A psychrometer is a type of hygrometer, a device that measures relative humidity. It uses two thermometers, one dry (dry bulb) and one covered in a fabric saturated with distilled water (wet bulb). Air is passed over both thermometers, either by a fan or by swinging the instrument, as in a “sling psychrometer.” A psychrometric chart is then used to calculate humidity from the dry and wet bulb temperatures. This circuit shows the LTC2991-based psychrometer. The two transistors provide the wet bulb and dry bulb temperature readings when connected to the appropriate inputs of the LTC2991.