Wet bulb temperature
If the percentage saturation of an air sample is less than 100, i.e. less than the rich, and he comes into contact with the water at the same temperature, there will be a difference in pressure vapor. As a result, some of the water evaporates. The latent heat required for the change of status will be taken from the sensible heat water, which will slightly cooled. This temperature difference of water provides the temperature difference and heat balance is reached where the flux sensible heat from air to water (Fig. 20.2) provides the latent heat to vaporize part of it.
The effect can be observed and measured by using the same two thermometers (Fig. 20.3), one of which has its own lamp enclosed in wet wick. The drier the air passing over them, the greater will be the rate of evaporation from the wick and the larger the difference between the two readings. In the case of air at 25C, 50% saturation, the difference will be approximately 6.5 K. measurement called the dry and wet bulb temperatures, and the difference in wet depression.
To constant conditions can be obtained, air velocity over the thermometers should be not less than 1 m/sec.
This can be done with mechanical aspiration fan (Assmann psychrometer) or rotary thermometers manually on the radius arm sling psychrometer, Fig. 20.4). If thermometers may not be in the moving airstream, they are protected from drafts perforated screen and only rely on natural convection. In this case, the wet bulb depression will be less and reading is called Masach wet-bulb thermometer.
Hence, the drier the air, the greater will be the difference between the dry bulb temperature, humidity and dew point temperature and, conversely, when the 100% saturation of these three match...
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Industrial 
