The pros and cons of thermocouples

 Thermocouples were perhaps the first industrial temperature sensor enabling engineers to fully close a temperature control loop. Its price has been reduced through the years and the market has produced different models for almost all applications.

How does a thermocouple works?

It is based on the voltage generated at the joint of two wires made of dissimilar alloys, this is: of different alloys. The voltage is generated by a change in temperature. Then, if you are able to measured the voltage, the temperature can be given as a function of the voltage.

Of course, you need to know the properties of the two wires in order to use the Seebeck voltage equation:

$\Delta eAB = \alpha \Delta T$        Eq. (01)

where $\Delta eAB$ is a change in the Seebeck voltage caused by a change in the temperature $\Delta T$ and $\alpha$ is the Seebeck coeffficient. 

Fig. 01 Here is a sketch showing a J-type thermocouple, its junction point, at which both ends are welded, and a multimeter to read the voltage.

On problem with thermocouples, when reading voltage on your own, is that the values are very small so that a high precision multimeter may be required. Once having a voltage reading you may use available tables to estimate the temperature.

The above works very well for a lab experiment but for industrial purposes model providing different functions other than just temperature are available.

Fig. 02 Hand held and thermocouple probe. In this case the sensor is a K-type thermocouple.

All thermocouples have different colours and the + and - wire are coloured as well so that you may correctly install it.

Fig. 04 Here is a thermocouple wire color chart taken from OMEGA. You can take this color code as a standard since all manufacturers use the very same code.

Some advantages of thermocouples

• Training is not needed (little, in fact) since reading is directly given in the screen
• It can be portableThere are handheld versions
• It is cheap. Although there are models that could be expensive manufactured by request
• Different models/designs are available in the market
• 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)
• Some models/designs may offer data loggers (which would increase its cost)
• It can be installed downwards or in any other position
• It can be part of a closed loop
• Some models/designs may offer wireless connection for remote data transfer or control over large distances
• Its screen make it easy to read the digits. Numbers have large format/presentation
• Number of decimals can be selected is desired (high precison)
• It can be installed in places with high vibration, in dusty environments or with high humifity or exposed to other elements (special case/protection can be added)
• Smaller number of mechanical parts so that effects of vibration is low
• Rapid response (in the order of ms)
• It can be installed in places of low or zero illumination

Some disadvantages of bimetallic thermometers

• 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 cannot be bought in small supply stores

• These can be used as test/reference instrument when compared with other technologies

• Lost of energy can be a trouble

Any question? Write in the comments and I shall try to help.

Other stuff of interest

• LE01 - AC and DC voltage measurement and continuity test
• The pros and cons of bulb thermometers
• The pros and cons of bimetallic 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

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Ildebrando.