User`s Manual - Buck Research Instruments LLC
Contents
1. is installed in the display connector you will see the following 4088 58 5 14 1 975 18 11 6 3 203 0 16 50 2008 03 16 14 42 12 Mixing ratio ppmv mirror temperature status flag pressure coldfinger temperature balance ePWM mirror flag Board temp date time Where Mixing ratio given in ppmv Calculated when there is a dew point status flag 1 Mirror temperature is the mirror temperature Status flag is 0 when not controlling on a dew point 1 when a dew point and 2 during a balance cycle Pressure in mb Coldfinger temperature given by diode on tip of coldfinger Should be lower than the mirror temp but can be higher when cooling because it has slower response than the mirror temp sensor Balance indicates if the servo is controlling Negative number means mirror too warm and needs to cool Positive number means mirror too cold and needs to heat PWM 255 to 255 indicates how much power is applied to TEC is cooling and is heating Mirror flag 0 clean mirror 1 mirror contaminated should be cleaned soon Board temp temperature of main PCB Date date in years months days Time time in 24 hours minutes seconds 24 Manual key stroke input to RS 232 decrease the contrast of the LCD display by 296 increase the contrast of the LCD display by 296 decrease the proportional gain coefficient by 1096 increase the proportional gain coefficient by 1096 decrease the derivative gain coefficient by 1096 inc2. Press Ipress 4 1 6 100 mbar 4 BALANCE When balanced BAL 5V or 12 5 mA 2 CONC 2 4 V 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 5 PRINCIPLES OF OPERATION 5 1 General The CR 2 is a chilled mirror condensation type hygrometer consisting of the following principle components a gold plated copper mirror with an attached stem an associated temperature sensor and heating coil a Stirling cycle cryocooler an optical system to sense condensing frost or dew 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 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 funda
3. be held by the cable coming out of it which is not removable The mirror is now exposed and can be cleaned 2 Moisten a soft cotton swab with mirror cleaning fluid acetone and gently wipe the swab over the mirror surface Immediately dry the surface with fresh cotton swab Inspection with flashlight may be helpful 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 mirror 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 17 turn off If either 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 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 perform
4. 115 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 EF 1 10 4 7 2 0 0320 5 9 x 10 6 EF 1 10 4 2 2 P 0 0383 6 4 x 10 6 2 where P is in millibars and T is in 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 0 596 21 APPENDIX 2 CONNECTOR PIN ASSIGNMENTS J2 SIGNAL CONNECTOR PIN OUT 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 2 Pressure 0 10 V 6 Pressure Return 7 Pressure 4 20 mA 8 Pressure 4 20 mA Return 9 VDF 0 10 V 10 Return 11 VDF 4 20 mA 12 VDF 4 20 mA Return 13 2 CONC 0 10 V 14 2 CONC Return 15 2 CONC 4 20 mA 16 2 CONC 4 20 mA Return 1 POWER CONNECTOR PIN OUT 2 28 VDC 3 28 VDC Return GND 4 Chassis GND J4 RS 232 CONNECTOR 9600 Baud 8 N 1 Requires Female to Female null modem cable 222 WARRANTY Manufacturer warrants that the it
5. BUCKRESEARCH INSTRUMENTS L L C MODEL CR 2 HYGROMETER OPERATING MANUAL Buck NSTRUMENTS LLC H20 Conc 18 2PPTU 2 D F Point 4 53 Display Module Pressure 856 526 Balance REBALANCE SERVICE MIRROR BALANCE INTERRUPT December 2009 BUCK RESEARCH INSTRUMENTS LLC PO Box 19498 Boulder CO 80308 Copyright 2008 Buck Research Instruments LLC All rights reserved TABLE CONTENTS 15 IntrOdUCHoD eta dede i fee don eee et cepa Certe edt Ud 4 1T General PU OM or oct irit te oon 4 MeS peci catons oou oe ood dava E d 3 2 Location of Principle Components 5 5 oou os boe ay II Quis Y FORI RED NEUE UM e ue In 6 2 1 Main 6 2 2 set aed a ies dann en das 7 3 Installation and Op ration eee to I EIER Ee 8 2 D ses Oi sre DL ose 8 3 2 Powerup Procedure te cesar eed 8 AON ose erred dr da 9 3 4 Power dowir Procedure 122525 9 10 10 4 2 Data Reduction Equations tette teet e enata Pn Roe eR Nora PN LO gn e ARE ed 10 5 Principles of Opera OM der va eo ae eed uode 11 EC US TS 11 11 5 2 1 Sampling Systefn IR EAS SR POUR S RU 11 5 22 Mito 14 523 Opt
6. TO position wait for the instrument to stabilize at the operating point The balance display should be within 200 counts of 0 and the Mirror T on the display should change to D F Point 4 The instrument is now ready for use NOTE It is always advisable to start measuring at relatively high humidity values above 409C to allow easy acquisition of condensation then go down in frost point temperature If it is necessary to begin operating at very low frost points initial acquisition of frost can be speeded up by spiking the pressure inside the sample chamber by quickly closing and opening a valve downstream of the outlet of the sample chamber This will cause the dew point of the gas inside the chamber to dramatically increase above the mirror temperature causing moisture to condense quickly on the mirror This can cause the balance reading to go from negative to positive very quickly which will cause the servo to heat the mirror If you regulate the mirror temperature by judiciously depressing and holding the cool button you can keep the layer on the mirror from being burned off and decrease frost acquisition time from many hours to 1 2 minutes 3 3 Operation During operation no special attention is required except for an occasional check of operating voltages to assure proper function If possible keep flow in the range 0 5 3 liters minute At very high dew frost point temperatures higher flow may be allowed Keep the sa
