Bimetallic strip – Working , advantages and Disadvantages
Bimetallic strip thermometers are mechanical thermometers. They are widely used in industry for temperature control because of their robustness, temperature range and simplicity. It consists of a bimetallic strip which is made of two dissimilar’ metals bonded together with one end fixed and the other free. A bimetallic strip is used to convert a temperature change into mechanical displacement. The principle is that as the temperature changes, one strip will expand more than the other causing the pair to bend at the free end. Most bimetallic strips use a high thermal expansion alloy such as steel or stainless steel coupled with a low thermal expansion alloy such as Invar. Steel and copper or in some cases, brass is also used instead of copper for bimetallic strips.
Figure 1.40 shows the configuration of bimetallic temperature controlled switch or thermostat. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and it is lower on the inner side when cooled. When the temperature of the switch is increased, the thermal expansion material will be higher than other side of low thermal expansion material. It causes the strip to bend upward in making contact so that the cunent can flow. By adjusting the size of the gap between strip and contact, the temperature can be adjusted or set.
Advantages of bimetallic strips
- Power source is not required
- Cost is low
- It is robust construction
- It is easy to use and it can be used up to 500°C.
Disadvantages of bimetallic strips
- It is less accurate
- It is limited to applications where manual reading is acceptable, e.g. a household thermometer.
- It is not suitable for very low temperatures because the expansion of metals tend to be too smaller. So, the device becomes a rather insensitive thermometer.
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