A device for calibrating electrical humidity sensors

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Publisher’s version / Version de l'éditeur:

Materials Research and Standards, 6, 1, pp. 25-29, 1966-03-01

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A device for calibrating electrical humidity sensors

Hedlin, C. P.

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Authorized Reprint from the Copyrighted Materials Researclz & Standards, V o l . 6, N o . 1

Published by the

American Society for Testing and Materials

A

Device for Calibrating Electrical Humidity Sensors

By C.

P.

HEDklN

This

device can calibrate Dunmore-type humidity sensors to within about

0.5

per cent relative humidity.

W H E R E A high degree of accuracy is not required, humidity sensors of the electrical-resistance type may be used for a considerable time without recalibration. Where it is necessary to know within well-defined limits the accuracv of the instrument, or where the sensor may inadvertently be subjected to harmful conditions, constant recalibration is necessary.

There are a number of humidity generators in North America [I-SI1 with which highly precise calibration might be done, but the cost, the time de- lay, the inconvenience of sending sensors to a central agency, and the need for iinmediate and frequent checks often make this procedure unsuitable. Con- sequently, there is a need for precise, relatively in- expensive calibrating devices that can be maintained by sensor users.

1 The italic numbers in brackets refer to the list of references a t the end of this paper.

A number of devices of this kind have been de- veloped. One type mixes dry and humidified air in the proportions required to give the desired humidity

[4].

A second type uses salts covering a wide range of humidities [5]. There still appears to be a need for equipment of this type providing flexibility in selection of both temperature and relative humidity over a wide range and a reasonably high level of accuracy.

I n the two-nressure device described here. a streant of air is saturated a t atmospheric pressure and then expanded to obtain the required humidity. The

CHARLES P. HEDLIN received a B.Sc. degree from the University of Saskatchewan and has obtained advanced degrees from the Uni- versity of Minnesota and the University of Toronto. Since 1960 he has been employed b y the National Research Council of Canada a t the Prairie Regional Station of the Division o f Building Research a t Saska- toan, Sask., where his primary interests have been in precise measure- ment of humidity and in the sorption properties of building materials.

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IVIercury Manometer Manometer ( A P , ) (

_A

~ 2 ) Electrical Leads - T h r e e Sensors V a c u u m P u m p Water Bath

Fig. 1-Two-pressure humidity calibrator.

ecluilibrium temperatures before arid after exparision are equal. A number of the principles and tech- niques used in this system were discussed by Weaver and Riley [6, 71 and by Weaver [8]. They meas- ured the water vapor content of an unknowri gas by comparing it with that of a standard gas. This was done by adjusting the pressure of one or both gases until they produced the same resistance in an electrical sensor.

Weaver [8] presented ari equation to relate the relative humidity to the total pressures before and after expansion, pa and p,. This equation includes a correction for the effect of air pressure on the solu- bility of water vapor i11 air

C ₌p, 1 _-

(

- 0.00O17pc _{. . .}

S

pa 1 -0.00017pa (1)

where C and S are the co~icentratioiis of water vapor a t pressures p, and p, (saturation), respectively.

For saturation a t a total pressure of 1 atmos this equation predicts that the maximum error in relative humidity will be approxin~ately 0.07 per cent a t a calibration chamber pressure of

+

atmos if the correction is neglected. This effect is small enough to be disregarded in the present case.

Equipment

The saturator consisted of two copper tanks (Fig.

I ) , 2 in. in diameter and 10 in. high, partially filled with 6-mm glass beads and water, or with chipped ice as required. These were connected in series. I11 each of them air entered through a +-in. brass tube, which extended through the chamber wall and hori- zontally across the chamber about

+

in. above the bottom. A series of +-in. holes was drilled in the under side of this tube to provide for passage of air into the chamber.

The pressure of the air was reduced after leaving the saturator by passing it through a needle valve. To avoid condensation from the resultant cooling, the air was heated before expansion. A 3-in. length of +-in. tube was wound externally with asbestos- covered Chrome1 C electrical heating wire and en- closed in a +-in. diameter outer tube. This assem- bly was covered with asphalt-base insulating tape. About 5 w of electrical power was sufficient for heat-

ing. If the air was not heated, sudden increases in e

the humidity in the calibration chamber occurred, probably due to re-evaporation of the condensate in the tubing on the low-pressure side of the valve.

A coil of +-in. copper tubing approximately 30 in. long was inserted between the needle valve and the calibration chamber to act as a heat exchanger to bring the air temperature back to the required value. This was attached to the needle valve and the calibration chamber by short lengths of rubber tubing.

