A thermometer can be used for measuring temperatures in a certain range but when you want to measure very high temperature a thermometer is not good enough and that is when you think about using a thermocouple because with the help of a thermocouple you can easily measure very high temperature.
The mercury-based thermometer which you are using or even if you use an electric thermometer has a certain limitation when you want to measure very high temperature and that is why in most of the applications where you had to measure very high temperature you have to think about using a thermocouple.
Not many people are familiar with the concept of using a thermocouple to measure very high temperature and therefore you are to 1st look at the basic working principle of a thermocouple which relies on electricity and heat. This basic working principle would be explained below.
Conduction of electricity and heat by metals:
If you have ever noticed when we are speaking about a good conductor it can be a good conductor of two things that is good conductor of heat or good conductor of electricity and in most of the cases it has been seen that if a metal is a good conductor of either of them then the metal is also a good conductor of the other one and there is actually a relation between this.
Let us take the example that the metal is actually a good conductor of electricity in which case the electrons which are carrying the charge that is the electric current would be able to move through the metal from one space to another but these electrons would also be generating heat while moving through the metal.
That is why the metal would be a good conductor of heat as well and this basic working principle is used into the working of a thermocouple as well.
This is the effect which is used in the working of the thermocouple and it is named behind the scientist which invented this effect.
Whenever you heat a piece of metal like an you would notice that even if you are and is further away from the heating point you would still have to let go of the metal rod pretty quickly because the heat would be travelling through the rod and soon the entire rod would get heated but what you did not know was that the electricity would also be travelling through the rod. This effect was discovered by Seebeck.
Whenever two and is of the same metal part are held at different temperatures there is a electric current passing through them and there is a voltage difference which is created as well. This is what is used in a thermocouple to measure the temperature is which are very high to measure with the help of a thermometer.
Instead of using a single metal rod Seebeck actually combined to metals with each other by joining them at a junction and therefore these two metal strips actually formed electronic circuit which was not broken at any place and the experiment which are Seebeck did was to put one of the junctions at a very high temperature like in boiling water and another junction at a very low temperature like in a beaker containing ice. Due to the temperature difference at both the junctions and due to the fact that the two strips now acted like an electric circuit which was continuous and which was not broken at any place electric current started flowing through the assembly and the quantum of this electric current was directly proportional to the difference in the temperature between the two junctions and this helps us in knowing the temperature at either of the junctions.
Working of a Thermocouple:
If you read through the above paragraph carefully then it would be very easy for you to understand the working of the thermocouple but if you have not then we would request you to again go through the above paragraph because that is the founding principal of a thermocouple.
In order to calibrate a thermocouple known temperatures are capped at both of the junctions and then the current is measured to know the formula which is required for finding out the temperature when the temperature is unknown at any of the junctions. Once you have the formula ready for calculating the unknown temperature the task actually becomes much more easier for measuring the higher temperatures.
For actually measuring the temperature what you have to do is that you had to keep one junction of the thermocouple at a reference point and this can be something called like a beaker containing ice and that other junction of the thermocouple is subjected to the unknown temperature which is to be found out. Then the voltage difference between the two junctions is measured with the help of a voltmeter. This voltage is then put into the mathematical formula which we have calculated above while calibrating the thermocouple and since the temperature at one of the junctions is already known it becomes very easy to know the temperature at the other junction and this is how a thermocouple works.
In Greek, the word thermos stands for measuring heat whereas the word couple is self-explanatory as the junction is made from two different strips of metals. This is how it got the name of a thermocouple.
Applications of thermocouple:
As we mentioned above for temperatures which are beyond the range of a normal thermometer thermocouples are used in these are widely used in the industry because the entire process of measuring the temperature and calculating the temperature can be completely automated because in addition to the voltage difference between the two junctions there is also a current flowing between two junctions and therefore this current can be easily measured by the computer as well which would ensure that we are able to automate the entire process of measuring the temperature. Due to this factor it is widely used in the industries to measure temperature over a very wide range.
As we explained about thermocouple only consist of a measuring element and two metal strips and therefore it is very affordable as well and this has given further impetus to the use of thermocouple in various industries.
If you’re wondering what are the metals which can be used to make a thermocouple then some of them are iron, nickel, rhodium, aluminium and some of the alloys which provide good conduction of electricity are also used. The final choice of the metal always depends on the temperature range which is to be measured and in some of the cases depends on the conditions in the industry as well because some of the times it has been seen that non-magnetic metal strips might be required whereas some other times it has been seen that noncorrosive metal strips might be required and depending on the requirement from the industry that type of metal strips are chosen.