes the steps of adjustably heating the gas sensing device generating a bias current controlling the temperature of the heated gas sens ing device on the basis of at least one operating condition of the sensing device and controlling the bias current gener ated by the heated gas sensing device on the basis of at least one operating condition of the sensing device In features of this method the temperature controlling step includes the step of maintaining the temperature of the heated gas sensing device at a predetermined absolute temperature the method further comprises the step of mov ing the sensing device into the presence of a gas of a predetermined type and the bias current controlling step includes the step of maintaining the magnitude of the bias current at a generally constant level during the moving step generating a signal at least partially representative of the temperature of the sensing device and monitoring the signal for an indication of the presence of at least one predeter mined gas the steps of controlling the temperature of the heated gas sensing device and controlling the bias current ge
50. o a single ended signal in synchronization with the off time of the modu lator 16 The output of the voltage potential converter 42 is connected to the first gain amplifier 44 the output of which is connected to the input of the temperature error subtractor 48 As described with regard to the second preferred embodiment of the present invention the input of the second gain amplifier 54 is connected to the cathode contact 30 of the sensing device 11 and carries a voltage level proportional to the bias current of the sensing device 11 The output of the second gain amplifier 54 is connected to one input of the bias current error subtractor 58 while the other input of the bias current error subtractor 58 is connected to the bias current US 6 644 098 B2 19 input means 56 as described previously The output of the bias current error subtractor 58 which thus carries a signal representing the bias current error is connected to the second processing means 60 The controllable switch 62 has one input connected to the output of the second processing means 60 and another input connected to the temperature input means 46 The output of the controllable switch 62 is connected to the input of the temperature error subtractor 48 The controllable switch thus is adaptable to route either the output from the temperature input means 46 to the temperature error subtractor 48 or the output from the second processing means 60 to the tem perature erro
51. odiment shown in FIG 4 the output of the modulator 16 is connected to the switch 15 in order to modulate the current generated through the anode coil 26 In operation the user turns the refrigerant detector 5 on and sets the controllable switches 70 72 to route the output of the first gain amplifier 44 and the signal from the temperature input means 46 to the multi purpose subtractor 66 Preferably both controllable switches 70 72 may be adjusted simultaneously using any suitable control apparatus or method such as a single mechanical control or transistor which is operatively connected to both controllable switches 70 72 on a command from a microprocessor or the like The user may then input a desired temperature into the refrigerant detector 5 using the temperature input means 46 and proceed to warm up the sensing device 11 quickly using the method described with regard to the first variation of the third preferred embodiment Once the refrigerant detector 5 has been warmed up the user may choose to simultaneously adjust the controllable switches 70 72 to route the output of the second gain amplifier 54 and the signal from the bias current input means 56 to the multi purpose subtractor 48 As described previously the switches 70 72 may be adjusted automati cally as soon as the desired operating temperature is reached and the user may then input a desired bias current into the refrigerant sensor 5 using the bias current input means
52. onding increase in the voltage across the resistor 36 This increase in the magnitude of the bias current from the sensing device 11 is then detected by the post processor 18 and the leak detection indicators and alarms 20 are utilized to inform the user of the presence of a leak Although a bias current increase may also be caused by an increase in the temperature of the sensing device 11 which commonly occurs when prior art refrigerant sensors are moved from a cooler area to a warmer one the temperature of the sensing device 11 of the present invention is main tained at a constant controllable temperature by the tem perature control loop 40 False readings caused by an increase in bias current generated as a result of a higher sensing device temperature are thus avoided as are false readings caused by fluctuations in the battery power supply 12 As a result an increase in the bias current may more dependably be interpreted by the refrigerant detector 5 as indicating the presence of halogen molecules rather than being a false reading In a second embodiment of the present invention shown in FIG 3 the control loops 22 include only a bias current control loop 52 The bias current control loop 52 which may also be implemented by either digital microprocessor and code or analog discrete components means includes a second gain amplifier 54 a bias current input means 56 a second subtractor 58 and a second processing means 60 The input