7. bsolute 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 f2 r x P 622 x 103 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 ppmw 622 103 x e P e e 622x103 x 1 DP DP 622x109 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 r 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 rx 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 Ib r x 0 007 Precipitable cm per km rho 10 20 NOTE 1 f1 DP and f2 e are variations on vapor pressure formulations found in Buck A J Appl Met 20 pp 1527 1532 1981 They are given by e vs DP or es vs T 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 eor T vs es dw 2 x bw s bw S 4 cw x s dw 2 over water di 2 x bi s bi s 4 ci x s di 1 2 over ice where aw 6 1121 ai 6 1
8. d away from the cold finger by the helium circulating from the cryocooler compressor Since the cryocooler has limited heat pumping capability the coupling must be very efficient and well insulated from external heat Therefore to improve thermal isolation the mirror stem and cold finger are enclosed in a sleeve containing cryogenic insulation and back filled with CO2 5 2 5 Thermistor Readout For obtaining the dew or frost point temperature from the temperature senor three readouts are provided VDF signal voltage RS 232 output and a direct temperature indication on the display Both are accurate within 0 1 9C 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 ma
9. duced accuracy Accuracy Response time Nominal operating range Electronics Temperature Pressure Sample gas Temperature Pressure Flow rate of sample Cryocooler type Operating lifetime Output signals Output format Construction Input voltage Power consumption maximum 0 19C Less than 40 sec from 100 to 409C frost Less than 1 sec from 40 C to 309 dew 40 to 409 100 to 2000 mb 2 to 30 psia 80 to 40 C 1 to 2000 mb 02 to 30 psia 0 2 3 liters minute Stirling cycle helium Approx 10 000 hours Dew frost point pressure mirror balance Coldfinger temperature RS 232 and 0 10 VDC Aluminum mounting hardware 316L Stainless steel sensor assembly 28 VDC optional 110 VAC 60 400 Hz lt 50 watts Approximate dimensions inches and cm Main unit Display module 10 25 cm w X 10 75 27 cm d x 3 75 9 5 cm h 8 20 cm 8 20 cm dx 5 13 Rack mounting optional Approximate Weight excluding cable and heat sink Main unit Display module 716 3 2 kg heat sink adds 3 Ibs 1 4 kg 3 25 lbs 1 5 kg Note These are approximate specifications Exact performance will vary depending on installation and operating environment 2 LOCATION OF PRINCIPLE COMPONENTS 2 1 Main Unit Figure 1 Signal connector J2 Display connector J3 Power J1 and RS 232 connector J4 Sensor assembly Consists of mirror module sensing c
10. e display will switch to D F Point from Mirror T This is normal and you will still be getting dew frost points when the mirror temperature looks stable even though the display shows Mirror T 3 4 Power down Procedure 1 If contamination is a likely hazard shut off flow through the sensor to protect the mirror 2 Switch the RUN INTERRUPT switch to INTERRUPT Disconnect power The mirror will remain cold for several minutes To avoid excessive condensation in the sensing chamber allow the instrument to reach room temperature before opening inlet or outlet lines 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 10v and 4 20 mA PRES Chamber pressure 0 10v and 4 20 mA 2 CONC Ib MMscf or ppmv 0 10v and 4 20 mA BAL Balance voltage 0 10v and 4 20 mA 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 CC 150 20 x VDF v 0 10V 1 CC 150 8 x IDF ma 4 20 mA Q PRESSURE Chamber pressure is calculated from the Vpress signal voltage by Press Vpress 1 6 156 25 mbar 3
11. e heat sink and fan if they were removed for aircraft installation The technology incorporated in the CR 2 is protected by domestic and foreign patents 1 1 General Description The CR 2 is a chilled mirror condensation type hygrometer Its high performance is achieved by cryogenically cooling a mirror using a closed cycle cryocooler and holding it at the frost point temperature by means of a heater control system Optical detectors are used for sensing condensate on a mirror and an ultra stable platinum RTD imbedded in the mirror is used to determine mirror temperature the dew or frost point Since operation of the CR 2 is based on a fundamental property of water vapor dew frost point it is intrinsically capable of long term accuracy and stability without the need for periodic recalibration The instrument s main unit consists of a sensor assembly cryocooler cryocooler drive circuitry and control readout circuitry A separate display module displays operational and output voltages and allows manual control of the instrument but is not required for operation The components of the CR 2 are Main unit containing sensor assembly Display module Display cable 9 ft Service kit containing tools spare parts power connector signal output connector and supplies Operating manual 1 2 Specifications Model CR 2 Cryocooled Hygrometer Measurement range Dew frost point temperature 1000C to 309 reads beyond with re
