By Corina Fiore
Updated Mar 24, 2022

A sling psychrometer is a reliable, field‑portable device that accurately determines relative humidity and dew point by comparing temperatures from a wet‑bulb and a dry‑bulb thermometer. The wet bulb contains a cotton wick saturated with room‑temperature water; evaporation from this wick cools the bulb. The dry bulb measures ambient air temperature. Both are mounted on a central dowel and spun through the air to promote evaporation.

Step 1

Moisten the cotton wick of the wet‑bulb thermometer with water at room temperature.

Step 2

Secure both thermometers on the dowel and swing the assembly briskly for at least one minute to ensure thorough air circulation.

Step 3

After swinging, record the temperatures of both the dry bulb and the wet bulb. The wet‑bulb reading will never exceed the dry‑bulb temperature; if it does, check that the wick water is not too warm or that the instrument is functioning correctly.

Step 4

Calculate the temperature difference (ΔT) between the dry and wet bulbs. For example, if the dry bulb reads 22 °C and the wet bulb 18 °C, ΔT = 4 °C.

Step 5

Determine the dew point by locating the intersection of the dry‑bulb temperature (y‑axis) and ΔT (x‑axis) on the dew‑point chart provided by NOAA or the National Weather Service. The corresponding temperature is the dew point in °C.

Step 6

Find the relative humidity by using the same chart. The intersection of the dry‑bulb temperature and ΔT yields the relative humidity percentage.

Things Needed

• Sling psychrometer (or two alcohol thermometers, one fitted with a cotton wick)
• Water at room temperature
• Relative humidity chart (e.g., NOAA Table 1)
• Dew point chart (e.g., NOAA Table 2)