53. onitoring steps until the desired temperature value has been reached the method includes the step of storing the desired temperature values in a memory and the sequence of desired temperature values is selected to create a ramp function of temperature versus time BRIEF DESCRIPTION OF THE DRAWINGS Further features embodiments and advantages of the present invention will become apparent from the following detailed description with reference to the drawings wherein FIG 1 is a detailed diagrammatic view of a prior art sensing device for use in various embodiments of the heated electrode refrigerant detectors of the present invention FIG 2 is a schematic diagram of a first preferred embodi ment of a heated electrode refrigerant detector according to the present invention FIG 3 is a schematic diagram of a second preferred embodiment of the heated electrode refrigerant detector of the present invention FIG 4 is a schematic diagram of a third preferred embodi ment of the heated electrode refrigerant detector of the present invention FIG 5 is a schematic diagram of a variation of the third preferred embodiment of the heated electrode refrigerant detector of FIG 4 and FIG 6 is a detailed diagrammatic view of an improved sensing device suitable for use in the primary detection circuits of FIGS 2 4 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings in which like numerals represent like com
54. onstant level on the basis of the first signal and monitoring the second signal for an indication of the pres ence of at least one predetermined gas In features of this method the second operating condition is a temperature of the sensing device the presence of a predetermined gas is indicated by a decrease in temperature the first electrode includes at least two ends and the step of generating a second signal includes generating the second signal at one or more of the ends of the first electrode the method further includes the step of moving the sensing device into the presence of one of the predetermined gases and the maintaining step includes maintaining the magnitude of the bias current at a generally constant level during the moving step the step of generating the bias current includes the step of generating the bias current according to a duty cycle and the step of maintaining the magnitude of the bias current at a generally constant level includes maintaining the magnitude of the bias current at a generally constant level according to the value of the duty cycle The present invention also includes a method of estimat ing the remaining useful life of a heated electrode gas detector for sensing the presence of at least one predeter mined gas the gas detector having first and second electrodes wherein the method includes the steps of adjust ably heating the first electrode to maintain a current in the second electrode of a pred
55. ount of time required to re polarize being inversely related to the amount of heat which is applied Thus the re polarization time may be minimized by maximizing the temperature to which the sensing device 11 is heated Unfortunately it is possible to over polarize a sensing device 11 particularly one which has been used only moments before and therefore requires little if any re polarization This may result in a sharp temperature drop when the bias control loop 52 is in operation or a sudden increase in the magnitude of the bias current when the temperature control loop 40 is in operation either of which may be improperly interpreted as a refrigerant gas detection Continued operation of the sensing device at the unneces sarily high peak temperature also contributes to the fore shortening of the sensing device lifespan It is thus critical to lower the temperature of the sensing device 11 to the desired operating temperature as soon as re polarization is complete and reliable operation may be ensured This tem perature adjustment may be based on the status of the bias current which decreases sharply in a significantly depolar ized sensing device 11 under constant temperature condi tions but settles out asymptotically to a constant magnitude US 6 644 098 B2 21 as re polarization is completed It has been determined that reliable operation of the detector 5 may be ensured once the bias current drops sufficiently close to its
56. ponents throughout the several views an improved heated electrode refrigerant detector 5 having one or more control loop in accordance with the preferred embodiments of the present invention will now be shown and described FIGS 2 4 are schematic diagrams of first second and third preferred embodiments of the improved heated electrode refrigerant detector 5 of the present inven tion In each preferred embodiment the heated electrode refrigerant detector 5 of the present invention comprises a primary detection circuit 10 a post processor 18 for post processing one or more signals a leak detection indicator and alarm 20 and at least one control loop 22 The primary detection circuit 10 includes a sensing device 11 a battery power supply 12 a current source 14 a switch 15 for bypassing the current source 14 a modulator 16 for modu lating the switch 15 according to a desired duty cycle determined by one or more of the control loops 22 and a number of basic circuitry components including first and second capacitors 32 38 a resistor 36 and an inductor 34 The sensing device 11 may be any conventional heated electrode refrigerant sensing device such as the one previ ously described and illustrated in FIG 1 or may alterna 10 15 25 30 40 45 50 55 60 65 12 tively be an improved sensing device such as the one described in conjunction with FIG 6 Further in an improved method of making the sensing dev
