A Device for Calibrating Electrical Humidity Sensors

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

A device for calibrating electrical humidity sensors

Hedlin, C. P.
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Authorized Reprint from the Copyrighted

Materials Researclz & Standards Vol . 6 N o .

Published by the

American Society for Testing

and

Materials

A

Device for Calibrating Electrical Humidity Sensors

By

C. P HEDklN

Th is

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-re-

sistance 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 recali-

bration 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.

The italic numbers in brackets refer to the list

of

references

at 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]

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 ac-

curacy.

In the two-nressure device described here. a streant

of air is saturated a t atmospheric pressure and then

expanded to obtain the required humidity. Th e

CHARLES P. HEDLIN received a B.Sc. degre e 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 by the National Research Council of Canada at the

Prairie Regional Station of the Division of Building Research at Saska-

toan Sask. where his primary interests have been in precise measure-

ment of humidity and in the sorption properties of building materials.

anuary 966 25


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T he calibration ch am ber was of bra ss, 3 in. in diam -

eter and 33 in. deep. i\/Iounting socke ts were pro-

vided for thre e sensors on t he u nder side of th e re-

mova ble lid of the chamb er. T h e necessarv wires

were int roduced into the chamber throughla tube

connected to th e lid and sealed with epoxy resin. T h e

lid was fastened to the chamber with bolts. silicone

rubb er gaske ts providing a tigh t seal. Conne ctions

were located on the chamber for pressure measure-

me nt, air entry, and air exit. Th e satu rato r and cali-

bration chamber, w ith connecting tubing , were sub-

merged in a water ba th provided with a therm ostat .

La ter equiplnent was modified in three ways. T he

sa tur ato r was made of brass tan ks. 6 in. insteacl of 10

in. high; th e l ine connecting the satu rato r to the

calibration ch alnber was mad e of stainless steel and

was attached rigidly to the latter; and the calibra-

t ion chamber was at tached to the saturator bv a

clamp, which permitted movement and adjustment.

The se changes resulted in a more comp act unit . This

unit was used to obtain th e 10 F results shown in Fig. 2.

Th e relative humidity was taken as the ratio of tota l

pressures in the calibration chamber and saturator

where:

p,

= baron ~etr ic ressure - 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 f rom th e saturator to the

calibration chamber (measured with a

U-tube mercurv manometer coated in-

ternally w ith antistatic f luid to improve

its accuracy by decreasing the adhesion

between the me rcury and th e glass).

Accurate m easure me nt of th e pressures is impor-

t an t .

An error of 1 mm H g in Ap, will result in an

error in relative hu m idity of appro xima tely 0.13 per

cen t. An error of 1 m m H g in Apl will result in an

error in relative humidity th at can be w rit ten

e = R H - R H = 100

( 1 3 )s 1

R e l a t i v e

H um i d i t y

I

Fig, 2-Results obt ain ed with the two-pressu re system com par ed with cali brati on curves solid lines) obt ain ed with the atmosphe re produc er.

The relat ive humidit ies at 10 are adjusted to make them correspond to supercooled water rather than to ice.)

I

anuary

9

27

L e g e n d

w = Q u ~ e s c e n t

H e ~ g h t

o f

W a t e r

Abo v e Air I n l e t

I n ) 80.6 O F

--

w ~ t h

a c k ~ n gwh e n

P a c k ~ n g

w a s Used

b = Dep t h o f

6

mm Be a d s ~ n

.

A a nd

B

-

s t a n d

2

n d

B

b w

b

A

w

+

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 o o

x o o o

I

I


7/8

If

p, = 760 nlnl Hg,

e =

Ap?(*0.00017), or k0.13

I)cr cent of the rlua ntity (100 - RH). Th e same in-

accuracy ~vill

occur

if th ere is an error of 1 mn l H g

in measuring the barometric pressure.

A vacuum pump was connected to th e calibratioil

c.hamber, and the rat e of air flow and relative hum id-

ity were regulated by the needle valve and a second

valve located betureen the calibration chamber ancl

thc vacuum pump.

R esu lts i d iscussion

This system was usccl to calibrate Duninore-type

k~uiniclity sensors.

Sensors were calibrated before

nricl after use in a two- temperature uni t [ I ] , and the

results obtained with the es~erimental evice were

wm pared with the calibrii tion data .

I t was assumed

th a t the op eration of the sensors was riot affected by

their being placed in a vacuum .

I t was important to hnow th e conditions required

for sub stan tial satu ratio n of th e air and to know also

whe ther wa ter d roplets n.oulc1 be en trained in th e air

Icaving the satu rator. Tests were carried out to

cictermine the effects of ~ v a tc revel and of th e rxes-

erice of packing ma terial.

