CR-1A User`s Manual
Contents
1. eaha 18 7 4 Troubleshooting Guide edes eS tO esu 19 Appendix 1 Humidity Conversion Equations 20 Appendix 2 Connector Pin Assignments 22 Appendix secto o etos T ad Maulana 23 Appendix 4 Mirror cleaning procedure 2 nd Ide 24 Fig 1 Main unit FIGURES Fig 2 Block diagram CR 1A chilled mirror hygrometer eee Fig 3 Sensor assembly block diagram eter e eta rua 1 INTRODUCTION The Model CR 1A Cryogenic Hygrometer is a high performance instrument capable of fast accurate measurements over an extremely wide humidity range Because these performance capabilities have not previously been available in a single instrument the CR 1A represents a significant advance in the state of the art The CR 1A differs from the original CR 1 primarily by having compact size light weight and a linear output signal There is a battery powered option for portable use for up to 8 hours There is also an optional version of the CR 1A with an autofill system that allows for continuous operation when connected to a large LN2 dewar and provide continuous use for up to a month at a time The technology incorporated in the CR 1A is protected by domestic and foreign patents 1 1 General Description The CR 1A is a chilled mirror condensation type hygrometer Its high performance is achieved by cryogenically cooling a mirror us2. BUCK RESEARCH Al INSTRUMENTS LL C MODEL CR 1A HYGROMETER OPERATING MANUAL Buck RESEARCH INSTRUMENTS CR 1A Chilled Mirror Hygrometer May 2008 BUCK RESEARCH INSTRUMENTS LLC PO Box 19498 Boulder CO 80308 Copyright 2008 Buck Research Instruments LLC All rights reserved TABLE OF CONTENTS 1 ieee eek ny eileen ata 4 1T General Description 4 5 2 Location OF Principal Components 6 c E E 6 3 Installation and Operation 8 Installation rc 8 3 2 Power up Procedure uoo lod o a a alee ees ede 8 TE EDT UNI TUM 9 3 4 Powerdown Proced re s ces des EO eu eoi eua HAS toT esu e 10 2 taria 11 41 Data Signals aco pee 11 4 2 Data Reduction Equations 11 9 Principles of Operation psi MN Dea PNE EUH 12 General d aee deu sce E 12 5 Techntesl 12 System eoe enses oet p dapes ata ford NE 12 2352 2 Mirr Module os store orci vet iet au ce cdd dat oum iuf 15 5 2 3 Optics 15 5 2 4 Temperature i eee eis Ua ao ride 15 6 Measurement Limitations lo 16 7 Maintenance and Troubleshooting 17 Ti Sample line Cleaning ione 17 aae ed cs 17 y WIInMPM
3. f2 r x P 622 x 109 r r f2 RH x f1 T 100 RH f2 rho x Tk 216 7 rho Vapor pressure e P 622 x 103 r RH x f1 T 100 RH rho x Tk 216 7 rho Mixing ratio r ppmw 622 103 x e P e 622 103 x f1 DP P f1 DP DP 622x103 x RH x es 100 x P RH x es RH 622 103 x rho x Tk 216 7 x P rho x Tk rho Relative humidity RH 100 x f1 DP f1 T DP 100 x e es e 100 x r x P 622x109 x es r 100 x rho x Tk 216 7 x es rho Absolute humidity rho 216 7 x f1 DP Tk DP 216 7 x e Tk e 0 2167 x r x P 622 001 x r x Tk r 216 7 x RH x es 100 x Tk RH mixing ratio by volume ppmv mixing ratio by weight ppmw x 1 6077 grains lb r x 0 007 Precipitable cm per km rho 10 20 NOTE 1 f1 DP and f2 e are variations vapor pressure formulations found in Buck J Appl Met 20 pp 1527 1532 1981 They are given by e vs DP ores vs T f1 DP EF x aw x exp bw DP dw x DP DP cw over water EF x ai x exp bi DP di x DP DP ci over ice DP vs e or T vs es 12 dw 2 x bw s bw s 4 cw x s dw 2 over water di 2 x bi s bi s 2 4 ci x s di 1 2 over ice where aw 6 1121 ai 6 1115 bw 18 678 bi 23 036 cw 257 14 ci 279 82 dw 234 5 di 333 7 In e EF In aw or ai EFw 1 10 4 7 2 P 0 0320 5 9 x 10 6 TH EF 1 10 4
4. If possible keep flow in the range 0 5 2 liters minute 0 25 4 scf h At very high dew frost point temperatures higher flow may be allowed Keep the sample line inlet protected from contamination This is best achieved by keeping the sample line closed when not connected to the desired sample WHEN MEASURING WATER CONCENTRATION IN NATURAL GAS A GLYCOL ABSORBING FILTER SUCH AS AN A GLYSORB FILTER MUST BE INSTALLED BEFORE THE INLET TO THE CR 4 OR ERRONEOUS READINGS WILL RESULT When making a large downward change in humidity it is better to make several intermediate steps rather than one large step to avoid losing the condensation layer on the mirror At low frost point values always allow time for the moisture levels in the lines and sensing chamber to equilibrate and for the D F point temperature to completely stabilize before taking a reading To avoid internal line condensation and resultant erroneous readings do not allow the inlet lines to cool below the expected frost point temperature 3 4 Power down Procedure Watch the display For autofill system See disconnection procedure in appendix When the mirror temperature starts to get warmer than the dew point you expect this indicates that there is no more LN2 in the dewar and you can proceed to step 2 Initiate a balance cycle by depressing the BALANCE switch for 5 seconds Watch the mirror temperature if the mirror temperature does not go below 0 C turn
