Measurement of air humidity

The humidity of air is an important environment's property in many industrial

processes and technologies, as well as it affects a men's health. There is always some
moisture in the air.
Absolute humidity of air Φ refers to water vapor in 1 m3 volume of air
mp
Φ= , (1)
V
where mp is the mass of water that is contained in the air of volume V. As it follows from
equation [1], the SI unit of absolute humidity is kg/m3.
Amount of water vapor contained in the air is delimited by the point of vapor
saturation at each temperature. The humidity reaches its maximal value Φmax at this point.
As can be seen from the table bellow, maximal value of humidity as well as vapor
pressure that corresponds to vapor saturation point increases with the temperature. When
the temperature of air with maximal value of humidity decreases, excess amount of water
vapor condenses (transforms its phase) into liquid water such that air humidity is equal to
its maximal value.
Relative humidity of air ϕ is the ratio of absolute humidity Φ to its maximal
value Φmax
Φ
ϕ= (2)
Φ max
Relative humidity is typically expressed in percents as
Φ
ϕ% = ⋅ 100 (3)
Φ max
Minimal value of relative humidity (0 %) corresponds to the dry air and maximal value
(100 %) corresponds to vapor saturated air. Relative humidity of air that falls between 50
and 70 % represents most pleasant environment for men.

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Air humidity measured by psychrometer

We will learn how to use Assmann's psychrometer (Fig. 1) to measure the air
humidity. It consists of two mercury thermometers; first of them is called dry
thermometer and it is used to measure the temperature of surrounding environment t1.
The bulb of second thermometer, called the wet thermometer, is covered with a wet
fabric. Evaporation from the wet thermometer lowers the temperature of surrounding air
t2 (i.e. t2 < t1) by the latent heat of liquid-to-vapor phase transition. Difference between
the two temperatures (t1 - t2), psychrometric difference, therefore depends on the air
humidity. Evaporation increases with decreasing humidity, which causes an increase in
temperature difference. For precise measurement, Assman's psychrometer includes a fan
that prevents water vapor from the formation of saturated vapor environment.

fan
wet thermometer
dry
thermometer

temperature
difference

flowing air

wet fabric

air intake vzduch

nasávaný

Fig. 1. Assmann's psychrometer

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 Water vapor contributes to atmospheric pressure by the partial pressure. It is
defined as the pressure of a vapor in dynamic equilibrium with its liquid form. Partial
pressure of saturated vapor is constant for each particular temperature. Tab. 2 shows its
values for several different temperatures. Relative humidity of air determined by the
Assmann's psychrometer can be then calculated from the partial pressure of water vapor
p1 (at the temperature t1) and partial pressure p2 (at the temperature t2) via relation
p 2 − Ab(t1 − t 2 )
ϕ= (4)
p1
where b is an atmospheric pressure in pascals (Pa) and A is a psychrometric constant. Its
value for psychrometer used in our laboratory is A=0,00066 K-1.

Equipment:
Assmann's psychrometer, barometer, test-tube with distilled water.

Experimental procedure:
1. Read the temperature t1 of the dry thermometer.
2. Wet the fabric that covers the bulb of wet thermometer by placing the test-tube with
distilled water into the opening underneath the thermometer for about 10 seconds.
Spin the string of fan in top part of psychrometer such that it will rotate during the
entire measurement. Watch the continuous decrease of temperature of the wet
thermometer and write down the value t2 after it becomes steady.
3. Wait until fan stops moving and temperature of wet thermometer increases again.
Repeat the measurement two more times and record all the values into the table.
4. Read the atmospheric pressure b of the barometer.
5. Calculate relative humidity ϕ according to the equation (4) and express it in percents
(ϕ%). Calculate absolute humidity Φ using the equation (2) and values from the
Tab. 2. Determine average values of ϕ% and Φ using all three measurements.

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Tab. 1. Temperature of dry thermometer t1, temperature of wet thermometer t2, partial
pressure of water vapor p1 at temperature t1 and partial pressure p2 at temperature t2,
relative humidity ϕ, maximal humidity Φmax and absolute humidity Φ of air at the
laboratory temperature t1.
Φmax Φ
measurement t1 [°C] t2 [°C] p1 [Pa] p2 [Pa] ϕ
[kg/m3] [kg/m3]
1
2
3

Tab. 2. Temperature dependence of the partial pressure of water vapor p and maximal
humidity of air Φmax.
t [°C] p [Pa] Φmax [kg/m3]

11 1307 10.0⋅10-3
12 1400 10.7⋅10-3
13 1493 11.4⋅10-3
14 1600 12.1⋅10-3
15 1707 12.8⋅10-3
16 1813 13.6⋅10-3
17 1933 14.5⋅10-3
18 2066 15.4⋅10-3
19 2173 16.3⋅10-3
20 2333 17.3⋅10-3
21 2493 18.3⋅10-3
22 2640 19.4⋅10-3
23 2813 20.6⋅10-3
24 2986 21.8⋅10-3
25 3173 23.0⋅10-3

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References:

1. Kopecký, F.: Physics for Students of Pharmacy I. Bratislava, UK 1999. 184 s. (in
Slovak).
2. Edition of Department of Physical Chemistry: Laboratory Practice in Physics for
Students of Pharmacy. Faculty of Pharmacy, Comenius University, Bratislava, UK
1991.
3. Oremusová J., Vojteková M.: Determination of air humidity by psychrometer.
Manual for laboratory practice. (in Slovak)

Manual written by RNDr.J.Gallová, CSc.

English version prepared by N. Kuþerka, PhD.