A
hygrometer is an instrument which is used to measure humidity around its
surroundings 11. There are different types of hygrometers that work on
different principles. We will talk about three different types of hygrometers
with different working principles in this report.

3.1
Psychrometer:

         Psychrometer is very easily accessible
and works on a basic principle. It is comprised of a one wet-bulb thermometer, one
dry-bulb thermometer, a cloth a handle as shown in figure 1. It follows the
principle of evaporation of water 12. The wet bulb-thermometer measures a
drop in the temperature when the water on the cloth on top of its tip evaporates
because of evaporation leads to cooling. At that time, the difference between
wet-bulb temperature and old-bulb temperature can be measured easily and that
difference in temperature is directly proportional to the amount of water
evaporated. A larger drop in the temperature suggests a relatively low humidity
level in the surroundings because amount of water evaporated will also increase
if the air is dry. If there is 100% humidity then there will no drop in the
temperature. The handle on the Psychrometer is used to generate the air flow
and so that the cloth on the Psychrometer will come in contact with more amount
of air.

Figure
1:
Psychrometer 12

3.2
Gravimetric Hygrometers:

         The second type of hygrometer that we will talk about
it Gravimetric hygrometer that works on the principle of separating the water
from the gas and hence finding the masses of both gas and water 13. The
gravimetric hygrometer consists of prover tubes and moveable pistons as shown
in the figure 2 below. As mentioned above, gravimetric hygrometric measures
humidity by finding the masses of both gas and water after separating them,
then water mole fraction is calculated. Once water mole fraction is calculated,
dew point and relative humidity can be calculated by taking the measurements
beforehand.

Figure
2:
Gravimetric Hygrometer 13

3.3
Chilled Mirror Hygrometer:

         The third type of hygrometer we will
talk about is Chilled mirror hygrometer that works on the dew point technique
to measure humidity 14. In this technique, a metallic surface is chilled to a
temperature so that the vapour pressure of both water and gas is in equilibrium,
ensuring that mass of water is not changing. Chilled mirror hygrometer consists
of metallic mirror, an inert metal, a thermoelectric cooler, light emitting
diode and photodetector monitor as shown in the figure 3 below.

Figure 3:
Assembly of Chilled Mirror Hygrometer 14

            In this assembly, gas flows over the
metallic surface forming dew droplets and scattering reflected light 14. The
amount of reflected light is proportional to photodetector output which
controls the pump and then a platinum resistance thermometer is used to observe
temperature at the dew point. Furthermore, to measure dew point, mirror is set
to equilibrium above 0º C.