Selecting the Right Humidity Testers for Clean Rooms and Calibrating Them Correctly

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The following instruments are humidity and dewpoint sensors:

1. Psychrometer

The psychrometer is basically a hygrometer consisting of two thermometers: a dry-bulb thermometer which measures the air temperature and a wet-bulb thermometer which is covered with a wet cloth and is cooled by the evaporation of water. Both the amount of evaporation and the latent heat generated by the evaporation depend on the air humidity. Using tables and charts, the difference between the two temperatures can be taken to work out the pressure of the water vapor in the ambient air and, from that, the relative humidity. The process is often used in laboratories and environmental test chambers.

2. Mechanical hygrometer

A mechanical hygrometer measures and records the humidity with the aid of a material which contracts and expands as the humidity changes. This method has been in use for many years although it is not particularly accurate.

3. Lithium chloride hygrometer

The principle of the LiCl hygrometer is based on the hygroscopicity of lithium chloride (its ability to attract water molecules). The sensor consists of a reel covered with an absorbent fabric and a bifilar winding (two insulated wires with currents travelling through them in opposite directions). The reel is coated with lithium chloride. An alternating current is passed through the winding and the lithium chloride solution, causing the reel to heat up. As a result, water evaporates from the lithium chloride solution at a rate which depends on the vapor pressure of water in the surrounding air. As the reel begins to dry, the resistance of the solution increases, and less current flows through the winding. Hence the reel cools down again. The heating and cooling of the reel eventually reaches an equilibrium point where it neither takes on nor gives off water. The temperature at this point is directly proportional to the dewpoint of the surrounding air.

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