5. balance routine 25
6. make up a thermo optical servo system which operates to maintain a constant layer of condensate When condensate is thus equilibrated mirror temperature is then at the dew frost point which is sensed by the imbedded temperature sensor Since the dew frost point temperature is a fundamental measure of humidity the CR 1A is intrinsically capable of long term accuracy and stability The development of this hygrometer follows the original work of H J Mastenbrook at NRL His work was adapted by the NOAA Geophysical Monitoring for Climatic Change GMCC program for balloon borne stratospheric water vapor measurements Buck Research has extensively redesigned and reconfigured the instrument for a broader range of measurements and applications incorporating proprietary new technical innovations in the process 5 2 Technical details A block diagram of the cryocooled hygrometer is given in Figure 2 and the sensor assembly is diagrammed in Figure 3 5 2 1 Sampling system The gas to be measured sample gas is brought to the sensing chamber through an inlet system and allowed to flow across the mirror surface in the sensor chamber At the exit of the sensor chamber the sample gas flows by a pressure gauge and is then returned to the original gas stream or exhausted as desired 12 BALANCE SIGNAL AIRFLOW TEMPERATURE DEW FROST POINT Fig 2 Block diagram CR 1A chilled mirror hygrometer 13 OPTICS BLOCK SENSING CHAMB
7. no gap between the sample chamber and the lens holder Do not use excessive force when tightening the screws Do a leak check now If there is a leak alternate tightening the 6 screws by tightening the screw 180 degrees from the one just tightened then move to the next screw and do the same thing If you number the screws 1 6 where is on the top go 1 4 2 5 3 6 You may need to use a right angle 9 64 hex key to get more torque if the ball driver will not tighten the screws any more Lastly put the optics block and cap back on using the 5 64 ball driver Insert the ball driver into the 2 holes on top of the optics cap to find the socket head screws in the optics block Make sure that there is no gap between the optics block and the lens holder 24 Addendum Key stroke input to RS 232 decrease the contrast of the LCD display by 2 increase the contrast of the LCD display by 2 decrease the proportional gain coefficient by 2 3rds increase the proportional gain coefficient by 5096 decrease the derivative gain coefficient by 2 3rds increase the derivative gain coefficient by 5096 decrease the integrator time constant coefficient by 2 3rds increase the integrator time constant coefficient by 50 ga Each repeated key press is progressive For example 5 C presses will increase the contrast by 10 5 p presses will decrease the gain by about 87 R resets all coefficients and contrast to original values B initiates a
8. power off If the mirror temperature goes below 0 C wait for the mirror temperature to warm up to 0 C before turning off power Turn off flow of gas to the CR 1A You can leave the power supply connected to recharge the batteries HOWEVER it is not recommended to leave the external power supply connected for long periods of time as the batteries are continually charged and will start to get hot We recommend disconnecting the external power supply after 24 hours if the CR 1A is not turned on again and used in that period of time 10 4 SIGNAL PROCESSING 4 1 Data Signals The following analog signals are available at signal connector J2 and vary over the following ranges VDF Mirror temperature Dew frost point 0 10 and 4 20 mA PRES Chamber pressure 0 10v and 4 20 mA H20 CONC Ib MMscf or 0 10v and 4 20 mA optional BAL Balance voltage 0 10v and 4 20 mA VAMB Ambient temperature 0 10v and 4 20 mA optional The 4 20 mA outputs sink current Connect up 24 VDC to the 4 20 mA returns either using the pins on the connector or from your data acquisition system The current flowing into the 4 20 mA returns corresponds with the equations below 4 2 Data Reduction Equations VDF Dew frost point temperature is determined from VDF by Taf CC 150 20 x VDF v 0 10V Taf CC 150 8xIDF ma 4 20 mA VAMB Chamber temperature is determined from V AMB by Tamb C 100 20 x VAMB v 0