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The calibration chamber was of brass, 3 in. in diameter and 33 in. deep. i\/Iounting sockets were provided for three sensors on the under side of the re- movable lid of the chamber. The necessarv wires were introduced into the chamber throughla tube connected to the lid and sealed with epoxy resin. The lid was fastened to the chamber with bolts. silicone rubber gaskets providing a tight seal. Connections were located on the chamber for pressure measurement, air entry, and air exit. The saturator and calibration chamber, with connecting tubing, were sub- merged in a water bath provided with a thermostat. Later equiplnent was modified in three ways. The saturator was made of brass tanks. 6 in. insteacl of 10 in. high; the line connecting the saturator to the calibration chalnber was made of stainless steel and was attached rigidly to the latter; and the calibration chamber was attached to the saturator bv a clamp, which permitted movement and adjustment. These changes resulted in a more compact unit. This unit was used to obtain the 10 F results shown in Fig. 2. The relative humidity was taken as the ratio of total pressures in the calibration chamber and saturator

where:

p, = baron~etric pressure - Apl,

p, = barometric pressure

-

Apl - Ap2, Apl = the pressure drop from the atmosphere to

the outlet of the saturator (measured with a U-tube manometer filled with oil having a specific gravity equal to unity a t 60 F), , , and

Ap:! = the pressure drop from the saturator to the calibration chamber (measured with a U-tube mercurv manometer coated in- ternally with antistatic fluid to improve its accuracy by decreasing the adhesion between the mercury and the glass). Accurate measurement of the pressures is important. An error of 1 mm Hg in Ap, will result in an error in relative humidity of approximately 0.13 per cent. An error of 1 mm Hg in Apl will result in an error in relative humidity that can be written

e = R H

-

R H , = 100

( 1 ( ps + 1 3 )

R e l a t i v e Humidity

I

Fig, 2-Results obtained with the two-pressure system compared with calibration curves (solid lines) obtained with the atmosphere producer. (The relative humidities a t 10 F a r e adjusted to make them correspond to supercooled water rather than to ice.)

I

January 7 966 ₂₇

-

_{- -}

Legend

-

w

= Q u ~ e s c e n t H e ~ g h t of W a t e r A b o v e

Air

I n l e t ( I n ) 80.6 O F -- w ~ t h P a c k ~ n g w h e n P a c k ~ n g w a s Used -

b

= Depth o f 6 mm Beads ( ~ n -. A and B - I s t a n d

2

n d - - - B

b w

b

- - - - - - A

w

-

0

+

I 0 4 5 2 4 . 0 0 4 9 0 0 0 4 4 - l o o 2 4 6 0 0 1 2 - . O O l / , I O 2 0 2 0

-

.

o

- x o o o I I

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If p, = 760 nlnl Hg, e = Ap?(*0.00017), or k0.13 I)cr cent of the rluantity (100 - RH). The same in- accuracy ~vill occur if there is an error of 1 mnl Hg in measuring the barometric pressure.

A vacuum pump was connected to the calibratioil c.hamber, and the rate of air flow and relative humidity were regulated by the needle valve and a second valve located betureen the calibration chamber ancl thc vacuum pump.

Results i d Discussion

This system was usccl to calibrate Duninore-type k~uiniclity sensors. Sensors were calibrated before nricl after use in a two-temperature unit [ I ] , and the results obtained with the es~erimental device were wmpared with the calibriition data. I t was assumed that the operation of the sensors was riot affected by their being placed in a vacuum.

I t was important to hnow the conditions required for substantial saturation of the air and to know also whether water droplets n.oulc1 be entrained in the air Icaving the saturator. Tests were carried out to cictermine the effects of ~vatcr level and of the rxes- erice of packing material. I n the first series of tests, at 80.6 F, only thcl srcond section of the saturator was used; the other sccation was left dry. Water depth (above the air inlet) was varied from approximately

+

to 4 in., and the clt.pth of 6-inm glass beads was twice that of the water in most cases (the water depth was measured with Leads in place). I n a second series, again using only the second section, no beads were used. Finally, both sections were used with and without beads. The rate of air flow ranged from 0.03 to 0.1 ft3/min. If the air flow rate was much in excess of the higher value, surging occurred ancl made accurate measurement of the pressure impossible.

The results are shown in Fig. 2. I t appears that with a water depth less than $ in. the air did not be- come snturatecl. For all other conditions, however, the results agreed, within about

+

per cent RH, with the calibration data obtained with the two-tempera- ture unit of Handegord and Till [ I 1. For approxi- mately 35 observations with three sensors the aver- age deviation was less than 0.2 per cent RH. This suggests that within practical limits saturation occurred without droplet carry-over.

Experiments were done a t 32 F using the same sensors as above. The experimental values of sensor conductance ancl relative humidity are plotted on logarithmic coordinates [9] in Fig. 2.

Less extensive tests mere carried out with other Dunmore-type serlsors a t relative humidities down to approximately 10 per cent. Accuracy was similar to that reported above.

111 later work a +in. depth of water and beads has been used in the first section, and roughly

+

in. of nrnter and 6 or 7 in. of beads in the second section. The decision to use these quantities was somewhat arbitrary, but was based on the need for a substantial reserve of water and the principle of using the first section for the bulk of the saturating and the second for a small amount of heat and moisture exchange that might be required.

T o obtain the results a t 10 F , s h o ~ ~ r n in Fig. 2, cne or both of the saturator chambers were filled to n , depth of about 4 in. with chipped ice. Screens were placed across the chambers, just above the air inlets, to support the ice. I t was found necessary to dry the incoming air to prevent plugging of the passage. The temperature in the calibration chamber was measuretl with a thermocouple. Temperature was affected by the variation in air pressure that accom- panied the establishment of a new relative humidity. Recause of this, and possibly because of sorption on the walls of the calibration chamber ancl tubing, roughly half an hour usually elapsed before equilib- rium was fully re-established in the sensor.

