The
compact Bacharach Sling Psychrometer uses the accurate wet bulb-dry bulb
thermometer principle to provide a simple, rapid measurement of relative
humidity without tables, charts, or formulas.
Portable and non-electronic, the sling
psychrometer consists of two thin bulb thermometers and a refillable
water reservoir in a high-strength plastic case. There is no need to wet
the wick each time a reading is made. The case is simply whirled freely
in the hand; wet and dry bulb temperature readings are taken; and
percent relative humidity is determined from the built-in slide rule on
the handle. When not in use, the thermometer case telescopes into the
handle for protection.

Thermometers in sling psychrometers are
filled with non-toxic red spirit (alcohol) and are calibrated in °F.