A sling psychrometer measures relative humidity using two thermometers, one wet and one dry. Evaporation from the wet bulb thermometer causes a temperature drop, indicating humidity. It can be made at home and is used by athletes and meteorology students. Accurate readings require facing the breeze and avoiding body heat interference.
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 spun in the air. The wet bulb thermometer has an absorbent cover which is soaked in water before using the instrument. A second thermometer, called a dry bulb thermometer, measures 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 ability of air to absorb moisture. Evaporation is slow in air with a high humidity; the opposite is true when the humidity is low. As water evaporates from the casing of the wet bulb thermometer, the temperature drops relative to the dry bulb thermometer. That’s why a hot, humid day is so much more uncomfortable than a hot, dry day. Evaporation on drier days helps to cool the body more efficiently.
Simply twirl the sling psychrometer for about a minute to measure the temperature difference between the two thermometers. The wet bulb thermometer temperature is read and recorded quickly before the dry bulb temperature reading. The rocking of the instrument can be repeated several times to obtain the lowest wet bulb temperature. The wet bulb and dry bulb temperatures are recorded and their difference calculated. A graph that has dry bulb temperature on one axis and temperature difference on the other is used to determine the relative humidity of the air.
While there are many sling psychrometers available at scientific equipment stores, students and others interested in meteorological instruments can make their own. All you need are two thermometers, a frame to hold them, a way to spin them, and some absorbent material to cover the wet bulb thermometer. In addition, a relative humidity table is required. Students in earth science and meteorology classes at all grade levels could 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 heat unsafe for athletic activities.
There are several simple mistakes that can lead to inaccurate measurements. When measuring outdoor relative humidity, the person taking the measurement should face the breeze, not away from the breeze. Also, the wick above the wet bulb thermometer should not dry out when taking repeated measurements. Another mistake is holding the sling psychrometer too close to the body, blocking the breeze and allowing body heat to interfere with the measurements. Speed is of the essence when reading the wet bulb thermometer, as the temperature may rise rapidly when the instrument stops.
Protect your devices with Threat Protection by NordVPN
