The Extech RH390 psychrometer measures humidity, air temperature, dew point, and wet bulb. It features a wide measurement range: 0–100% RH, -22 to 199°F dew point, 32 to 176°F wet bulb, and -4 to 199°F temperature. Accuracy is ±2% RH and ±1.8°F (±1°C) for temperature, with 0.1% RH and 0.1°F resolution.

The dual backlit LCD simultaneously displays humidity and one other parameter (temperature, wet bulb, or dew point). Response time is under 30 seconds. Additional features include data hold, min/max functions, auto power off (with disable), and low battery indicator. The slim design with rubberized sides facilitates one-hand operation.