12. ed 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 proceeding 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 sup
13. ems 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 limited 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 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 RS 232 output stream Connect RS 232 output using null modem cable to computer that has HyperTerminal Set HyperTerminal for 9600 8 N 1 with flow control set to none If the LCD display not connected and the key
14. et lines to avoid stress and possible damage to the CR 2 NOTE do not overtighten Swage or VCR type fittings Overtightening can destroy their sealing ability Make sure each VCR connection has one but no more than one metal gasket installed 3 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 5 This is especially important when operating the instrument in a humidity environment that is very different from that of the sampled air 3 2 Initial Power up Procedure 1 Temporarily connect the display module to J3 of the main unit using the cable supplied Make sure that the RUN INTERRUPT switch is in INTERRUPT position 2 Apply power to The display should show some diagnostic screens then initiate a balance routine When the balance routine is finished flip the RUN INTERRUPT switch to the RUN position Monitor the coldfinger temperature by depressing the COOL Switch and reading the temperature on the second line of the LCD display If you do not have a pressure sensor then coldfinger temperature is automatically displayed on line 2 of the LCD display With the COOL switch depressed continuously or alternatively flowing very dry gas 70 C or drier through the sample chamber the cold finger temperature should read at least 100 C after 10 minutes 3 Then with the HEAT AUTO COOL switch in AU
15. ew to frost as long as the mirror temperature does not go above 0 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 used 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 2 Mirror check and cleaning Sect 7 2 3 Leak checking Sect 7 3 4 Cryocooler and cryo sleeve check Sect 7 4 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 5 64 hex ball driver into holes and loosen each screw The screws will remain captive inside Once screws are loose remove optics block It will
16. ge the assembly 7 5 Troubleshooting Guide Display shows 2 Conc XXX X 1 This is normal A concentration will not display until a dew or frost point is reached 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 2 will continue to function normally but be prepared to clean mirror soon Oscillation of output some oscillation at dew point above 0 9C is normal 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 509C cavity resonance interaction with contaminants in the chamber may be occurring Clean sensing chamber with the CR 2 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 e 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 a
17. hamber and optics module Outlet on right Base plate Heat sink and fan assembly Can be removed for aircraft installation Lab legs Remove for aircraft installation CR 2 Hygrometer Las C M LM Fig 1 Main unit 2 2 Display Module Figure 2 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 acquired Displays internal main PCB temperature when HEAT COOL switch depressed HEAT AUTO COOL momentary Provides additional heat to partially clear the mirror of condensate or full cooling to allow additional frost to collect on the mirror Normal operation is AUTO BALANCE momentary If held down for 5 seconds initiates a balance routine REBALANCE LED Lights when a balance routine is performed If it stays light after balance finishes this indicates that mirror is becoming contaminated and will need to be cleaned soon SERVICE MIRROR LED This LED flashes to indicate that mirror is too contaminated for proper operation and needs to be cleaned RUN INTERRUPT Turns off power to only the cryocooler Figure 2 Display module 3 INSTALLATION AND OPERATION 3 1 Installation 1 Inspect the instrument for mechanical or other damage 2 Connect the inlet and outlet gas flow lines to the sensor assembly Make sure there is some flexibility in both inlet and outl
