The improvement of lithium chloride dew-point hygrometer for direct reading and controlling of relative humidity

Environmentlnternational, Vol. 12, pp. 471-474, 1986

0160-4120/86 $3.00 + .00 Copyright © 1986 Pergamon Journals Ltd.

Printed in the USA. All rights reserved.

THE IMPROVEMENT OF LITHIUM CHLORIDE DEWPOINT HYGROMETER FOR DIRECT READING AND CONTROLLING OF RELATIVE HUMIDITY Zhang Xi-zhong and Chu Yu-lian Institute of Health, China National Center for Preventive Medicine, 29 Nan Wei Road, Berjing, China

Hu Xi-qi Department of Weapon Industry, Beijing, China (Received 26 February 1985; Accepted 13 September 1985) The lithium chloride dew-point hygrometer has many advantages over other types of hygrometers. However, it only reads and controls the dew-point temperature of air instead of the relative humidity, which is more important in industry, agriculture, food storage, and hygiene. This paper describes a new hygrometer which is based on the same principle as the lithium chloride dew-point hygrometer, but it can read and control the relative humidity directly. The instrument is quick in response and the ranges of temperature and relative humidity are quite large. Its accuracy is normally within 3% RH and its precision is within 2% RH.

Introduction

coil, there is another insulated layer and then a glass fiber sleeve (c). Two gold wires (d) are parallelly wound around the glass fiber sleeve. Two legs (e and f) are connected to the coils (b and d). The glass fiber is soaked with lithium chloride solution. The construction o f the sensitive element is different from the ordinary construction, in which the temperature device is usually inside the metal tube. This new construction has low thermal inertia, and its rate of response is twice as fast as that o f the ordinary sensitive element. Nelson (1965) has already given a detailed description about the principle of the lithium chloride heated electric hygrometer. By this principle, there is a dewpoint of the air that corresponds with a sensitive element temperature. The relative humidity of the air can be calculated from the air temperature and its dewpoint temperature by the following equation:

The lithium chloride dew-point hygrometer has the advantages o f simplicity in construction, wide dewpoint range, stability in performance, and no need for calibration. However, it only gives the dew-point o f air and does not give the relative humidity. Yet in industry, agriculture, food storage, hygiene, and many other fields, the meaning of relative humidity is often more important than the dew-point. This paper will introduce a new construction o f a lithium chloride dew-point hygrometer which can read and control relative humidity directly. The range of the instrument is wide, and it can be adjusted at will. For instance, it can be used in the ranges 4 0 - 6 0 °C, ~ = 1 5 % - 1 0 0 % , 0 - 4 0 °C, ~ = 1 5 % - 1 0 0 % , - 2 0 - 1 0 °C, ~ = 3 5 % - 1 0 0 % .

The Construction of the Sensitive Element and the Principle of Measuring the Relative Humidity relative humidity =

The construction o f the sensitive element is shown in Fig. 1. A thin metal tube (a) has its outside surface insulated. Outside the insulated layer, a copper coil (b) used as a temperature sensor. Outside o f the copper 471

the water vapor pressure of the air at the dew-point temperature the saturated water vapor pressure at the air temperature

x 100070. (1)

472

Zhang Xi-zhong, Chu Yu-lian, and Hu Xi-qi Table 1. The constants for calculating the vapor pressure. Temperature (°C)

A

B

-40-0 0-40 40-60

2.636 x 101° 1.336 x 109 8.452 x l0 s

6145 5319 5177

td = a t o + b,

(5)

T~ = c T o + d,

(6)

C

or

e d

where tp or To is the probe temperature and td or Td is the dew-point temperature. The values of a, b, c, and d for different temperature ranges are shown in Table 3. Substituting Eq. (6) into Eq. (4), we have Fig. 1. Construction of the sensitive element.

The saturated vapor pressure o f air (mm Hg) is the function o'f the absolute temperature of air; detailed information can be obtained from G o f f (1965). This relation can be shown approximately with Eq. (2) in a limited temperature range: P

= A

(2)

e e:r

The values o f A and B for different ranges are listed in Table 1. Let t, be the air temperature and td be the dew-point temperature. By substituting them into Eqs. (1) and (2), the relative humidity of air is obtained. 1/r.) X 100°7o,

(3)

In ~ = - B ( 1 / T d - 1/Ta),

(4)

=

e -nil/r,-

or

where Ta and Ta are the absolute temperatures of air and dew-point. The relations between the probe temperatures and dew-point temperatures of air can be caliberated. Our calibration device is based on the principle o f two temperatures (Zhang et al., 1964). The relations are shown in Table 2. The relation is approximately linear in each limited range and may be expressed with Eqs. (5) and (6):

lnfJ=

-B

( cT, ,+ d

1)

T,

(7)

The relative humidity is a function o f probe temperature Tp and air temperature Ta. The Analoguo Gireuit Equation (4) shows that by choosing two sets of resistors and setting their variations due to temperature changes to be proportional to the values of T, and Ta, then the relative humidity o f air can be obtained from an analogue circuit. The analogue o f air temperature Ta In Fig. 2, let R, be a copper resistor for measuring the air temperature and let R~ be a manganin resistor; then R a is the sum o f R, and R~. Assuming Ro' is the resistance of R, at 0 °C and et its temperature coefficient at temperature to, we have

Ra = R0'(1 + otto) + R l.