9. the dewar when the CR 1A is off Note These are approximate specifications Exact performance will vary depending on installation and operating environment 2 LOCATION OF PRINCIPAL COMPONENTS 2 1 Main Unit Figure 1 Front Balance Switch Buck RESEARCH INSTRUMENTS _ Display CR 1A Chilled Mirror Hygrometer Heat Cool Switch Power Switch Rebalance LED Service Mirror LED Top Dewar top with 4 Black plastic dewar built in funnel gt lid with built in brass pressure relief valve Back RS 232 WARMING P am WES Signal Output J2 Sample chamber pressure connection 3 Inlet to Sample and outlet Chamber Fig l Main unit 6 T m Eqs BALANCE SWITCH momentary Depress for 5 seconds to initiate a balance cycle This will heat the mirror up to 30 C to clear off condensation and rebalance the optics circuit HEAT COOL SWITCH momentary Provides additional heat to partially clear the mirror of condensate or full cooling to allow additional frost to collect on the mirror Also used to speed initial layer formation See section 3 2 REBALANCE LED Lights during balance cycle Also stays lit after balance cycle if there is a small amount of contamination on the mirror DISPLAY Backlit LCD display that displays H2O concentration mirror temperature Mirror T or D F Point when dew or frost point acquired pressure in millibar and balance 200 when dew frost point acq
10. 10V Tamb CC 100 8 x IAMB ma 4 20 mA PRESSURE Chamber pressure is calculated from the Vpress signal voltage by Press Vpress 1 6 1 5625 O 10 bar sensor 10 for 1 bar sensor Press Ipress 4 1 6 0 10 bar sensor 10 for 1 bar sensor BALANCE When balanced BAL 5V or 12 5 mA H20 CONC 2 4 0 1000 ppbv 4 6 0 1000 ppmv 6 8 V 0 1000 PPTV Conversion to Other Humidity Units To convert dew frost point readings to other humidity units refer to Appendix 1 11 1 2 3 4 5 6 5 PRINCIPLES OF OPERATION 5 1 General The CR 1A is a chilled mirror condensation type hygrometer consisting of the following principle components a rhodium plated copper mirror with an attached stem an associated temperature sensor and heating coil a liquid nitrogen system for cooling the mirror an optical system for sensing condensate mirror reflectance and control circuitry for controlling mirror temperature via the heating coil Operation is based on maintaining equilibrium vapor pressure over a water or ice surface on the mirror Above the equilibrium temperature mass transport is away from the surface and below the equilibrium temperature it is onto the surface When the surface is just at the dew frost point temperature the mass of condensate on the surface remains constant As is the case with conventional cooled dew point devices the mirror optics and electrical circuit
11. 2 2 P 0 0383 6 4 x 10 6 2 where P is in kPa and T is in C NOTE 2 RH is defined here using es with respect to ice below freezing However RH is also frequently defined using es with respect to water even below freezing NOTE 3 These conversions are intended for use with moist air rather than pure water vapor They therefore include EF the enhancement factor which corrects for the slight departure of the behavior of water in air from that of a pure gas NOTE 4 The definitions f1 and f2 for ice agree with an extrapolation of NBS values down to 120 deg C within 0 5 21 APPENDIX 2 CONNECTOR PIN ASSIGNMENTS J2 16 pin Signal connector Pin 1 Balance 0 10 V 2 Balance Return 3 Balance 4 20 mA DC Power 4 Balance 4 20 mA Return for self powered loops 5 Pressure 0 10 6 Pressure Return 7 Pressure 4 20 mA 8 Pressure 4 20 mA Return 9 VDF 0 10 V 10 VDF Return 11 VDF 4 20 mA 12 VDF 4 20 mA Return 13 Dew Point 0 5 V 14 Dew Point Return 15 Dew Point 4 20 mA 16 Dew Point 4 20 mA Return J4 RS 232 connector 9600 Baud 8 N 1 Flow control set to NONE Use null modem cable with Female Female ends Included APPENDIX 3 WARRANTY Manufacturer warrants that the items delivered shall be free from defects latent and patent in material and workmanship for a period of one year after acceptance of the specific goods by Buyer The Buyer s sole and exclusive remedy under this warranty shall be lim
