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IMTC 2003 -Instrumentation and MeasurementTechnology ConferenceVail,CO, USA, 20-22May 2003

Custom Made Automated System for Determination of Humidity andTemperature Gradientsin Climatic ChambersJ ovan Bojkovski, Domen Hudoklin, JankoDrnovSek, Igor PuSnik,DuSanA greiUniversity of Ljubljana, Faculty of Electrical Engineering,Laboratory of Metrology and Quality, Trzaska 25, 1000 Ljubljana, SloveniaTel: +386 14768 224, Fax: +386 14264 633, Email: ovan.boj kovski@e.uni - l j . si , http://lmk.fe.uni-lj.si

Absfrocl -The objective OJ lh is paper is to present method fordeterminat ion o humidity and temperature gradients i n climnticchambers, independent OJ th e size of the cham ber. I n the papercustom made auto mated system is discussed as well. The system isdeveloped to satisfi all th e requirements concerning determinationof humidity and temperature gradients in climatic ch ambers. Dataacquired with this sysfem can be afierwards used for calibrati oncerfii/icate OJ the c l imat ic chamber i t s er o r as a basis for theuncertainty contribution oJthe clim atic chamber in the process ofcal ibrat ion OJ the relative humidity sensors inside the chamber.Th e system itself can beused und er the laboratory conditions, aswell as fo r th e cal ibrat ion on-si te.

I. INTRODUCTIONThe objective of this paper is to present method fordetermination of humidity and temperature gradients inclimatic chambers, independent of the size of the chamber.Climatic conditions have a great impact on productcharacteristics during the transport, storage and use. It istherefore in many cases necessary to perform climatic testson the product. This is typically done in climatic chambers

either according to the relevant standard covering certainproduct or according to the empirical procedures developedby the producers. Climatic chambers are widely used forvarious purposes in industry, research and development,calibration and testing laboratories etc. The usual temperaturerange of climatic chambers is from -100 "C up to 300"C,easily covered by the platinum resistance thermometers fortemperature measurements as well as for evaluation of suchchambers. T he size of these chambers is typically up to 1000liters but they can also he larger custom made spaces forspecial purposes like storage of the drugs in pharmaceuticalindustry (climatic rooms). In a case that a chamber is toosmall or that a required level of uncertainty is not so high,thermocouples can also be used. The automated systemdeveloped in our laboratory can be used both with thestandard types of thermocouples and platinum resistancethermometers, [l]. On the other hand, usual humidity rangeof such climatic chambers, in the temperature range from 10"C up to 95 "C, is between I O % r.h. and 98% r.h. Humidityprofile inside such chambers can be measured withappropriate number of calibrated relative humidity sensors.Namely, before performing any test, climatic chamber needs

to be evaluated, therefore depending on the size of thechamber, tipically 9 sensors (in each comer of the chamberand one in the geometrical center) are used. In addition, twoother principles of measwement of humidity can be used.First method requires dew point meter as a referenceinstrument, second method requires psychrometer as areference instrument, as described in details in [2]. In ourcase we use dew point meter at the reference point (thegeometrical center of the chamber), which enables themeasurement of the dew point inside the climatic chamber.Since we can assume that the pressure inside the climaticchamber is the same across all the test space, relativehumidity distribution inside the test space can be calculatedfrom the dew point measured at the reference point and fromthe measured temperature distribution, using Sonntagformula as described in details in [3]. In addition tomeasurement of temperature and humidity gradients insidethe chamber, stability in time of both properties isinvestigated. The position of sensors is shown in Fig. 1.

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Fig. 1.The position of sensors For measurementof temperatureand humidity propertiesof climatic chamberwith dew-point sensoras areference senMr.

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11. MEA SUREMENT SET-UP AND EV AL UATIONPROCEDURESThe measurement set-up consists of computer, whichcontrols all the measurement equipment connected to it eitherwith serial or GPIB communication interface. The controllingprogram is designed in LabView, which also enables us toconnect to central database. The database among other dataholds necessary information about all the equipment andhistory of measurements performed, enabling us to performtraceable measurements and better analysis afterwards. A llthe changes of the database are password protected andrecorded to the special log file. Thus the integrity of thedatabase isassured. In the case that the evaluation system is

taken for work on-site, only the part of the central database,which has information about instruments used for on-siteevaluation, is exported and then nm locally on the laptopcomputer on site. The software is developed and testedaccording to the software standards for accreditedlaboratories, as described in [4]. The objectives forautomation of such systems are primarily the reliability ofmeasurements, repeatability, consistency, and minimizationof various influences causing gross measurement errors.Temperature probes are attached to K eithley 7001automatic scanner with two plug-in cards, which alternatelyconnects all the sensors to the volt/ohm-meter. The selectionof the scanner card depends on the required uncertainty(selection of the type of the sensor), size of the chamber(number of sensors) and other parameters. For thevoltagehesistance measurements a precision Hewlett Packard34420A nanovoltimicroohm meter is used. Both instrumentsare connected to aPCvia the IEEE-488 bus. The HP 34420Ameter is chosen because it enables accurate measurement ofthermal voltage as well as resistance. The ultimate solution inprecision would be a combinationof a resistance bridge and ananovoltmeter. This solution is applicable only to the

