In bimetallic piece thermometers the various rates of development of steels when boil is provided to measure up the temperature!


Operating principle

Temperatures can be meassured based upon the rule of different thermal development of solids. For this purpose, two steel strips (e.g. Steel and also brass) with different degrees of thermal expansion are firmly joined together. When heated, the metal strip deforms as result of the different levels of expansion. The deformation is a measure of the temperature and can be review off a calibrated scale.

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Figure: Structure and also principle the a bimetallic strip

In bimetallic piece thermometers the various rates of expansion of metals when boil is offered to measure up the temperature!

Animation: operating principle that a bimetallic strip thermometer

Due to their metallic materials, bimetallic thermometers deserve to be used at temperature from listed below -100 °C to much more than 500 °C.

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Bimetal strips are created by roll the various metal sheets one on optimal of the other and then heater them so the the metals bond at the share by diffusion processes (cold welding).

Note: as result of the electrical conductivity that the bimetallic strip, it is also frequently used as a safety and security component for automatic shutdown. Bimetallic strips have the right to be uncovered in kettles, for example, i m sorry switch turn off the kettle automatically by opened the electrical circuit once the temperature is too high (or when the boiling point is reached).

Types that bimetallic thermometer

The longer the bimetal, the better the bend and thus the sensitivity because that temperature measurement. For this reason, a long bimetal piece is regularly wrapped right into a coil. Relying on whether the bimetallic coil is twisted choose a spiral or a screw, 2 different types can it is in distinguished.

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Figure: Bimetallic strip wrapped right into a coilAnimation: Bimetallic piece wrapped right into a coil

Spiral kind bimetallic thermometer

The simplest architecture of a bimetal thermometer is come wrap the bimetallic strip right into a spiral. The inner finish of the spiral is firmly associated to the housing. A tip is attached come the outer end of the spiral. The measure temperature deserve to then be read off a calibrated scale.

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Figure: Bimetal thermometer (spiral type)

Such a architecture using a bimetallic spiral is no only very space-saving but also cost-effective. However, the disadvantage is that the dial and also the temperature sensor space not separated from every other. The entire bimetal thermometer must thus be located directly in the medium whose temperature is to it is in measured. Together thermometers are used, for example, in refrigerators or freezers or to determine the room temperature.

Animation: operation principle the a spiral form bimetal thermometer

Helical type bimetallic thermometer

In many situations it is necessary to spatially separate the indicator (pointer) from the sensor (bimetallic coil). For example, if the water temperature is to it is in measured in a heater pipe, together is normal in heater systems. The temperature sensor should then be located inside the pipe, when the screen for the temperature need to be external the pipe. Or in the food sector it is additionally necessary to different the display screen from the sensor if, because that example, the temperature inside the food has to be measured (“piercing thermometer”).

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Figure: Bimetal thermometer (helical type)Figure: operation principle of a bimetal thermometer (helical type)

In this cases, bimetal thermometers space equipped through a bimetal strip wrapped right into a helical coil. The helical bimetal is firmly linked at one end to the inside of a measuring tube (the bimetal is attached come a cylindrical pin, i m sorry is pressed firmly right into the stem). A rotatable metal rod is guided with this helical coil, i beg your pardon is associated to it in ~ the loosened end. A tip is attached come the upper finish of the metal rod. If the measuring tube is now heated, the helical bimetal winds up and rotates the steel rod. Top top a calibrated range the equivalent temperature have the right to be review off.

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Figure: style of a bimetal thermometer (helical type)
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Figure: operation principle of a bimetal thermometer (helical type)

Such bimetal thermometers can additionally be equipped with switch contact which near an electric circuit once a particular temperature is gone beyond or undershot; an electrical signal is triggered accordingly. Switch contacts deserve to thus perform regulate tasks, e.g. As a thermostat because that underfloor heater systems, i beg your pardon switches off the heater pump for safety reasons when temperatures are too high and switches that on again once the temperature falls below a certain level.