57. pplied to the sensing device 11 and that power is directly related to the value of the duty cycle Thus any change to the temperature of the sensing device 11 may be seen first as a change in the duty cycle of the modulator 16 Thus the occurrence of a rapid reduction in the duty cycle of the sensing device 11 may be detected by the post processor 18 and the leak detection indicators and alarms 20 are utilized to inform the user of the presence of a leak It should be obvious to one of ordinary skill however that the post processor 18 may alternatively be adapted to receive information related directly to the temperature of the sensing device 11 in which case a decrease in the temperature thus directly indicates the presence of halogen molecules at the sensing device 11 In a further feature of the present invention a method is also provided for determining the remaining useful life of a detector 5 having a bias current control loop 52 Because over time the bias current generated by the sensing device 11 would naturally tend to decrease as the sensing device 11 is used up the temperature must regularly be increased in order to compensate for this natural decrease However 10 15 20 25 30 35 40 45 50 55 60 65 18 every sensing device 11 has a maximum safe operating temperature above which it cannot be safely operated with out significantly increasing the risk of damage to the detec tor 5 and injury to
58. r subtractor 48 as desired or on the basis of one or more particular operating condition As used herein the term operating condition may include without limitation a desired or actual temperature a desired or actual bias current magnitude a period of time the amount of moisture in the sensing device 11 and the like The output of the first subtractor 48 is connected to the first processing means 50 as in the first preferred embodiment and the output of the first processing means 50 is connected to the input of the modulator 16 Finally the output of the modulator 16 is connected to the switch 15 in order to modulate the current generated through the anode coil 26 In operation the user turns the refrigerant detector 5 on and sets the controllable switch 62 to route the temperature input means 46 to the temperature error subtractor 48 The user may then input one or a series of desired temperatures into the refrigerant sensor 5 using the temperature input means 46 In a preferred method of warming the sensing device 11 up quickly while at the same time minimizing the amount of stress placed thereon it has been found that the first temperature entered may be zero or its equivalent followed by a series of successively higher set point values chosen to create a ramp function until the sensing device 11 reaches a particular peak temperature The temperatures in the series may be generated during warm up using an algorithm based on tim
59. rature for the sensing device 11 If a microprocessor is utilized then the preferred temperature input means 46 would be either a 10 15 20 25 30 35 40 45 50 55 60 65 14 preprogrammed set point or a pre defined temperature vs time profile with the former being used for fixed tempera ture operation and the latter for the preferred warm up procedure described herein and or for normal operation Either the preprogrammed set point or the temperature time profile could be programmed into a microprocessor in order to eliminate user intervention Additionally in the microprocessor controlled system it should be clear that the input from the temperature input means 46 and the output of the first gain amplifier 44 may both be digitized and so the values processed by the first subtractor 48 may be digitized values rather than actual voltages If the temperature control loop 40 is instead implemented in discrete components then the temperature input means 46 preferably includes a keypad for numerically inputting a particular desired temperature which may then be automati cally converted to a corresponding voltage level but it should be clear that other devices may be used to input a particular number and that alternatively a user could input the voltage level corresponding to a particular temperature directly without any need for conversion Alternatively the temperature input means 46 could include an adjustm
60. rent control loop 52 converts and amplifies the voltage potential across the resistor 36 and subtracts that signal from the desired set point signal provided from the bias current input means 56 US 6 644 098 B2 17 The resulting error signal is amplified and phase compensated by the second processing means 60 in order to optimize settling time overshoot and ringing The output of the second processing means 60 is a duty cycle set signal which is provided as an input to the modulator 16 The modulator 16 then adjusts the duty cycle of the modulation to raise or lower the amount of heating applied to the sensing device 11 as described with regard to the temperature control loop 40 Because the magnitude of the bias current is directly related to the temperature of the sensing device 11 a rise or drop in the temperature of the sensing device 11 results in a corresponding respective rise or drop in the magnitude of the bias current Thus any change in the magnitude of the bias current is detected by the bias current control loop 52 and counteracted by a corresponding adjust ment to the temperature of the sensing device 11 to bring the bias current back to the specified level Because this process occurs continuously the bias current from the sensing device 11 is always maintained reasonably close to the set point regardless of any external influence or conditions including the presence or absence of halogen molecules at the sensing device 1
