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K-Type Thermocouples reviewed

2017-12-23 21:47  
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A thermocouple is a sensor that generates an electrical potential related to the temperature. The operating principle of the sensor is base on the fact that any electrical junction between two different metals generates an electrical potential that depends on the temperature and the metals that are 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 can thermocouple withstand is 1000?°C without any damage.

How ever thermocouples have a drawback. It is because connecting thermocouple to circuit creates addition junctions between different metals who generate additional potentials. Generally speaking measuring thermocouple potential output is measuring not direct function of absolute temperature, but a difference between temperature at measurement point and temperature at the connection point.

K-Type Thermocouples

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

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

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 in boiling water, indicator should show 100°C.


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