The 225-5230 Assmann Psychrometer utilizes two precision mercury thermometers with 3-point calibrations. A calibration certificate gives the correction required at each point. Corrected accuracy is better than ±0.1°C. The double-tube thermometers are protected by a chrome-plated brass case. The thermometer bulbs are guarded by double-walled radiation shields thermally isolated from the main housing by a plastic bushing. Aspiration of the bulbs is accomplished by a spring-wound fan which will provide eight minutes of aspiration per winding.
Two models are available, the 225-5230 measures over a range of -30° to +50°C and the 225-5231 has a range of -20° to +130°F. Each instrument is supplied with a wind shield, a mounting support, a syringe to moisten the wet-bulb wick, extra wicks, psychrometric tables, and a carrying case.