61. rocess as indicated by the temperature of the active sensor compared to that of the reference sensor This characteristic makes the active sensor of the 724 patent impervious to fluctuations in absolute temperature due to ambient conditions However in heated electrode refrigerant detector systems the presence of the gas sought is indicated generally by an increase in bias current which is also affected by the ambient temperature of the sensor As a result a heated electrode refrigerant sensor using the temperature control system of the 724 patent would still be affected by ambient conditions because it is incapable of controlling the absolute temperature of the sensor In addition the absolute temperature of the sensor cannot be controlled to prevent damage during warm up of the system and the like Thus a need exists for a temperature control system suitable for use with a heated electrode refrigerant detection system which does not make use of a reference sensor and which may be utilized to control the absolute temperature of the heated electrode US Pat No 3 912 967 to Longenecker the 967 patent discloses a circuit for providing regulation of the absolute temperature of a heater anode of a refrigerant gas sensor power supply outputs two different DC voltage levels one of which is connected through a transistor switch to the heater anode coil of a heated electrode gas sensing element The circuit monitors the approximate
62. t of the second gain amplifier 54 and the signal from bias current input means 56 and makes the difference available at its output The output of the second subtractor 58 is con nected to the second processing means 60 both of which may easily be constructed by one of ordinary skill in the art of signal processing methods and apparatuses Like the first processing means 50 the second processing means 60 is an analog or digital filter whose coefficients may be determined empirically by one of ordinary skill and may be utilized to amplify and phase compensate the signal from the second subtractor 58 The output of the second processing means 60 is connected to the input of the modulator 16 to provide a duty cycle set signal to the modulator 16 which may be identical to the pulse width modulator described with regard to the first preferred embodiment As with the first preferred embodiment the output from the modulator 16 is connected to the switch 15 in order to modulate the current generated through the anode coil 26 at a frequency of approximately 20 kHz and a duty cycle ranging from approximately 3 to 8 In operation the user turns the refrigerant detector 5 on and a bias current of a desired magnitude is provided to the refrigerant detector 5 in one of the manners described above using the bias current input means 56 The bias current control loop 52 supplies a pulse width modulation signal to the switch 15 at a set duty cycle The bias cur
63. t electrode is dependent on a duty cycle and wherein the step of maintaining the first electrode at substantially the predetermined absolute temperature includes the step of adjusting the duty cycle 10 The method of claim 4 further including the step of monitoring the actual temperature of the first electrode and wherein the step of maintaining the first electrode at sub stantially the predetermined absolute temperature includes the steps of reducing the temperature of the first electrode upon determining that the actual temperature exceeds the predetermined absolute temperature and raising the tem perature of the first electrode upon determining that the actual temperature is below the predetermined absolute temperature 11 A gas detector operative in conjunction with a power source for sensing the presence of at least one predeter mined gas the gas detector comprising an anode cathode assembly coated with a ceramic material the anode cathode assembly including a cath ode wire and an anode wire at least partly surrounding the cathode wire wherein the anode wire has opposing ends and wherein one of the anode wire ends is electrically connected to the power source a pair of supply contacts electrically connected to the respective ends of the anode wire a pair of temperature sense contacts electrically connected to respective ends of the anode wire a cathode contact electrically connected to an end of the cathode wire a te
