While we talk about temperature measurement sensors then RTD and Thermocouple come first in our minds. The thermocouple is the widely used temperature sensor in the industries.
In this blog, we will see, What is a thermocouple? How do thermocouple work? Types of the thermocouple, basic thermocouple function, etc.
So without wasting much time, let’s start this blog.
What is a Thermocouple? How do they work?
The thermocouple is defined by Wikipedia,
A thermocouple is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature.
In simple words, I can say that,
The thermocouple is a device that is used to measure the temperature of a specific point. The thermocouple consists of two dissimilar metals connected together to create two junctions, the one junction is connected to the body whose temperature is to be measured and called hot or measuring junction. The other junction is connected to the body whose temperature is known and called cold or reference junction.
Working Principle of the Thermocouple:
The working principle of the thermocouple mainly depends on these three effects: See back, Peltier and Thompson.
See beck effect:
See beck effect states that when two dissimilar metals are joined together at two junctions and when any of one metal is heated up, electrons start flowing from hot metal to cold metal.
The Peltier effect is opposite to the see beck effect and stats that when we provide potential difference among any two dissimilar metal can force the temperature difference between them.
The Thomson effect states that when two dissimilar metals are joined together then there must be a potential difference exist within the circuit due to the temperature gradient along the entire length of the conductor.
How does Thermocouple work?
A thermocouple is mainly built with two different metals. Generally, Iron and Constantan are used. By connecting these metals it makes two junctions, one is connected with a voltmeter or called cold junction and the other is connected to whom we want to know the temperature and called hot junction.
As shown in the above figure if the temperature at two junctions is the same then there is the same emf but in reverse polarity there and no current is flowing through the circuit.
But when the temperature at both junctions are different then emf is induced in the circuit and there is a current flow through the circuit. The total current or induced emf through the circuit is measuring through the measuring device.
The device for measuring current or emf is connected within the circuit of the thermocouple. The amount of emf developed within the circuit is very small, generally, in millivolts, that’s why we have to use a very accurate instrument to measure the amount of emf. Generally, an ordinary galvanometer or voltage balancing potentiometer is used.
Types of Thermocouple:
In this type of thermocouple, a thermocouple is entirely isolated from the probe wall. The major advantage of this type of thermocouple is that it eliminates ground loop problems as well as the life of the thermocouple also increases due to the reduced stress effect. The response to this type of thermocouple is a little slower.
The conductive metals and probe walls are directly connected in this type of thermocouple. The advantage of fast response time due to very good heat transfer.
In this type of thermocouple, conductive metals are exposed to the target medium. They provide the best heat transfer and very quick response. It is not suitable for corrosive target media and where quick response time is a necessity then it will have the first priority. It is suitable for gas temperature measurement.
Thermocouples are available for different metal combinations. Each different combination has a different temperature range. The most common is the “Base Metal” thermocouple known as K, J, T, E, and N types. The high-temperature thermocouple combination is S, R, C, and GB Type.
Common thermocouple types and temperature range:
How do I choose the thermocouple?
Here are some factors that should be considered while selecting the thermocouple.
Operating temperature is the full ambient temperature range over which it gives the linear output. This range will decide which metal combination of thermocouple you require for your application. The higher the temperature range will add extra cost to your budget.
There are three types of thermocouples: Exposed, Grounded, and Ungrounded. An exposed thermocouple gives you the fastest response time but if the probe will be exposed to corrosive gas or pressure then an exposed thermocouple should not be used.
An ungrounded thermocouple gives you a slow response time but still, it will be the best choice if your application is desirable to have a thermocouple electronically isolated and shielded by the sheath.
Accuracy is very important for any instrument. Accuracy is the indication of how close the measured value to the actual temperature value. This is also referred to as error or tolerance.
Chemical, Abrasion, or Vibration Resistance
The thermocouple sheath material should be chemically resistant when using in a corrosive environment.
Advantages and Disadvantages of Thermocouple:
Advantages of Thermocouple:
- low cost as compared to other temperature devices
- larger temperature range
- fast response time
- robust and can be useful in harsh environment condition
- No self-heating effect
Disadvantages of Thermocouple:
- low accuracy
- re-calibration is difficult
So this is it, I hope you like this blog. You get all the details about What is a thermocouple? How does a thermocouple work? Basic thermocouple function, What are the advantages of a thermocouple? How to choose the best thermocouple for your application? ,etc.
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