Bimetallic thermometers are perhaps, after bulb thermometers, the subsequent advancement in temperature measurement for industry. It has survived to these days due to its mechanical robustness.
How does a bimetallic thermometer works?
This is not as easy as for the bulb thermometers. Despite its simple working principle there are several designs avalillble in industry so that the physics may be a little tricky.
Their working depend on the thermal dilatation/expansion coefficient of a two-different metals sheet (strip). A temperature increment produces a deformation on the bimetallic sheet. If you are able to relate the strip deformation with the temperature deformation then you can draw a scale for temperature.
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| Fig. 01 The relationship between temperature and thermal expansion in a bimetallic sheet |
Figure 01 shows how the strip deforms as temperarure increases. The bimetallic thermometer is then based on this behavior.
Then, how do you build a bimetallic thermometer based on the presented physical mechanism? A thermometer of this kind would need to embed the bimetallic strip into a small space suitable for different applications. Therefore, a common design consists of a helix made of this strip.
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| Fig. 02 Parts of a bimetallic thermometer |
In Fig. 02 the components of a bimetallic thermometers are presented.
Some advantages of bimetallic thermometers
Training is not needed (little, in fact)
It does not need electric energy in order to work
No batteries needed
It can be portable
There are handheld versions
It is cheap
Different models/designs are available in the market
Coloured gauges may be available
There are impact resistant models so that it is not easily broken (because of its metal casing)
Some models/designs may offer thermal switches (which would increase its cost)
It can be installed downwards or in any other position
Some disadvantages of bimetallic thermometers
Hard to read
Numbers can be too small
Precision is not trustable due to the scale (human intervention)
It needs to be in contact with the fluid/object to be measured
It should be installed in a place with very low or zero vibrations (because of its mechanical assembling)
Data can not be transferred from the instrument to a controller or another device
Not suitable for closed loops
It shall be installed in well illuminated places
Slow time response
Further calibration is required due to loose in its mechanical assembling
Some interesting stuff on bimetallic thermometers
Commonly used for lab, industry or home applications
Different alloys can be used to extend/change the range of measurement
These can be bought even in small supply stores
These can be used as test/reference instrument when compared with other technologies
Poor eyesight can be a trouble
Other stuff of interest
- LE01 - AC and DC voltage measurement and continuity test
- The pros and cons of bulb thermometers
- Some examples of temperature instruments
- Minor losses - Formulas
- What is a process variable?
- What are the most important process variables?
- Time dependence of process variables
- A list of process variables
Ildebrando.
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