Bimetallic thermometer is prone to failure at high temperature the limitations of itself?

Bimetallic thermometer is fast, has good linearity and high temperature performance, but many users find bimetallic thermometer met after high temperature, would reduce the accuracy of measurement, what reason is this? Don't bimetallic thermometer is suitable for working under high temperature? Bimetallic thermometer is failed or the limitations of the thermometer itself? As we all know, bimetallic thermometer temperature sensing element is composed of two layers of different linear expansion coefficient, made of sheet metal folding welding together, linear expansion coefficient of large pieces of metal is called the active layer, and the other one is called passive layer, one end of the element is fixed, the other end of the free end, when the measured temperature changes, as a result of the two layers of metal linear expansion coefficient is different, the free end by the combination of torque and bending ( Or deformation) , the change of curvature and physical properties of the bimetallic strip material, length of the thickness of each layer, temperature, and has nothing to do with the width. When temperature is designed, the bimetallic strip of material and geometry size, so the change of curvature is only related to temperature, if on the free end is equipped with drive mechanism, pointer and dial indication in temperature, so you can directly show the temperature value. Through the careful analysis of the above, we can find because thermometer USES a metal material, so there will be a certain limit. The thermal expansion coefficient of metal only can approximate think when in low temperature is constant, by high temperature, 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. These detail aspects of using a thermometer to us there is a help, otherwise it will cause blind measurement, and in the process of use, must be to test the thermometer. Verification equipment mainly include reading glass, thermostat, freezing trough, reading telescope, etc. Verification method is: put thermometer detecting element into the containing ice, water temperature tank or freezing point, after waiting for a stable value for reading.