12. ER Fig 3 Sensor assembly block diagram 14 The sampling system must be carefully sealed to prevent room or cabin air from contaminating the measurements Stainless steel materials must be used throughout the inlet portion to minimize outgassing during low humidity sampling 5 2 2 Mirror Module The mirror assembly consists of a mirror a mirror support and the RTD heater and sensor that are attached to the mirror A small ultra stable RTD is installed beneath the mirror face to measure the dew frost point temperature Heating is provided by a resistive heater coil wound around the mirror stem 5 2 3 Optics Module The mirror surface is maintained continuously and automatically at the frost point temperature by an electro optical control system This system measures the quantity of light specularly reflected from the mirror condensate and maintains a constant reflectance at the mirror surface thus providing the condensate equilibrium for the frost point temperature The optics module consists of a photodiode pair and a light emitting diode LED One photodiode maintains constant LED intensity the other photodiode provides a current output that is proportional to the light reflected from the mirror The bias circuit is set so that when the proper condensation layer is on the mirror about 80 of the light emitted by the LED is received at the detector 5 2 4 RTD Readout For obtaining the dew or frost point temperature from the temper
13. ature senor three readouts are provided VDF signal voltage RS 232 output and a direct temperature indication on the display All are accurate within 0 15 C throughout the measurement range 15 6 MEASUREMENT LIMITATIONS Under field operations measurement errors can arise from a number of causes Any deviation of the mirror temperature from the frost point temperature will of course cause error Perhaps the most common error source is from outside air leaking into the hygrometer sampling system Therefore it is important that the instrument be leak tested periodically and with each relocation of the instrument especially if components of the instrument have been exchanged or serviced Long exposure of the sampling system to high humidities or condensation of water which occurs if cold surfaces are exposed to ambient air which has a higher dew point than the temperature of the surfaces causes temporarily high readings until the walls have completely outgassed The lines may take a very long time to dry enough to allow accurate readings when measuring frost points below 50 C Any hygroscopic material in the lines or chamber such as dust further lengthens this time It is therefore advised to keep the lines clean and dry Calibration of the mirror temperature sensor and associated electronics is required to accurately determine dew frost point temperature Buck Research Instruments has done this Once calibrated the temperature se
14. ay changes from Mirror T to D F Point and 2 Conc Will change from XXX X to a value 5 The instrument is now ready for use Connect up the flow of gas Keep the flow to 2 slpm or less to improve stability and keep the heat load on the mirror to a minimum The higher the flow rate the faster the LN2 will boil off NOTE It is always advisable to start measuring at relatively high humidity values above 20 C to allow easy acquisition of condensation and then go down in frost point temperature If it is necessary to begin operating initially at very low frost points acquisition of frost can be speeded up by use of the HEAT COOL SWITCH and by spiking the pressure inside the sample chamber To do this stop flow through the sample chamber for no more than 1 2 seconds by putting a finger over the outlet of tubing connected to the outlet of the sample chamber or closing a valve downstream of the outlet of the sample chamber This will spike the pressure inside the chamber causing a large amount of moisture to collect on the mirror Watch the BALANCE value on the display It will go positive As soon as the value stops increasing depress the COOL switch and hold it This will keep the frost layer on the mirror from being burned off and enable readings within 1 or 2 minutes instead of 10 to 20 minutes 3 3 Operation During operation no special attention is required except for an occasional check of operating voltages to assure proper function