In soine of the tests, a thermocouple was inserted into the upper portion of the second section of the saturator. Generally the temperature there agreed closely with that of the bath and the calibration chamber. To ensure accurate results, calibrated tl~ermocouples, or suitable wells to receive a mercury thermometer, should be incorporated near the outlet of the saturator and in the calibration chamber. If such wells are used, it is important that they be de- signed so that the measured temperature will not be affected by thermal conduction along the well or the t c>mperature sensor.

A short series of tests was carried out to assess the effect of the moisture content of the air entering the saturator on the humidity in the calibration cham- her. The inconling air was alternately dried by passing it through a desiccant and saturated a t a temperature above that of the two-pressure system saturator. Each condition was allowecl to esist for about half an hour before reverting to the other. The results, as indicated by a Duninore-type sensor in the calibration chamber, did not appear to be significantly affected by the treatment of the incoming air.

In previous tests the temperature control of the bath was very close; variation probably did not es- ceed a few hundredths of a degree Fahrenheit. To determine the approximate effect of wide variation, the control was arranged so that the bath temperature fluctuated approximately $ F above and below the null point. The coriductance of the Dunmore-type sensor varied about the correct value by about 2 pa (corresponding roughly to *$ per cent RH). I t appeared that the calibration chamber and sensor responded to the changes in bath temperature more slowly than did the saturator. This result suggests

that the ideal bath temperature should not fluctuate c by more than about *0.10 F , or that an arrangement

for damping the variations should be incorporated.

Summary and Conclusions B

The two-pressure system described here is suitable for routine calibration of Dunmore-type humidity .sensors. Air is saturated a t atmospheric pressure and expanded to a lower pressure in the calibration chamber. The humidity in the calibration chamber is regulated by adjusting the pressure there. Series of tests using calibrated Dunmore sensors were carried out at 80.6, 32.0, and 10.0 F. At 80.6 F , the

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average deviation froill the calibration curve was less than 0.2 per cent RH, and a t the lower temperatures it was less ihan 0.3 per cent R H .

The two-pressure vacuum principle has several characteristics which reconlmend it as a calibrating device. It performs satisfactorily over the complete range of temperatures and humidities for which Dunmore sensors are suitable. I t is simple to con- struct and, if reasonable care is taken, gives reliable results. Despite the vacuum ill the calibration chamber, very little difficulty with leakage was en- countered.

It is probable that the temperatures in the calibration chamber and the saturator will be equal. T o ensure accurate results, it is desirable to deterilline whether this is actually the case. This car1 be done by introclucing calibrated thernlocouples or a therino- pile into suitable wells in the chambers.

The author wishes to express his appreciation to G. 0. Halldegord for his suggestions regarding the developilleilt and testing of this apparatus, and to D. G. Cole for constructirig the equipment.

This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Division.

[ I ] G 0. Hsndegord and C. E. Till, "New Humidity Stand- ard," Transactions, Am. Soc. Heating, Refrigerating, and Air Conditioning Engrs., Vol. 66, 1960, pp. 288-308.

[I] A. Wexler and It. D. U:l~liels, Jr., "Pressure-Humidity Apparatus," Journal o j Research, Nat. Bureau Standards, Vol. 48, No. 4, Apr~l, 1951, pp. 269-274.

[S] E . J. Amdur and R. W. White, "Two-Pressure Relative Humidity Standards," Humidity and Moisture, Measure- ment and Control i n Science and Industrzy, Vol. 3, Reinhold Publishing Corp., New York, 1965, pp. 445-454.

141 V. Vaisala, "Mixing Hygrostat for Calibration of Hygro- scopic Hygrometers," Ibid., pp. 473-477.

[5] R. G. Wylie, "The Properties of Water-Salt Solutio~ls in Relation to Humidity," ibid., pp. 507-517.

[6] E. R. Weaver and R. Riley, "Measurement of \I-ater in Gases by Electrical Conduction in a Film of Hygroscopic Material and the Use of Pressure Changes in Calibration,"

Jor~rnal o j Research, Nat. Bureau Standards, Vol. 40, No.

3, March, 1948, pp. 169-214.

[7] E. R. Weaver and R. Riley, "Measurement of Water in Gases by Electrical Conduction in a Film of Hygroscopic illaterial-Use of Pressure Changes in Calibration,"

Analytical Chemistry, Vol. 20, No. 3, March, 1945, pp 216-229.

[8] E. R. Weaver, "Electrical Measurement of Water Vapor With a Hygroscopic Film," ilnalytzcal Chemistry, Vol. 23, No. 8, August, 1951, pp. 1076-1080.

[ g ] C. P. Hedlin, "A Resistance-Humidity Relationship for Sensors of the Ihnrnore Type," H ~ i n ~ i d i t y and h[oisture, Measureme?~t and Control zn Science and Industry, Vol. 1, Reinhold Publishing Corp. New York, 1965, pp. 273-279.