These hygrometers are certified to be accurate within ± 3 percent RH. The dial indicates the complete range of 0 to 100 percent relative humidity. Each instrument has been tested at three different positions of the dial at temperatures ranging from 32 to 230F. The large easy-to-read dial. The durotherm humidity sensor is certified to be accurate within ±3 percent RH. This sensor is constructed of a man-made fiber that expands and contracts as the humidity changes and mechanically linked through precision bearings and gears to the indicator. A calibration screw is accessible through a slot in the case for periodic calibration. The thermometer features a bimetallic coil sensor and is accurate to 1 percent of scale. Available with a Brass or stainless steel case perfect for indoor and outdoor use large easy-to-read 5 inch diameter dial case is drilled for wall mounting no batteries or external power source required tested at three different humidity positions and includes a certificate of accuracy.