I n th e first series of te sts ,

a t 80.6 F , only thcl srcond section of t h e sat ura tor was

used; the other sccation was left dry . W ate r de pth

(above the air inlet) was varied from ap proximately

to 4 in. , and th e clt.pth of 6-inm glass bead s was twice

th at of th e water in most cases ( the wate r dep th 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

witho ut beads. T he rate of air flow ranged from

0.03 to 0.1 ft3/m in. If th e air f low rat e was much in

excess of th e higher value, surg ing occurred ancl ma de

accu rate mea surem ent of t he p ressure impossible.

Th e results are shown in Fig. 2.

I t appear s tha t

with a wate r depth less tha n in. the air did not be-

come snturatecl. Fo r all other conditions, howev er,

the results agreed, within abo ut per cent R H , with

the calibration da ta obtained with th e two-tempera-

ture unit of Handegord and Till [ 1. Fo r appro xi-

mately 35 observations w ith three sensors the aver-

age deviation was less than 0.2 per cent R H . This

suggests that within practical l imits saturation oc-

curred without droplet carry-over.

Experiments were done at

3

F using the same

sensors as above. T h e 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

Dunm ore-type serlsors a t relative hum idities down t o

approximately 10 per cent. Accuracy was similar

to th at repor ted above.

111

later work a +in. de pth of water an d beads has

been used in the first section, an d roug hly in. of

nrnter an d 6 or 7 in. of be ads in t he second se ction.

The decision to use these quantit ies was somewhat

arbi t rary, but

was

based on the need for a substan-

tia l reserve of w ater a nd t he principle of using the first

section for the bulk of the sa tura ting and t he second

for a small am oun t of heat and m oisture exchange

th at m ight be required.

To ob t a i n t he r e s u l t s a t 10 F , s ho~~r nn Fig. 2,

cne or both of the sa tura tor ch ambers were fi lled to n

de pth of abo ut 4 in. with chipped ice.

Screens were

placed across the cham bers, just abo ve the air inlets,

to support the ice. I t was found necessary to dry

the incom ing air to preve nt plugging of the passage.

The temperature in the cal ibrat ion chamber was

measuretl with a thermocouple. Tem perature was

affected by the variation in air pressure tha t accom-

panied t he establishm ent of a new relative hum idity.

Rec ause of this, an d possibly because of so rptio n 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 th e uppe r portion of the second section of th e

satu rato r. Generally the temp erature there agreed

closely with th at of th e bat h an d the calibration

chamb er. T o ensure accurate results , calibrated

tl~ermo coup les, r suitable wells to receive a m ercury

thermome ter, should be incorporated near th e ou tlet

of the sa tur ato r and in the calibration cham ber. If

such wells are used, i t is importan t t ha t they be de-

signed so th at the measured tem peratu re will not be

affected by thermal conduction along the well or the

t c>m perature sensor.

A sho rt series of tests was carried ou t to assess the

effect of th e moisture co nte nt of t h e air entering the

saturator on the humidity in the calibration cham-

her. Th e inconling air was alternately dried by

passing i t through a desiccant and saturated at a

temperature above that of the two-pressure system

sat ur ato r. Ea ch 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 incom-

ing air.

In previous tests the tem perature control of the

ba th was very close; variation probably did not es-

ceed a few hun dredth s of a degree Fah renhe it . To

determine the approximate effect of wide variation,

the control was arranged so that the ba th temp erature

fluctuated approximately F above and below the

null point. T he 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 th e changes in b ath temperature more

s lowly than did the satu rator .

Th is result suggests

tha t the ideal bath tem perature should not f luctuate

by more than abo ut *0.10 F , or tha t an ar rangement

for damping th e variations should be incorporated.

Sum mary an d Conclus ions

The two-pressure system described here is suitable

for routine ca libration of Dun mo re-ty pe hum idity

.sensors. Air is satur ated a t atmospheric pressure

and expanded to a lower pressure in the calibration

chamber . T he humidi ty in the calibration chamber

is regulated by ad justing the pressure there. Series

of te sts using calibrated D un mo re sensors were car-

ried out at 80.6, 32.0, and 10.0 F. At 80.6 F , the

28

Materials Research

&

Standards


8/8

average deviation froill the calibration curve was

less than

0.2

per cent RH, and at the lower tempera-

tures it was less ihan

0.3

per cent RH.

The two-pressure vacuum principle has several

characteristics which reconlmend it as a calibrating

device.

It

performs satisfactorily over th e 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 cali-

bration chamber and the saturator will be equal. To

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 oj Research

Nat. Bureau Standards,

Vol. 48, No.

4,

Apr~l, 951, pp. 269-274.

[S] E. J. Amdur and

R.

W. White, Two-Pressure Relative

Humidity Standards, Hum idi ty and Moisture Measure-

me nt 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 n

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

Materia l and t he Use of Pressure Changes in Calibration,

Jor~ rnal 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 Ihnrnor e Type,

H ~ i n ~ i d i t ynd h[oisture

Measurem e?~t nd Control zn Science and In dust ry

Vol. 1,

Reinhold Publishing Corp. New York, 1965, pp. 273-279.

lanuary 9

Documents

A Device for Calibrating Electrical Humidity Sensors