The Extech RH390 is a high-accuracy handheld psychrometer that simultaneously displays relative humidity and one of three temperature parameters: ambient air temperature, dew point, or wet bulb temperature. Featuring a slim ergonomic design with rubberized sides for secure one-handed operation, a dual backlit display, and fast response time (<30 seconds for RH), the RH390 delivers laboratory-grade precision in field applications. With ±2% RH accuracy—the highest in its class—it’s ideal for HVAC, industrial hygiene, environmental monitoring, and building diagnostics. The unit includes Data Hold, Min/Max recording, auto power-off (with disable option), and comes with a carrying case and 9V battery.
| Relative Humidity | 0 to 100% RH | 0.1% RH | ±2% RH |
| Air Temperature | –22 to 199°F (–30 to 100°C) | 0.1° | ±1.8°F (±1°C) |
| Dew Point | –22 to 199°F (–30 to 100°C) | 0.1° | Derived from RH & temp |
| Wet Bulb Temperature | 32 to 176°F (0 to 80°C) | 0.1° | Derived from RH & temp |
| Response Time (RH) | <30 seconds | — | — |
| Display | Dual backlit LCD | — | — |
| Power Source | 9V battery (included) | — | — |
| Dimensions | 7.8 x 1.7 x 1.3" (200 x 45 x 33 mm) | — | — |
| Weight | 7 oz (200 g) | — | — |
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The Extech RH390 is a high-accuracy handheld psychrometer that simultaneously displays relative humidity and one of three temperature parameters: ambient air temperature, dew point, or wet bulb temperature. Featuring a slim ergonomic design with rubberized sides for secure one-handed operation, a dual backlit display, and fast response time (<30 seconds for RH), the RH390 delivers laboratory-grade precision in field applications. With ±2% RH accuracy—the highest in its class—it’s ideal for HVAC, industrial hygiene, environmental monitoring, and building diagnostics. The unit includes Data Hold, Min/Max recording, auto power-off (with disable option), and comes with a carrying case and 9V battery.
| Relative Humidity | 0 to 100% RH | 0.1% RH | ±2% RH |
| Air Temperature | –22 to 199°F (–30 to 100°C) | 0.1° | ±1.8°F (±1°C) |
| Dew Point | –22 to 199°F (–30 to 100°C) | 0.1° | Derived from RH & temp |
| Wet Bulb Temperature | 32 to 176°F (0 to 80°C) | 0.1° | Derived from RH & temp |
| Response Time (RH) | <30 seconds | — | — |
| Display | Dual backlit LCD | — | — |
| Power Source | 9V battery (included) | — | — |
| Dimensions | 7.8 x 1.7 x 1.3" (200 x 45 x 33 mm) | — | — |
| Weight | 7 oz (200 g) | — | — |