The best-designed sling psychrometer we've seen for student use. Durable, rugged, and mercury-free this instrument will hold up quite well in the field. The entire instrument is packaged in a self-contained plastic case, which easily opens up to a form the sling psychrometer. A slide rule type design built into the case makes conversion of the wet and dry bulb measurements to the percent of relative humidity quick and easy. Thermometer range is 20 to 120 ºF.