Bimetallic thermometer why working under high temperature disorder?

Why bimetallic thermometer work disorder at high temperatures, this is due to the nature of is. Bimetallic thermometer is a kind of suitable for measuring the low temperature ( - 40+550C） The testing instrument. Bimetallic thermometer working principle is to round shape into threaded heat bimetallic strip as thermal devices, and put it in a protective sleeve tube, one end is fixed, known as the fixed end, the other end of the connection on a thin shaft, called the free end. Spool at the free end is equipped with a pointer. When the temperature changes, the resulting thermal devices the free end of the rotation, drive the thin axis from the Angle of the pointer changes, corresponding temperature indicator on the dial. In addition, because the thermal expansion coefficient of metal only when in low temperature ( - 40+550C） Can approximate thought is constant, the high temperature ( More than 550 c) , the thermal expansion coefficient of metal will not remain constant, then to metal heat bilges cold shrink principle of bimetallic thermometer to measure the temperature is not accurate.