64. t invention being limited only by the claims appended hereto and the equivalents thereof What is claimed is 1 A gas detector operative in conjunction with a power source for sensing the presence of at least one predeter mined gas the gas detector comprising a detection circuit the detection circuit including a sens ing device having first and second electrodes wherein the first electrode is connected to the power source for heating the first electrode a temperature controller operatively connectable to the detection circuit for maintaining a temperature of the first electrode at a predetermined magnitude a current controller operatively connectable to the detec tion circuit for maintaining a electrical current in the second electrode at a predetermined magnitude and a switch adjustable between at least two positions wherein in a first switch position the temperature controller is operatively connected to the detection circuit and in a second switch position the current controller is operatively connected to the detection circuit the position of the switch is determined on the basis of an operating condition of the gas detector 2 The gas detector as in claim 1 wherein the switch is located internally to the gas detector 3 The gas detector as in claim 1 wherein the switch is located externally on the gas detector 4 A method of controlling a gas detector for sensing the presence of at least one predetermined gas the
65. tage potential converter 42 first and second gain ampli fiers 44 54 temperature and bias current input means 46 56 a multi purpose subtractor 66 a multi purpose processing means 68 and a pair of controllable switches 70 72 Simi larly to the variation of the third embodiment shown in FIG 4 the voltage potential converter 42 is connected to the anode contacts 28 and the modulator 16 and converts the differential temperature signal present at the anode contacts 28 into a single ended signal in synchronization with the off time of the modulator 16 The output of the voltage potential converter 42 is connected to the first gain amplifier 44 the output of which is connected to one input of the first controllable switch 70 The other input of the first control lable switch 70 is connected to the output of the second gain amplifier 54 the input of which is connected to the cathode contact 30 of the sensing device 11 The output of the first controllable switch 70 is connected to the input of the multi purpose subtractor 66 the other input of which is connected to the output of the second controllable switch 72 The respective inputs of the second controllable switch 72 are connected to the temperature and bias current input means The output of the subtractor 66 is connected to the input of the multi purpose processing means 68 the output of which is connected to the modulator 16 Like the modulator 16 of the variation of the third emb
66. termined at the time of manufac turer 17 The system as in claim 13 further comprising means for entering the predetermined absolute temperature into the gas detector 18 The system as in claim 12 wherein the amount of heat applied to the first electrode is dependent on a duty cycle and wherein the means for maintaining the first electrode at substantially the predetermined absolute temperature com prises means for adjusting a duty cycle 19 The system of claim 12 further comprising means for monitoring the temperature of the actual temperature of the first electrode means for reducing the temperature of the first electrode upon determining that the actual temperature exceeds the predetermined absolute temperature and means for raising the temperature of the first electrode upon deter mining that the actual temperature is below the predeter mined absolute temperature 20 The gas detector as in claim 19 wherein the internal switch is implemented with discrete components 15 20 25 30 40 45 50 UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO 6 644 098 B2 Page 1 of 1 DATED November 11 2003 INVENTOR S Dennis Cardinale et al It is certified that error appears in the above identified patent and that said Letters Patent is hereby corrected as shown below Title page Item 56 References Cited FOREIGN PATENT DOCUMENTS insert WO 9924887 A 05 20 99 WIPO Sign
67. this method the method further includes the steps of selecting the predetermined absolute temperature and while the detector is being operated providing an indication of the predetermined absolute temperature to the gas detector the step of providing an indication of the predetermined absolute temperature includes the step of predefining the predetermined absolute temperature during manufacturing the step of providing an indication of the predetermined absolute temperature includes the step of entering the predetermined absolute temperature into the gas detector the amount of heat applied to the first electrode is dependent on a duty cycle and the step of maintaining the first electrode at substantially the predetermined absolute temperature includes the step of adjusting the duty cycle the method further includes the step of monitoring the actual temperature of the first electrode and the step of maintaining the first electrode at substantially the predetermined absolute temperature includes the steps of reducing the temperature of the first electrode upon determining that the actual temperature exceeds the predetermined absolute tempera ture and raising the temperature of the first electrode upon determining that the actual temperature is below the prede termined absolute temperature The present invention also includes a method of control ling a heated electrode gas detector for sensing the presence of at least one predetermined gas t