15. chamber Also the lens sticks out of the lens holder so do not place the lens holder lens side down Always have the lens facing you when setting the lens holder down You should be able to reuse the gasket about 10 times and if you are lucky and do not touch it the gasket will stay in place Cleaning the Mirror Once you have access to the mirror dip a cotton tipped applicator in acetone and clean the mirror gently Use each applicator only once After the mirror has been cleaned with acetone wipe the mirror with a dry applicator Then dip another applicator in water and wipe the mirror off again Finally wipe the mirror off with a dry applicator to complete the cleaning process Replacement Now put the lens holder back on the sample chamber Use the index marks to get the correct alignment Start middle 2 screws that are 180 degrees apart Leave gap for the gasket if the gasket has separated from the sample chamber Next insert the bottom 2 socket head screws to prevent the gasket from falling through Drop the gasket into the 4 gap making sure that it is in the correct position before tightening the screws any further If the gasket is sitting too low a small metal strip or a metal ruler can be used to lift the gasket or optional viton o ring back into place Tighten the captive screws first using the 9 64 ball driver Alternate back and forth between both screws Tighten the other 4 socket head screws making sure that there is
16. destroy their sealing ability Adjust the flow rate of the gas so that it is less than 2 4 scf h Initially and as often thereafter as necessary check that all electrical and mechanical connections are secure It may be advisable to test for leaks using one of the methods in Section 7 This is especially important when operating the instrument in a humidity environment that is very different from that of the sampled air 3 2 Power up Procedure Connect the external power supply to the power connector on the back of the instrument Flip the power switch to the ON position If unit is supplied with internal batteries external power supply connection is not required However if LOW BATTERY Is displayed in place of BALANCE on the display after the balance cycle in step 2 below completes reconnect the power supply as the batteries are almost completely discharged and the instrument will soon shut down After initialization of the electronics a balance cycle will be initiated The Rebalance LED will light and the display will show an increasing mirror temperature until 30 is reached Dewar with liquid nitrogen Please see separate instructions in the appendix if you have the autofill system First unscrew the black plastic threaded lid that contains a brass pressure relief valve This will expose a funnel shaped lip on the top of the dewar Use the included NISSAN STAINLESS thermos to fill the dewar If you put
17. ed on a cryogen Dewar and electronics to provide power control displays and analog output signals all contained in a single chassis Great care is taken to ensure maximum reliability and safety of operation including coolant overpressure protection and dual protection against overheating of the mirror assembly The components of the CR 1A are Main assembly containing dewar sensor and electronics Power cable and 24 VDC brick power supply Service kit with spare o rings and gaskets NISSAN STAINLESS thermos for filling dewar Null modem cable and connector for J2 Operating manual 1 2 Specifications Model CR 1A Cryocooled Hygrometer Measurement range Moisture concentration Dew Frost point Dew Frost point reading accuracy Response time Nominal operating range Ambient Temperature Gas Pressure Flow rate of sample Construction Dewar capacity Operating time per filling With optional autofill system Loss rate when not in use Input voltage optional Power consumption Weight Dimensions chassis 0 01 to 27 000 ppmv at SP 120 9C to 30 9C 0 15 10 30 sec typical 40 to 40 C 0 to 150 psia 0 10 bar 5 2 liters minute 316L stainless steel 0 6 liter 6 0 10 0 hours 1 month about 0 6 L day 100 240 VAC 50 60 Hz brick supply 4500 mAh NiMH battery 25 watts maximum 12 5 lbs 5 7 kg including batteries 8 W x 12 Dx 11 H 20 x 30 x 27 5 cm This is just for reference DO NOT leave LN2 in
