Humidity measurement using Dew point method

What is Dew point?

The dew point is the temperature at which a mixture of air and water vapor saturates. The classic method for determining the dew point is to slowly cool a polished surface until condensation occurs. The temperature of the surface when the first drop appears is considered the dew point.

How to measure the humidity using dew point method?

You can use this method to determine the absolute humidity or partial pressure of the vapor.A widely used approach to continuously measure the dew point is based on the temperature of the vapor equilibrium. It measures the temperature at which a saturated solution of a hygroscopic salt (lithium chloride) achieves a balance of vapor with the atmosphere. You use electric heating to reach the temperature of the saline solution, since it is much higher than the temperature of the pure water. In terms of the structure of the spray measurement system, a tube containing a temperature measurement element is wrapped with a glass fiber that has been moistened with a saturated solution of lithium chloride salt. Two conductors are wrapped around the assembly in contact with the wick and are supplied with low voltage alternating current (25 v). The flow of current through the salt solution generates heat, which raises the temperature.

When the temperature of vapor equilibrium is reached, water evaporates, reducing current flow and heat input. The temperature cannot go any higher because all the water would evaporate and heat input would cease.
It cannot fall because all the salt would then go into solution, and too much heat would be generated. Therefore, equilibrium is reached with a portion of the lithium chloride in solution and conductive and the remainder
dry and non conductive. Thus, heat input is balanced with heat loss. The thermometer bulb, when placed inside the metal tube, will measure temperature or dew point. This is also a measure of absolute humidity and can be expressed in grains of moisture per pound of dry air, percentage of water vapor by volume, and other units.
Since there is no provision for cooling with this method, it can be used only for conditions where the equilibrium temperature of lithium chloride is above ambient temperature. This corresponds to a minimum of approximately
12 percent to 15 percent relative humidity over ordinary temperature ranges. The method can be used up to saturation, or 100 percent relative humidity. Ambient temperatures may vary from 200°F to –30°F. You must measure dew points at higher temperatures on a cooled sample

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