laboratory work and is not suitable for the on-site work,because of the size of a 'resistance bridge and specificenvironmental conditions demanded. On the other hand, thetypical performance of a chamber is not so good that wewould need to perform measurements with platinumresistance thermometers and a resistance bridge. For thereference dew point meter we can use two different meters,Michell S4020or General Eastern RHB-2.Two main characteristics of chamber are variations inspace and fluctuations in time. In a steady-state conditionsome areas inside a chamber are warmer or colder thanothers. This is due to the temperature variations (gadients)inside a chamber. It is well known that relative humiditydepends on absolute water content and air temperature.Usually, the largest gradients are near the climatic chamberdoor and near the ventilating fan. The system automaticallyfinds the largest gradient compared to the reference point andrecords the information about the reference temperature, thevalue of a gradient and the position of the other thermometerwith the largest gradient from the reference point into ameasurement report. Usually the temperature fluctuations intime are small compared to the temperature variations insidea chamber. The system continuously measures all the pointsfor a required time (usually more than 15 minutes, themeasurement time as well as the time period betweensamples can be set l?om the program) and then records areference temperature, a reference relative humidity, thelargest fluctuation in time and the position of the largestfluctuation.

111. RESULTSThe automated system was tested on the Voetsch climaticchamber (volume of the chamber is 1000 liters; temperaturerange is-70 "C to 180 "C; climatic operation: 10"C to95"C,humidity range I O O h r.h. to 98 O h r.h.). For the temperaturemeasurements 9 small platinum resistance thermometers

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were used. They were calibrated by comparison in oursecondary laboratory together with nanovolthicroohm meterand scanner.Table1.Ust of typica uncertainbf contributions to the total uncertainty of a measuring system

1 Measuring Scanner Sensor used with expanded Total uncertainty Dew pointInstrument uncertainty uncertainty0.02 "C 0.001 "C PRT 0.01"C 0.03 O C 0,15 "C0.02O C 0.001"C Small PRT 0.05 "C 0.07"C 0,15 "C0. 05"C 0.01 "C ThermocoupleK type0.09 O C 0.1 "C 0,15 OC

Eight sensors were placed in each comer of the climaticchamber and ninth was placed in the geometrical center ofthe chamber, reference point. For the dew point temperaturemeasurement we used Michell 4020 dew point meter. Asexplained above, as measuring instrument the HewlettPackard HP 34420A nanovol thicroohm meter incombination with K eithley 7001 scanner and 7067 4-wire I Ochannel scanner card. The li st of uncertainties for usedequipment is presented in Table1.The measurements were performed with Voetsch climaticchamber set up point 20 "C and 90% r.h. In the Fig. 2 thebehavior of the dew poini temperature during one hour of thestable state can he seen. The arithmetic mean value of thedew point temperature is 19,12 "C with the standarddeviation 0,035 O C . Fig. 3 shows temperature variation insidethe chamber during one hour of the stable state. The sensorwith serial number 1.081.199 was placed in a reference pointinside the chamber, very close to the dew point sensor. Thearithmetic mean value of the reference point temperature was20,53 "C with the standard deviation 0,06 O C . The largestdifference of any two sensors inside the chamber was 0,243OC .

V o e t s c h 20 ' C and 90 % r .h .

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Fig.2. Measurementof th e de w paint

V o e t s c h 20 'C and 90 XI

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Fig.3. Measurementof the temperature variation

From these two sets of measurement data, the relativehumidity distribution inside the chamber was calculated. Thisdistribution is presented on the Fig. 4.V o e t s c h 20 ' C and 90 % r.h.

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17:lO 17.18 1726 1734 17:42 1750 )--.-&;;y"")Tim.

Fig.4. Caculated relativehumiditydistribution inside the chamber

The average relative humidity at the reference point was91,6 % r.h. with the standard deviation of 0,5 % r.h. Thecalculated largest difference of any two sensors was 2 YOofr.h.Based on our own procedure, which was developed from[5]and our own experience in evaluating climatic chambers,the automated system was developed. The system can heused io the laboratory as well as for on-site calibrartions. It isapplicable for the temperature and humidity range, which ismost used in industry, and it can be used for evaluations oflarge climatic test spaces. In this cases also the uncertainty

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due to thermometer cables shall be taken into account.Compared to other avaliable systems, the automated systemfor evaluation of the climatic chambers enablesmeasurements at more points inside the chamber. Thenumber of points can be easily expanded with additionalscanner cards. Furthermore, it is completely automated andconnected to the database. The uncertainty of the usedequipment represents the optimal combination in both cases,no matter if we want to evaluate the chamber withthermocouples or platinum resistance thermometers, anddew-point meter.

REFERENCES[I ] Bojkovaki J ovan,.PuSnik Igor, DmovSek J anko, Hudoklin Domen,lEEE irons. imlmm.meas.,vol. 50,no. 6, pp. 1599-1603,200l[2] C. R auq F. Helgesen, T. M agnusen, "Investigation of theMeasurement Uncertainty in a Climatic Chamber Used for Calibrationof the RH Sensors:, Proceedingsof 8" TEMPMEK O 2001, Berlin, pp.917-922,2001[3] Sonntag, D., Meborologico Zeitschrfl, voI3, pp51 -66, 1994141 TasiC Tanasko, Bojkovski lovan, PuSnik Igor, DmovSek J anko,

Reliability design s f a colibrorion laboraiory momalion so/hwre,Conference on Precision Electromagnetic Measurements, CPEM2000, Sydney, Australia, May 2000Society of Environmental Engineers, "A Guide to Calculating theUncertainty of th e Performance of the Environmental Chambers".public daft, 1999 ,I S]

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