18. ics 14 3 2 4 Cooling System n a E REIR IRE ERA 14 3 2 5 Thermistor Reddo t sos 14 6 Measurement 15 7 Maintenance and Troubleshooting 4 16 16 Mirror CIO e nmi i A add 16 Leak D 17 7 4 Cryocooler and Cryo Sleeve Check 18 T Troubleshooting Guide sts ig ved rg 18 Appendix 1 Humidity Conversion Equations 19 Appendix 2 Connector Pin Assignments 2l FIGURES Erg T escas big o 5 cen m ut ere Fig 3 Block diagram CR 2 frost point hygrometer essere Fig 4 Sensor assembly block ia Sram dis uode MODEL 2 HYGROMETER OPERATING MANUAL 1 INTRODUCTION The Model CR 2 Cryocooled 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 1 CR 2 and CR 3 family represents a significant advance in the state of the art The CR 2 is designed for use with research aircraft When using in the lab make certain to provide adequate cooling for the instrument by reinstalling th
19. mental measure of humidity the CR 2 is intrinsically capable of long term accuracy and stability The development of this hygrometer follows the original work of 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 4 and the sensor assembly is diagrammed in Figure 5 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 LED Z LENS THERMISTOR AIRFLOW MPERATURE DEW FROST POINT Fig 3 Block diagram CR 2 frost point hygrometer 28 THERMISTOR est SENSING CHAMBER Fig 4 Sensor assembly block diagram 14 The sampling system must be carefully sealed to prevent room air from contaminating the measurements Stainless steel materials must be used throughout the inlet portion to minimize o
20. mple line inlet protected from contamination This is best achieved by keeping the sample line closed when not connected to the desired sample gas To avoid internal line condensation and resultant erroneous readings do not allow the inlet lines to cool below the expected frost point temperature The Display Module provides convenient displays for most parameters as an alternative to the signal outputs available at J2 Both dew frost point mirror temperature indications are accurate to 0 1 9C see Appendix 3 The Display Module should be connected only when the power is off Normal operation is indicated by BAL settling to the range 200 however at lower frost points loss of equilibrium can result in only a small departure from its previous balance point 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 balance voltage BAL to completely stabilize before taking a reading Due to higher physical gain at high dew points some oscillation is likely to occur at dew points above 0 9C with amplitudes of a fraction of a degree It is rather rapid and can easily be filtered out during data processing Also it is possible that the oscillations put the balance voltage outside the range where th
21. ply 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 96 of pressure differential per minute Leaks can then be located by overpressuring the instrument as in Method 2 18 7 4 Cryocooler and Cryo Sleeve Check To check whether adequate insulation exists in the cryo sleeve it is best to check minimum mirror temperature capability as follows using the display module 1 Stop sample airflow through the instrument 2 Push COOL switch and hold and observe mirror temperature It should go below 95 OC If not the instrument is still operational but it is beginning to lose significant insulating ability 3 If the cryocooler or cryo sleeve must be serviced contact Buck Research Instruments LLC Do not disturb either the crocooler or the cryo sleeve and do not open the cryo sleeve without instructions from Buck Research Instruments LLC doing so will void the warranty and possibly dama
22. rease the derivative gain coefficient by 1096 decrease the integrator time constant coefficient by 1096 increase the integrator time constant coefficient by 1096 ga v0 0o 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 40 R resets all coefficients and contrast to original values B initiates a balance routine G polled output of RS 232 data when LCD display switch is OFF optional F Forces frost below 0 C by cooling the mirror below 40 C 25 8 1 ul Z WW pG2 S238u2u OT 4 54 S6 sau2u GZ uu 80 ww Sau2u S
23. utgassing during low humidity sampling 5 2 2 Mirror Module The mirror assembly consists of a mirror a mirror support and the thermistor heater and cryodiode which are attached to the mirror stem A small ultra stable thermistor is installed in the mirror face to measure the dew frost point temperature Heating is provided by a resistive heater coil wound around the mirror stem A diode located at the bottom of the stem monitors the cryocooler coldfinger temperature which is controlled by its own control circuit 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 phototransistor pair and a light emitting diode LED One phototransistor maintains constant LED intensity the other phototransistor provides a current output that is proportional to the light reflected from the mirror The bias circuit 15 set so that when the proper condensation layer is on the mirror about 85 92 of the light emitted by the LED is received at the detector 5 2 4 Cooling System The mirror stem is thermally coupled to the cold finger by a thermally conductive flexible coupling In operation heat is pumpe
24. y take a very long time to dry enough to allow accurate readings when measuring frost points below 509C 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 sensor 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 d
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