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K-Type Thermocouples reviewed

A thermocouple is a sensor that generates an electrical potential related to the temperature. The sensor’s operating principle is based on the fact that any electrical junction between two different metals generates an electrical potential that depends on the temperature and the metals used. The principle applies equally well if three metals are used. In that case, there are two junctions in series and the net potential results from the series addition of the two individual potentials. For example, if a copper iron junction is in series with an iron-tin junction, the net potential is the same as for a copper-tin junction. However, that is only true if both junctions are at the same temperature.

The K-Type thermocouple is usually made of Chromel (+) and Alumel(-). The voltage generated from this sensor is 4mV/100°C. The max temperature that can thermocouple withstand is 1000°C without any damage.

However, thermocouples have a drawback. It is because connecting thermocouple to circuit creates additional junctions between different metals that generate additional potentials. Generally speaking, measuring thermocouple potential output does not measure absolute temperature but a difference between the temperature at the measurement point and temperature at the connection point.

If we look at the circuit picture, the cold joint is usually located on the circuit board. If we can measure temperature on the circuit board, then it is simple to calculate the temperature at the measurement point. Onboard temperature can be measured with any temperature sensor like AD7416. We can assume that the temperature inside the chip is the same as on the board.

To make measurements more accurate, try to solder thermocouple joints as close to the onboard temperature sensor as possible. And of course, if you want to extend the thermocouple, always use Chromel wire on the positive side and Alumel on the negative. If you use regular copper wires, there will be additional junctions and additional potentials that create measurement errors. Of course, other types of thermocouples generate different potentials.

The easy method to test thermocouple is to use water. If you put it in ice water, you should see 0°C, and if you put it in boiling water, the indicator should show 100°C.

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