(8)

Rl = Ro' (273.2 et - 1)

(9)

Let

and Ro = 273.2 aRo',

Table 2. Relationship between the dew-point temperature and probe temperature. Dew-point temperature (°C)

-20

- 10

0

10

20

30

40

50

Probe temperature (°C)

5.3

20.3

35.3

48.5

63.1

78.2

94.1

106.8

(10)

473

Reading and control of relative humidity Table 3. Constants a, b, c, d in different temperature ranges.

Dew-point temperature (°C)

a

b

c

d

-40-0 0-40 40-60

0.666 0.682 0.784

-23.52 -23.18 - 33.8

0.666 0.682 0.784

67.52 63.73 25.11

then R~ = R~ (1 + ata) + R~ (273.2 oc = Ro (1 + to/273.2),

1), (11)

= Ro TolTo.

Since R o / T o is a constant, then R, is proportional to the temperature T,. T h e a n a l o g u e o f d e w - p o i n t t e m p e r a t u r e Td In Fig. 3, let Rp be a c o p p e r resistor for measuring

the probe temperature and R E a m a n g a n i n resistor; then Re is the sum of R , and R~. Assuming Ro" be the resistance o f Rp at 0 °C, at temperature Tp we have R , = Rg (1 + t~tp) + R2.

(12)

L..--i

\

~

!

/

ad

Fig. 3. Analogue of dew-point temperature. It is clear that if R~, R2, and Ro" can satisfy certain relations, we are able to obtain two c o m p o u n d resistances R~ and Rd that are proportional the absolute temperature T, and Ta. The resistance Ro" now used is 300 12 at 0 °C and its temperature coefficient is ct = 0.0043 1/°C, then we can obtain Ro", R~, and R2 at different temperature ranges, as shown in Table 4. Analogue circuit After Ra and R d are obtained, Eq. (4) can be easily

analogued by using a single operational amplifier as shown in Fig. 4; we have V, =

-

E R3 (1/gd

-

(16)

lIRa).

Putting Eqs. (11) and (13) into Eq. (16), we have

Let

(17)

V, = - R3 To E (1/Td -- 1/Ta)/Ro.

(13)

Rd = Ro Td/To

Let

and substitute Eq. (6) into Eq. (13), we have Ra = c Ro (To + tp)/To + d R o / T o .

R3 To E/Ro = B ,

(14)

and c o m p a r i n g Eq. (4) with Eq. (17), we have

C o m p a r i n g Eq. (12) with Eq. (14), and setting

ln ft = V, = - B

Ro" eLtp = tp c go~To

(1/Ta-

(18)

I/T,,).

Therefore the output V~ is the function o f relative humidity. Since the relation is logarithmic and nonlinear, an antilogarithmic amplifier stage is used in the circuit.

and Ro" + R2 = c Ro + d Ro/To,

Performance of the Instrument

we m a y have

An instrument operating according to the above principle is constructed. It can measure, record, and control the relative humidity. The accuracy of measurement is usually within _ 3% R H (some individual points m a y reach -+4%). The control precision m a y reach _+2%.

Ro" = c RO/aTo

and R2 = c Ro + d RO/To - Ro", = dRo/To + c R o ( a T o

-

1).

(15)

Table 4. Values of R[, R1, and R2 when Rg = 300 Ohms.

Temperature (°C)

Resistance

at

RI

\

/ Ra=RI +R 1

Fig. 2. Analogue of air temperature.

fl

40-60

0-40

- 40-0

R~ R, R2

382.5 66.75 93.66

440 76.83 172.95

450 78.56 183.02

474

Zhang Xi-zhong, Chu Yu-lian, and Hu Xi-qi R3

Ra

Rd Fig. 4. Analogue circuit.

This instrument has been used to control the relative humidity of a variable temperature and humidity chamber which is 12 m in diameter and 10 m in height. Its temperature should go to 60 °C and the relative humidity should go to 95 ___3%. This is a difficult task for an ordinary hygrometer. Yet th4s. mew designed instrument can control the relative humidity quite well.

Conclusion This instrument is developed by basing on the principle o f the lithum chloride dew-point hygrometer. It has many advantages over other types o f hygrometer. The accuracy of measurement is usually within ___3% RH and the control precision may reach ___2% RH.

References Goff, J. A. (1965) Saturation pressure of water on the new Kelvin scale, in Humidity and Moisture, Measurement and Control in Science and Industry, vol. 3, pp. 283-292. Nelson, D. E. and Amdur, E. J. (1965) The mode o f operation of saturation temperature hygrometer based on the electrical detection of Salt solution phase transition, in Humidity and Moisture, Measurement and Control in Science and Industry, vol. 1, pp. 617-626. Zhang, X. Z., Cui, Y. L., and Zhen, T. W. (1964) The manufacturing and calibration of resistance type hygrometer MeteoroL Tech. Instrument Manufact. 2, 7 - 9 (In Chinese).