68. tor as a function of the difference between the operating temperature value and the maximum operating temperature value the method further includes the step of predetermining the maximum operating temperature the step of predetermining the maxi mum operating temperature is done empirically and the maximum operating temperature is a maximum safe oper ating temperature of the gas detector and or the maximum operating temperature is a maximum effective operating temperature of the gas detector In another aspect of the present invention a gas detector for sensing the presence of at least one predetermined gas US 6 644 098 B2 9 and operative in conjunction with a power source includes an anode cathode assembly coated with a ceramic material the anode cathode assembly having a cathode wire and an anode wire at least partly surrounding the cathode wire wherein the anode wire has opposing ends and wherein one of the anode wire ends is electrically connected to the power source a pair of supply contacts electrically connected to respective ends of the anode wire a pair of temperature sense contacts electrically connected to respective ends of the anode wire a cathode contact electrically connected to an end of the cathode wire and a temperature sensing circuit electrically connected to at least one of the temperature sense contacts for monitoring the temperature of the anode cathode assembly In features of this aspect the power sour
69. wn in the industry as poisoning the sensor and no good solution to the problem has yet to be proposed Finally despite their limited lifespan prior art refrigerant detectors provide no means of monitor ing or checking the sensor to determine its remaining life Some solutions to these problems have been proposed For example the H10Xpro Refrigerant Leak Detector avail able from the Yokogawa Corporation of America of Newnan Ga is a refrigerant leak sensor of the heated electrode type Like other sensors of this type the Yokogawa sensor becomes less sensitive over time The Yokogawa sensor allows users to increase the sensitivity of the sensor by increasing the heat which is applied to the electrode Because the magnitude of the bias current is dependent not only on the voltage potential between the anode and cathode and the amount of refrigerant present but is also dependent upon the temperature of the electrode and because the sensitivity of the sensor is related to the magnitude of the bias current the sensitivity of the sensor may be improved by raising the temperature of the electrode during operation of the sensor Yokogawa allows this to be done by manually turning a screw a small amount presumably to adjust the operating voltage of the electrode Further there is a great danger that the user may forget to return the sensor tem perature to the manufacturer s setting when he replaces a depleted sensor with a new one ther
70. y any heated electrode refrigerant sensor implementation in which accu rate information about the temperature of the sensing device 111 is desired However if the improved sensing device 111 is not readily available or if other factors make its use undesirable it should likewise be clear that the various control loops described herein may instead make use of an ordinary three terminal sensing device as previously described It will therefore be readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application Many embodiments and adap tations of the present invention other than those herein described as well as many variations modifications and equivalent arrangements will be apparent from or reason ably suggested by the present invention and the foregoing description thereof without departing from the substance or scope of the present invention Accordingly while the present invention has been described herein in detail in relation to its preferred embodiments it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention The foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiments adaptations variations modifica tions and equivalent arrangements the presen
71. y power supply 12 a greater amount of the time The effect of this is to raise the temperature of the sensing device 11 to the desired tempera ture input using the temperature input means 26 On the other hand when the measured temperature of the sensing device 11 is higher than the desired temperature then the duty cycle set signal represents an instruction to the modu lator 16 to decrease the duty cycle thus leaving the switch 15 on for a lesser proportion of the period of each cycle and allowing the anode coil 26 to be heated by the battery power supply 12 a lesser amount of the time The effect of this is to lower the temperature of the sensing device 11 to the desired temperature which was input using the tempera ture input means 46 By constantly monitoring the actual temperature of the sensing device 11 and adjusting the amount of applied power accordingly the temperature of the sensing device 11 may be held substantially constant Once the refrigerant detector 5 is operative the user may utilize it to detect the presence of halogen molecules and accordingly to identify a refrigerant leak To detect a leak the refrigerant detector 5 may first be reset in a location which is known to be free of halogen molecules The refrigerant detector 5 may then be moved to the desired test location If the sensing device 11 is moved into the presence of halogen molecules the bias current will correspondingly increase resulting in a corresp

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