Thermocouples – Thermo Electric Effects

The most common method of temperature measurement uses the thermocouples. It is based on seeback effect. The thermocouple temperature measurement is based on creation of an electromotive force (e.m.f).

Basic Principle :

The basic principle of thermocouple is when two dissimilar metals are joined together an e.m.f will exist between the two points A and B, which is primarily a function of the junction temperature. The above said to be principle is Seeback effect.

Construction :

The thermocouple consists of one hot junction and one cold junction. The hot junction will be introduced into the place where temperature is to be measured. The other cold junction is maintained at constant reference temperature. Also one voltage measuring instrument is connected to the free ends of the thermocouple.

Operation :

In thermocouple, the known temperature is called reference temperature. The temperature which is to be measured is introduced in the thermocouple’s hot junction.

A common arrangement for establishing the reference temperature is ice bath. The reference temperature is controlled to be at a constant temperature of 0°C.

The temperature difference between the ice bath and the hot junction temperature will create an e.m.f. This e.m.f production is measured in the millivoltmeter. So, this change in e.m.f rate will give the measure of temperature. The output voltage is equal to, (E)=A T+\frac{1}{2} B T^{2}+\frac{1}{3} C T^{3}

Where T is the temperature in °C and A, and B are constant depending upon the thermocouple material.

1. Law of intermediate metals.
2. Law of intermediate temperature.

Law of Intermediate Metals

If a third metal is introduced into a thermocouple circuit, as shown in fig, the net e.m.f of the circuit is not affected as long as the new connections are at the same temperature.

Thermopile

The thermocouples connected in series to measure the temperature are called thermopile. The thermopile arrangement is useful for obtaining a substantial e.m.f for measurement of a small temperature difference between the two junctions. When thermocouples are connected in series, it is very important to ensure that the junctions are electrically insulated from one another. This thermopile arrangement is shown in fig.

The four junctions are all maintained at different temperatures connected in series. Assume that junction ‘A’ will produce a potential drop in the plus to minus direction. Junctions ‘B’ and ‘D’ will produce a potential drop in the opposite direction.

The junction ‘C’ will generate a potential drop in the same direction as junction A. So, the e.m.f is measured at the potentiometer, (E)=e_{A}+e_{B}+e_{C}-e_{D}

The value of E is zero when all the junctions are at the same temperature.

Parallel connection of thermocouples :

The fig shows the parallel connection thermocouple and it is used for obtaining the average temperature of a number of points.

Each of four junctions may be at different temperatures and will generate a different e.m.f. The average temperature of the junctions is calculated in the potentiometer.

The total e.m.f in parallel connection thermocouple is, (E)=\frac{E_{1}+E_{2}+E_{3}+E_{4}}{4}=\frac{E}{n}

Where ‘n’ is the number of junction pairs and ‘E’ is the total reading of the thermopile.

Advantages of thermocouple temperature measurement:

1. It can measure the temperature at the last point of electric contract of the dissimilar materials.
2. The junction size is very small. Theretore, transient response will be high.
3. Its application is ranging from measurement of room-air temperature to that of a metal bath.
4. Simplicity and low cost.

Disadvantages:

1. Due to faulty soldering in junctions, the thermocouple does not conform to the standard e.m.f temperature tables.
2. Thermocouples may be used outside their applicable range.
3. Junction compensation may be required in the reference junction.
4. There may chance of using wrong type thermocouple for the temperature measurement.
5. Installation faults may occur.