18. ed when measuring natural gas 16 7 MAINTENANCE AND TROUBLESHOOTING The following maintenance items should receive attention as required 1 Cleaning of sample lines depending on use Sect 7 1 1 Mirror check and cleaning Sect 7 2 2 Leak checking Sect 7 3 7 1 Sample line Cleaning To keep sample lines clean thus improving response at very low humidities wash with water or acetone and blow dry with a mild pressure from a dry air or nitrogen source It may be desirable to heat the lines for a few moments to drive off residual water 7 2 Mirror cleaning The mirror should be cleaned when the Check Mirror LED is flashing 1 Make certain mirror is at or near the room temperature and power has been shut off There are 2 holes in the flat part of the optics block Insert hex ball driver into holes and loosen each screw The screws will remain captive inside Once screws are loose remove optics block It will be held by the cable coming out of it which is not removable The mirror is now exposed and can be cleaned 2 See Appendix 4 for detailed procedure 3 If necessary repeat process If contamination persists clean again with acetone followed by water using a small amount Never use alcohol in the sensing chamber as this affects the hygroscopic properties of the mirror surface for some period of time In the absence of acetone distilled water alone or used after acetone or MEK can be effective CAUTION The mir
19. ing liquid nitrogen or other cryogen and holding it at the frost point by a heater control system Optical detectors sense condensate on mirror and a thermistor imbedded in the mirror is used to determine mirror temperature the dew or frost point Since operation of the CR 1A is based on a fundamental property of water vapor dew frost point it is intrinsically capable of long term accuracy and stability The CR 1A features an autobalancing function that automatically adjusts for mirror contamination To keep a constant layer of dew or frost on the mirror the reflectivity of the mirror must be measured when the mirror is completely cleared of condensation and when it is completely obscured by condensation Once this is done 20 of this span is calculated and set to be the control point This is achieved by heating the mirror to a high enough temperature to remove condensation then turning on and off the LED that shines on the mirror If contamination ever gets to be enough that the control point cannot be achieved another balance function will be initiated and the control point reset Once the contamination becomes too great an LED is illuminated to indicate that the mirror needs to be cleaned The CR 1A is microcontroller based The microcontroller uses flash memory that is field programmable and updateable If this is required please contact Buck Research Instruments LLC for instructions The instrument consists of a sensor assembly mount
20. ited to repair or replacement Defective goods must be returned to the Manufacturer promptly after the discovery of any defect within the above referenced one year period Transportation expenses to return unit to Manufacturer shall be borne by the Buyer Return shipping to Buyer shall be borne by Manufacturer for valid warranty claims This warranty shall become inapplicable in instances where the items have been misused or otherwise subjected to negligence by the Buyer NOTWITHSTANDING ANY OTHER PROVISION OF THIS CONTRACT NO OTHER WARRANTIES WHETHER STATUTORY OR ARISING BY OPERATION OF LAW EXPRESSED OR IMPLIED INCLUDING BUT NOT LIMITED TO THOSE OF MERCHANTABILITY OR FITNESS FOR PARTICULAR PURPOSE SHALL APPLY TO THE GOODS OR SERVICES PROVIDED HEREUNDER OTHER THAN THE REPAIR AND REPLACEMENT WARRANTY ABOVE SELLER SHALL IN NO EVENT BE LIABLE TO BUYER OR ANY THIRD PARTY FOR ANY DAMAGE INJURY OR LOSS INCLUDING LOSS OF USE OR ANY DIRECT OR INDIRECT INCEDENTAL OR CONSEQUENTIAL DAMAGES OF ANY KIND 23 Appendix 4 CR 1A Mirror Cleaning Procedure Removal Insert 5 64 ball driver through the 2 top holes of the optics cap to find the 2 56 socket head screws inside Loosen those screws to remove the optics block Using the 9 64 ball driver completely remove all 6 8 32 socket head screws from the lens holder Be careful when removing the last screw as there is a nickel gasket or rubber o ring in between the lens holder and the top of the sample
