Think of hygrometers like a gas gauge: without one, you are risking total catastrophe. Stripped down to their most basic function, hygrometers measure the amount of moisture in the air using a metric known as relative humidity, or RH, expressed as a percent. Analog hygrometers look a bit like a thermometer, and usually include an adjustment screw on the back to calibrate it using the "salt test." Digital units come in a variety of shapes and sizes, from flat to round, and some even include a digital thermometer. Shop our selection of hygrometers and buy them for the lowest prices online.