5230 and 5231 Assmann Psychrometer
Description

The Assmann psychrometer utilizes two precision mercury thermometers with 3-point calibrations. A certificate is provided with each thermometer, giving the correction required at each point. Corrected accuracy is better than +/-0.1°C. The double-tube thermometers are supported and protected by a chromeplated 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 springwound fan which will provide 8 minutes of aspiration per winding.
Two models are available

The Model 5230 measures over a range of -30° to 50°C; the thermometer graduations are 0.2°. The Model 5231 has a range of -20° to 130°F with graduations of 0.5°. 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 sturdy carrying case.

Assmann psychrometer—A special form of the aspiration psychrometer, developed by Assmann.
Two mercury thermometers, mounted vertically side by side in a chromium- or nickel-plated polished metal frame, are connected by ducts to an aspirator. Each thermometer is located inside a pair of coaxial metal tubes, highly polished inside and out, that screen the bulbs from solar radiation.
Middleton, W. E. K., and A. F. Spilhaus, 1953: Meteorological Instruments, 3rd ed., rev., Univ. of Toronto Press, 102–104.