Working principle of industrial bimetal thermometer

It works on the principle that two different metals expand at different degrees when the temperature changes. In order to improve the sensitivity of temperature measurement, the metal sheet is usually made into a spiral shape. When the temperature of the multi-layer metal sheet changes, the amount of expansion or contraction of each layer of metal varies, causing the spiral coil to roll up or loosen. Because one end of the spiral coil is fixed and the other end is connected with a pointer that can rotate freely, when the bimetallic piece feels the temperature change, the pointer can indicate the temperature on a circular graduation scale. The temperature measurement range of this kind of instrument is 200~650℃, and the allowable error is about 1% of the two steps of the scale. The purpose of this thermometer is similar to that of a rod-shaped glass liquid thermometer, but it can be used under conditions that require higher mechanical strength.