The Fieldpiece model ARH4 Digital Sling Psychrometer and Hygrometer Accessory Head measures both relative humidity and ambient air temperature. It calculates dew point and wet bulb. It’s great for calculating target superheat as well as logging %RH, temp, WB or DP over time. The head converts %RH, ambient temperature, dew point temperature, and wet bulb temperature to a voltage at a rate of 1 %RH or degree to 1mVDC and your reading is displayed on your meter. The ARH4 snaps directly onto Fieldpiece stick meters, data logger, electronic handles and adapters.