  13 April 2010
Author: Giorgos Lazaridis
How Thermocouples (TCs) Work

History A very simple thermocouple wire. One wire is made of Chromium and the other of Aluminum. Both are Nickel alloys.

In 1821 Thomas Johann Seebeck (1770-1831) noticed that, if two wires from different material are connected at their endings performing two different junctions, and these junctions do not have the same temperature, then placing these wires near by a compass, would cause the compass to move. At first he believed that the temperature is the reason to change the magnetic field. Later on, he understood that a current was flowing within the wires that generated a magnetic field and moved the compass. That called the Seebeck Effect

The Seebeck Effect is one of the three thermoelectric phenomena and the first to be discovered. Along with the Peltier effect and the Thomson effect, it explains how the temperature difference can create a voltage.

How Thermocouple works - A simple example

I have also a page with thermocouple experiments. Visit this link: Experimenting with Thermocouples.

The Seebeck effect is present whenever two dissimilar metals -of any material- performs a junction. Nevertheless, there are some metal pairs that have a predictable voltage according to temperature, and have also larger temperature gradients. These pairs are named with a letter, as for example the type-E thermocouple. The most popular type of thermocouple is the type-K. This type is made of Nickel-Chromium versus Nickel-Aluminum wires and has a very wide range of temperature gradients (from -200oC up to 1300oC) where the voltage changes almost linear.

The following drawing demonstrated a thermocouple. The left junction is heated. The right junction is kept at room temperature. Thus, a current flows within the wires: As someone would expect, if the right junction does not exists, a voltage difference shall be created across the wire endings: The three Thermo-Electric Laws

There are three empirical Thermo-Electric Laws to explain the operation of the thermocouple:

1. The Law of Homogeneous Circuits
An electric current cannot be sustained in a circuit of a single homogeneous metal

Look at the two following images. The two circuits are identical and the temperature in the junction is equal. Both will generate a voltage difference. On the right circuit, a second heater is placed under one of the two thermocouple wires and heat it up. The first law states that, because the second heater heats ONLY one wire and NOT the junction, the output voltage of both circuit will be the same. The output voltage is only affected by the junction temperature and not the temperature of the wires. That is because, any temperature change to a homogeneous wire will create no voltage.  2. The Law of Intermediate Metals
If two dissimilar metals performs a thermocouple, and a third dissimilar metal is introduced to the circuit, as long as the temperature along the entire length of the third metal is kept uniform, the output voltage will NOT be affected

Look at the following circuit. The blue wire of the thermocouple is joined with a third dissimilar metal. If the temperature of this wire across it's entire length is the same, then the output voltage will NOT be affected by this insert. 3. The Law of Intermediate Temperatures
If a thermocouple with 2 junctions with temperatures T1 and T2 produces a voltage difference V1, and voltage difference of V2 in temperatures T2 and T3, then voltage generated when the temperatures are T1 and T3 will be V1+V2.

This law is very important to understand how to acquire the temperature from a thermocouple. Usually, the temperature to voltage characteristic of a thermocouple (as you will see later on) is given for 0oC (32oF) reference temperature. Look at the following 2 circuits. The thermocouple is a K-type thermocouple, and the voltmeter uses copper wires. At the point that the voltmeter is connected to measure the voltage, two new thermocouple junctions are created! This is a major problem of thermocouples and will be discussed in details later on. At the left circuit, these new junctions are kept into 0oC (32oF). This way, the measured voltage can be directly converted into temperature, as the table to convert it is usually with reference temperature 0oC. But in real life, the reference temperature is not 0oC. Looking at the right circuit you can see a more realistic example. The reference temperature now it 20oC. The voltage cannot be directly converted into temperature as the reference junction (where the voltmeter is connected) is not at 0oc. According to the 3rd law, if we know the reference temperature, then we can calculate the measured temperature, by adding the measured voltage and another voltage named VREF. This VREF is the voltage that would be created by the thermocouple, if the non-zero reference voltage was measured by a thermocouple junction with reference temperature 0oC. I know it is complicated. Frankly, it is not!  An example: For the left circuit, suppose that T1 = 0oc (first reference temperature). According to the measured voltage, we can directly convert to temperature for T2. In our example, for a K-type thermocouple, the measured voltage would be 4.095mV, that corresponds to 100oC. But a measuring instrument is rarely placed in 0oC. On the second circuit (right side), the measuring instrument is placed in a room with 20oC. Although the temperature T2 is still 100oC, the measured voltage will be now 3.297 mV! Why? Because the junctions where the voltmeter is connected is NOT in 0oC, thus they introduce another opposite current to the circuit!

To calculate the T2, we need to compensate T3! According to the 3rd law, we need to add to the measured voltage, the voltage that would be generated if T2 was T3 and T3 was 0oC. In this case, the voltage can be directly found from the table (as we are talking about 0oC reference temperature) and would be 0.798 mV. So, we add this value to the measured voltage and the total voltage is 0.798+3.297 = 4.095 mV, and this corresponds to 100oC!

The Problem of measuring temperature with thermocouples

Any junction between two dissimilar metals, will perform a thermocouple! That is where the whole problem begins. Suppose that we have a K-type thermocouple. We will need to connect both wires of the pair to copper wires, either because we want to extend the wires, or because we want to connect the multimeter. These connections will create another thermocouple pair. The following drawing demonstrate this circuit: The red and blue wires comes from the K-type thermocouple. The orange wires are copper wires. Thus, due to the first law, T4 and T5 will not create any difference as the wires are always copper. So, we can re-write the circuit as follows: The problem is that, at the points where the thermocouple wires are connected with the copper wires, two new thermocouples are created! Thus, these thermocouples will create EMF (Electro Motive Forces) to the circuit and alter the readings.

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