The use of the heated electrical hygrometer with sodium chloride

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The use of the heated electrical hygrometer with sodium chloride

- Ser TA1 s92 no.

67

c * 2 BLDG

THE

USE

O F

_THE

_HEATED

_ELECTRICAL

HYGROMETER WITH SODIUM CHLORIDE

C.P. Hedlin and R.G. Nicholson

'1

I

!

I

D I V I S I O N O F B U l L D l N G R E S E A R C H N A T l O M k L R E S E A R C H C Q U H C I L O T T A W A C A N A D A i '

THE

U S E

OF THE

HEATED ELECTRICAL HYGROMETER

WITH SODIUM CHLORIDE

C .

_P.

_{Hedlin and}

_R.

_G.

_Nicholson

The- heated electrical h y g r o m e t e r usually consists of a pair of _elec-

trodes wound aver a fabric covering on a tubular upp port.

Ta

b e g i n oper- ation a solution of salt, tlsually lithiwn chloride, is distributed over the surface, A voltage is applied to _{the electrodes and the heating that results} from conduction through the salt solution raises the temperafxr e _{and d r i v e s}

off some of the moisture.

The

resistance

-

moisture-content c u r v e changes s h a r p l y at a point corresponding to the saturated solution, and a p r o p e r l y

functioning unit w i l l reach and operate at: this condition, the temperature of

the hygrometer adjusting automatically sa that the water vapour p r e s s u r e

of the saturated solution is in _{equilibrium with the ambient water vapour}

pressure.

At _{all other relative humidities above the equilibrium relative humi-} d i t y of the salt (about 11 p e r cent for lithium chloride) the temperature of the s e n s o r is above t h e ambient temperature.

At

ambient humidities below the critical value, the conventional sensor w i l l not function. A s the h n m i d i t y in-

creases, the differential between sensor and ambient temperature also in- creases. The temperatux e s _{obtained under normal} conditions ar e _quite ac -

ceptable.

I£

this instrument is employed at conditions of high temperature and high humidity, however, t h e r e will be a rapid deterioration of t h e sensor.

A common upper limit is considered to be 160' F dewpoint temperature for the lithium chloxide system.

For a l i m i t e d number of applications in which high humidities at

higher equilibrium relative humidity, thus r e s u l t i n g in a Lower instrument

temperature for a _{given dewpoint temperature. For t h i s reason, consider-} ation w a s given to t h e use of sodium chloride,

which

has an e q u i l i b r i m

relative humidity of about 75 per cent. In these experiments t h e salt solution

was made by combining one p a r t of salt to one part of water b y weight. The c h a r a c t e r i s t i c s of the start-up were similar to those f o r l i t h i u m chloride, the high temperature being reached quickly and then falling roughly to the

equilibrium condition. A characteristic start-up required 10 t o 15 minutes.

The necessary environment was provided b y saturating air at a r a t e

a£ approximately 1/ 1 0 cu f t per minute, and conveying it to the t e s t chamber. The pressure drop between the saturator and the test chamber was regulated

s o as _{to provide relative humidities which ranged from} 9 8 _{per c e n t down to} 80 per cent. Although the ventilation rates obtained in this way are probably substantially below the s u g g e s t e d 5 0 f E per minute, the sensor gave the a p -

pearance of operating in a s a t i s f a c t o r y manner during the s h o r t - t e r m t e s t s .

T h e dewpaint temperature ranged from about 5 0 ° F to 2 0 0 " _{F ; the correspond-}

i n g s e n s o r ternperakres ranged from about 5 8 ° F to 2 1 5 ° F (Figure 1 ) . I n

addition to plotting t h e dewpoint versus sensor ternperatur e, the d s f e r e n t i a l

between the d e w p o i n t and sensor temperature is plotted against the dewpoint

temperature to show the t y p e of scatter observed.

The line of b e s t fit w a s found f o r these data, using t h e m e t h o d of l e a s t

squares, This resulted in t h e equation:

w h e r e the subscripts dp and s indicate dcwpoint and sensor temperatures, respectively, ( " F).

T h e sodium c h l o r i d e system was subjected to l o n g - t e r m t e s t s and

t h e s e indicated that a substantial drift occurred, the sensor ternperatur e

gradually r i s i n g as time went by. On the basis of these t e s t s , it was cencluded that the sensor should be washed and r e c o a t e d e v e r y t w o t o t h r e e w e e k s .

CONCLUSIONS

O v e r a restricted area of temperature and humidity, the use of sodium chloride _in place of lithium chloride appears to provide a solution

to the problem of exc e s sive sensor temperature with resultant deterioration. This s y s t e m i s useful only at relative humidities exceeding 7 5 p e r cent. For data in the dewpoint range from 50 to 20Q0F, the average difference between observed figures and the Line of _{b e s t f i t was 0 . 4 " F . The system does} not

appear to b e suitable for long- term measurement as drift occurs and periodic washing and recoating a r e required.

8 0 ₁₀₀ 120 140 160 180 200 220

SENSOR

TEMPERATURE,

"F

F I G U R E

H

S E N S O R T E M P E R A T U R E

MINUS D E W P O I N T

T E M P E R A T U R E V S S E N S O R

T E M P E R A T U R E ( T O P ) ;

D E W P O

l

NT

T E M P E R A T U R E V S

S E N S O R

T E M P E R A T U R E

( B O T T O M )

F O R

S O D I U M

C H L O R I D E

A S

A

H E A T E D

ELECTRS

C A L H Y G R O M E T E R .

4 7 0