Sling Psychrometers are a very reliable instrument to accurately determine percent relative humidity (RH) without the necessity of consulting complex tables. The unit contains a slide rule calculator which correlates wet and dry bulb thermometer indications for direct reading of relative humidity. There is no need to wet the wick each time a reading is taken; this unit has a built-in water reservoir and can be whirled freely in the hand. Calibrated to give accurate readings from 25°F to 120°F with a RH reading accurate to within ±5%.