21. ng to the next connection Method 2 If the vacuum pumping system is unable to evacuate the inlet plumbing to a level that will produce an on scale reading on the vacuum gauge disconnect the vacuum pump from the gas inlet port and replace it with a low pressure air supply with a needle valve for regulation Slowly pressurize the gas inlet tubing being careful to limit the pressure applied to no more than two atmospheres Dampen the inlet tubing connections with soap solution or other leak detection solution and watch for air bubbles forming at each connection The presence of any air bubbles indicates a leak at the connection Repair any connections found leaking and recheck for leaks When no more bubbles can be found disconnect the low pressure air supply and reconnect the vacuum pumping system Repeat the preceding vacuum leak testing procedure CAUTION Overpressure within the above limits will not damage the pressure sensor However slight calibration adjustments may be necessary after any overpressure Overpressure limit 100 of span Method 3 Plug one end of the sensing chamber Attach an ordinary pump with a shutoff valve to the other end Lower the pressure as much as possible Close the shutoff valve and monitor pressure inside the sensing chamber to determine leak rate With proper sealing the pressure change rate should be less than 0 2 of pressure differential per minute Leaks can then be located by overpressuring the instrume
22. nsor has been found to exhibit no measurable drift even after years of use Low drift components are used throughout the temperature sensing circuit to ensure long term accuracy However recertification and recalibration is recommended yearly to ensure proper operation Errors can arise from failure to correct for differences between chamber pressure and ambient values This is only important if you are measuring ambient dew frost point and not 2 concentration The system must be allowed to fully equilibrate before accurate readings can be obtained When measuring very low frost points equilibration can take much longer Within twenty degrees below freezing the existence of supercooled water on the mirror can cause the temperature to read low as the instrument is measuring dew point Eventually the dew will turn to frost To speed this up press the COOL switch and allow the mirror to cool 5 10 degrees below the dew point reading then release COOL switch This will convert the dew to frost as long as the mirror temperature does not go above 0 C after COOL switch is released Contamination of the mirror by salt or other electrolytes can vary the relationship between vapor pressure and dew frost point Raoult error Other chemical contamination may cause similar error This is particularly relevant when measuring in natural gas as glycols contained within it can condense out on the mirror This is why a glycol absorbing filter must be us
23. nt as in Method 2 7 4 Troubleshooting Guide 18 Display shows Mirror needs cleaning Service Mirror LED blinking 1 Mirror needs cleaning Clean mirror 2 Optics cable is disconnected Reconnect optics cable Rebalance LED stays lit after balance cycle finishes 1 Mirror starting to get contaminated CR 1A will continue to function normally but be prepared to clean mirror soon Oscillation of output 1 Reduce sample flow until oscillation stops then gradually increase flow again 2 Turn instrument off allow to warm up and clean mirror 3 If oscillation is slow 10 20 sec period and most pronounced in the region 30 to 50 C cavity resonance interaction with contaminants in the chamber may be occurring Clean sensing chamber with the CR 1A cleaning fluid supplied 19 APPENDIX 1 HUMIDITY CONVERSION EQUATIONS Revised 7 96 Computer efficient algorithms for converting among several humidity units as used in HCON are given here They utilize vapor pressure formulations developed by A Buck 1981 DP dew or frost point in deg C vapor pressure in millibars es saturation vapor pressure in millibars P pressure in millibars r mixing ratio by weight in ppm RH relative humidity in percent rho absolute humidity in g m3 rhos absolute humidity at saturation T temperature in deg C Tk absolute temperature in K Saturation vapor pressure es f1 T e RH Dew frost point DP f2 e e
