Question

define thermocouples

Answer 1

Thermocouples are one type of electrical device which is made of two electrical conductors of dissimilar electric conductivity. These two materials are joined together which forms electric junction which varies with temperatures. When this junction experience a change in temperature, a voltage is produced due to the thermoelectric effect. By measuring this voltage we can get the exact value of the junction. That's why thermocouples are widely used as Temperature sensor.

To know accurately how thermocouples are working, we need to know about the Seebeck Effect.   German physicist Thomas Seebeck discovered that when two metals of different electric conductivity are joined and if there is a temperature difference between the joints, a magnetic field is produced due to thermoelectric current and these current is produced due to an electromotive force. Under open circuit condition, this electromotive force is directly proportional to the temperature gradient ().

If the electromotive force is then

Now this S is a Temperature dependent constant which differs from metal to metal called Seebeck coefficient.

So to measure a particular temperature we need a reference temperature which is known. Then this temperature gradient will produce a voltage. By measuring this voltage we can determine the unknown temperature. This is the basic principle of thermocouples.

There are different types of thermocouples:

• Nickel-alloy thermocouples
• Tungsten/rhenium-alloy thermocouples
• Platinum/rhodium-alloy thermocouples
• Platinum/molybdenum-alloy thermocouples
• Chromel–gold/iron-alloy thermocouples
• noble-metal alloy thermocouples
• Iridium/rhodium alloy thermocouples

They also have subcategories. But I think that discussion isn't relevant for this.