The working principle of bimetallic thermometers
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This post was last edited by HaiLeXingGong on 2011-5-27 at 20:40. The temperature shown by the bimetallic thermometer in use currently differs significantly between the field and the control room, resulting in inaccurate readings. How should this be corrected? Could anyone else explain the working principle of a bimetallic thermometer?
It is firmly bonded together by two pieces of metal with different coefficients of expansion; one end is fixed, while the other end is connected to a pointer through a transmission mechanism. When the temperature changes, due to different expansion coefficients, the bimetallic strip undergoes angular displacement, which causes the pointer to indicate the corresponding temperature. This is the working principle of a bimetallic thermometer.
It can be removed for calibration in the laboratory; if a bimetallic thermometer is inaccurate, it can simply be replaced without the need for calibration (it’s not cost-effective to do so – it’s better to buy a new one). If you want to calibrate it yourself, you can use a blank dial and mark the scales on it again according to the standard calibration table. The sensing element of a bimetallic thermometer is made by welding two layers of metal sheets together, each with a different coefficient of linear expansion. The metal sheet with the higher coefficient of linear expansion is called the active layer, while the other is known as the passive layer. One end of the element is fixed, while the other end is free. When the temperature being measured changes, due to the differing coefficients of linear expansion of the two metal sheets, the free end bends (or deforms) as a result of the combined torque. The degree of bending depends on the physical properties of the materials making up the metal sheets, the thickness of each layer, and the temperature; it does not depend on the width. Once the temperature range is determined, along with the materials and geometric dimensions of the metal sheets, the degree of bending depends only on temperature. If a transmission mechanism, along with a pointer and a scale marked with temperature values, is installed at the free end, then the temperature value can be displayed directly.
The temperature-sensing element of a bimetallic thermometer consists of two layers with different coefficients of linear expansion (i.e., the active layer and the passive layer). When the temperature being measured changes, the difference in these coefficients causes the bimetallic strip to undergo angular displacement, which in turn moves the pointer to indicate the corresponding temperature.