24. ror surface has a coating that scratches easily However moderate scratching does not prevent normal operation Use only soft flexible cotton swabs to clean the mirror Apply minimal pressure 4 Turn instrument power on and wait and see if Service Mirror LED and Rebalance LEDs turn off If either or both LEDs stay lit try cleaning the mirror again Make sure the mirror is completely dry before turning power on 5 If the Service Mirror LED cannot be turned off by cleaning the mirror remove the optics block and clean the lens and entire sample chamber 17 7 3 Leak Testing The introduction of even small amounts of room air into the sampling system will cause errors in low frost point readings Therefore it is desirable that leak testing be performed on the instrument package and sampling system after initial assembly and after any maintenance activity that involves disassembly of the instrument or interconnecting tubing Method 1 Connect a vacuum pump and vacuum gauge to the sampling system inlet port and close or cap the outlet port or vice versa Evacuate down to the minimum attainable pressure A reading of 100 microns Hg or less indicates the system is adequately sealed To locate a leak place a few drops of alcohol on each tubing connection and watch the vacuum gauge pressure reading If the reading abruptly increases there is a leak Allow some time for the vacuum readings to recover after each upscale deflection before proceedi
25. the lip of the thermos to the lip of the dewar lid and tilt slightly the dewar should fill the fastest possible way If you tilt too much there will be splatter of the LN2 You will need to fill the thermos between 2 and 2 1 2times to fill the dewar When you can see LN2 bubbling at the top of the dewar where the stainless steel bellows is you are done Be careful not to overfill Wait until there is no more boiloff before you screw the lid back on This should take no more than 5 10 minutes CAUTION Use extreme care with liquid nitrogen Liquid nitrogen has a sea level boiling point of 196 9C with a large expansion ratio in volume from liquid to gas approximately 700 to 1 Very brief contact with fluids at cryogenic temperatures can cause injury similar to thermal burns The eyes are especially vulnerable to this type of exposure Use a proper transfer container to transfer cryogen to the instrument and avoid direct contact with 8 any part of body or clothing wearing safety glasses and clothing as necessary Caution fabric gloves can hold the liquid nitrogen and must be avoided Avoid venting to a confined small space possibly displacing the air and causing oxygen deprivation 4 Then wait for the instrument to stabilize at the operating point The mirror temperature will decrease until the Balance value gets close to 0 Then the mirror temperature will increase until the Balance voltage stays between 200 at which point the displ
26. uired Displays internal main PCB temperature when HEAT COOL switch depressed POWER SWITCH SERVICE MIRROR LED Flashes when there is too much contamination on mirror Mirror must be cleaned to restore normal operation of instrument OUTLET AND Optional PRESSURE TRANSDUCER Connect sampling pump or vent here 1 8 tubing goes to pressure transducer inside instrument OPTICS BLOCK AND CABLE Contains LED and detector that looks at mirror through lens Loosen 2 screws inside 2 holes in top of cap to remove optics block to clean mirror Screws stay captive inside cap Cable is also captive POWER CONNECTOR Connect included power supply to this connector RS 232 Connect to data acquisition system or PC running Hyperterminal INLET Connect tubing to inlet SIGNAL OUTPUT 0 10 V and 4 20 mA outputs for various signals See section 4 DEWAR PLUG Use dewar plug operator with this plug if the vacuum needs to be recharged If the mirror will no longer make 120 the vacuum may need to be recharged Please contact Buck Research Instruments LLC for instructions on how to do this 3 INSTALLATION AND OPERATION 3 1 Installation Inspect the instrument for mechanical or other damage Connect the inlet and outlet gas flow lines to the sensor assembly Make sure there is some flexibility in both inlet and outlet lines to avoid stress and possible damage to the CR 4 NOTE do not over tighten Swagelok type fittings Over tightening can
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