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A sling psychrometer is a simple meteorological instrument used to measure the relative humidity of the air. It consists of two thermometers on a structure that can be whirled in the air. The wet-bulb thermometer has an absorbent cover that is soaked in water before using the instrument. A second thermometer, called the dry-bulb thermometer, measures the temperature in the usual way. Relative humidity and dew point are determined using the difference between the temperatures recorded by the two thermometers.
The wet-bulb thermometer measures the air’s ability to absorb moisture. Evaporation is slow in air with a high humidity level; the opposite is true when the humidity is low. As water evaporates from the covering of the wet-bulb thermometer, the temperature drops compared to the dry-bulb thermometer. This is why a hot and humid day is so much more uncomfortable than a dry, hot day. Evaporation on the drier day helps to cool the body more efficiently.
Twirling the sling psychrometer for about a minute is enough to measure the difference in temperatures between the two thermometers. The temperature of the wet-bulb thermometer is quickly read and recorded before the dry-bulb temperature is read. Swinging the instrument can be repeated several times in order to get the lowest wet-bulb temperature. The wet-bulb and dry-bulb temperatures are recorded and their difference computed. A chart that has dry-bulb temperature on one axis and temperature difference on the other is used to determine the relative humidity of the air.
Although there are many sling psychrometers available from scientific equipment stores, students and others interested in weather instruments can make their own. All that is needed are two thermometers, a structure to hold them, a way to whirl them, and some absorbent material to cover the wet-bulb thermometer. In addition, a relative humidity chart is necessary. Students in earth science and meteorology classes at all grade levels might gain a deeper understanding of the concept of relative humidity by conducting experiments with sling psychrometers. Athletes and their coaches also use a sling psychrometer to determine when humidity levels make hot weather dangerous for athletic activities.
There are several simple mistakes that can lead to inaccurate measurements. When measuring the outdoor relative humidity, the person taking the measurement should stand facing into the breeze, not away from the breeze. In addition, the wick over the wet-bulb thermometer should not be allowed to dry out when taking repeated measurements. Another mistake is holding the sling psychrometer too close to the body, both blocking the breeze and allowing body heat to interfere with the measurements. Speed is essential when reading the wet-bulb thermometer, as the temperature might rise quickly when the instrument comes to a stop.