The thermometer should be marked in divisions of at most 0.5 F if it is used in conjunction with a wet-bulb thermometer. The thermometer should be shielded from radiant heat sources such as motors, lights, external walls, and people. Hand-held thermistor, resistance bulb, or thermocouple thermometers can also be used. They are more expensive than a glass thermometer but are not necessarily more accurate. These instruments can be purchased with a sharp probe allowing them to be used for measuring product pulp temperature.
Air is forced over the wick, causing it to cool to the wet-bulb temperature.
The thermometer sensitivity required to determine an accurate humidity varies according to the temperature range of the air. More sensitivity is needed at low than at high temperatures. For example, at 150 F a 1 F error in wet-bulb temperature reading results in a 2.6 percent error in relative humidity determination, but at 32 F that same error results in a 10.5 percent error in relative humidity. Before wetting the wick of the wet-bulb thermometer, operate both thermometers long enough to determine if there is any difference between their readings. If there is a difference, assume that one is correct and adjust the reading of the other accordingly when determining relative humidity.