The uniquely compact Bacharach Sling Psychrometer accurately determines percent relative humidity without the necessity of consulting complex tables. No need to wet the wick each time a reading is taken; a built in water reservoir holds sufficient water for several hours of testing. The Sling Psychrometer contains a slide rule calculator which correlates wet and dry bulb thermometer indications for direct reading of relative humidity. Thermometers are constructed of shock resistant glass and stems have deeply etched numerals with 1° scale divisions for easy reading. When not in use, the thermometer case telescopes into the handle for protection.