All psychrometers derive an environment’s wet-bulb and dew point temperatures from a precise measurement of relative humidity. The original psychrometer had two thermometers mounted together with a handle attached on a chain. One thermometer was ordinary. The other had a cloth wick over its bulb and was called a wet-bulb. New, digital versions—like General’s EP870X family— improve on this basic design, offer HVAC technicians a better tool for recharging the refrigerant of a residential or commercial A/C system.