Thermocouples are probably the simplest, cheapest and most reliable temperature sensors available.
But like many pieces of great technology, their simplicity hides a mystery!
A thermocouple is made of two different kinds of wire, joined at one end and connected to a voltmeter at the other end.
When heated a thermocouple generates a voltage approximately proportional to the temperature difference between the junction of the two wires and the two loose ends of the wire.
This is really useful – and by using standard types of wire – humanity can measure temperatures in a simple way.
And the mystery? The mystery is that none of the voltage you measure is generated at the tip of the thermocouple!
How a thermocouple works
In 1821 Thomas Johannes Seebeck discovered that accompanying every temperature difference in a metal, a small voltage was generated: a ‘thermo-voltage’.
We now know that it is caused by the differing extent to which the electrons in the metal are disturbed at different temperatures.
Seebeck noted that the voltage generated for a given temperature difference depended on the type of metal.
So a copper wire stretched between two temperatures generated one voltage, (V1), but a nickel wire stretched between the same two temperatures generated a different voltage (V2).
In a long wire, a voltage is created across the length of the wire, and one can work out the total voltage measured by adding together all the small voltages (ΔV) due to all the small temperature changes (ΔT).
Interestingly – and this is at the heart of the mystery – because the ΔVs are only generated by ΔTs – it doesn’t matter how long the wire is, or which route it takes!
The thermo-voltage is proportional to the overall temperature difference between the two ends of the wire.
Joining two types of wire together
A ‘thermo-couple’ is made by joining two dissimilar wires together. Because the two wires are different, the voltages V1 and V2 generated by each ‘leg’ of the pair don’t cancel, and there is a net voltage (V1 – V2) characteristic of the two types of wire, and the temperature difference from one end to the junction.
So if you know the temperature of your voltmeter, then you can work out the temperature of the tip of the thermocouple by measuring the thermo-voltage.
The ‘thermo-voltage’ is usually tiny, typically only 40 microvolts per 1 °C of temperature difference, but that’s enough to make a measurement with an uncertainty of about 1 °C in many circumstances.
From the explanation above it should be clear that the ΔVs are generated along the entire length of the wire – but no voltage is generated at the junction!
If one puts a thermocouple in a furnace – then the ‘thermo-voltage’ corresponds to the temperature at the tip of the thermocouple. But all the delta ΔVs are generated as the thermcoouple goes through the wall of furnace!
If one pulls the thermocouple through the wall, then a different piece of wire generates the voltage.
So in order to get reproducible results it is important that the composition of the wire is uniform along its length. This is one of the major problems in the making thermocouples and being confident they are reading correctly..
A thermocouple thermometer
A thermocouple thermometer is actually two thermometers in one!
- First the device has a thermometer inside – usually an electrical resistance thermometer called a thermistor – that records the temperature of the electrical terminals.
- Secondly the device has a sensitive voltmeter that records the ‘thermo-voltage’. Based on the type of wires from which the thermocouple is made, the device works out how much hotter or colder the tip of the thermocouple is than the electrical terminals.
Combining the results of the two temperature measurements together gives the temperature of the tip of the thermocouple
Interesting places to stick a thermocouple
Because thermocouples are small and tough and light, you can stick them in places that you can’t easily stick other thermometers. You might like to try these experiments:
- Let some ice warm up to 0 °C – and then press it down on some salt with a thermocouple trapped underneath. The temperature will fall to roughly -16 °C – really cold!
- Try putting just the tip of the thermocouple in a candle flame. You should get an answer close to 1000 °C!
- Try working out just how hot a cup of tea is when its just right – for me it’s close to 60 °C.
It’s hard not to love a scientific instrument that can do all that!