Temperature Sensor - The Thermocouple

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        A thermocouple is a junction formed from two dissimilar metals.  Actually, it is a pair of junctions.  One at a reference temperature (like 0 oC) and the other junction at the temperature to be measured.  A temperature difference will cause a voltage to be developed that is temperature dependent.  (That voltage is caused by something called the Seebeck effect.)  Thermocouples are widely used for temperature measurement because they are inexpensive, rugged and reliable, and they can be used over a wide temperature range.  In particular, other temperature sensors (like thermistors and LM35 sensors) are useful around room temperature, but the thermocouple can


The Thermocouple
    Let us look at some other types of base-metal thermocouples.  Type T thermocouples are widely used as are type K and Type N.         A polynomial equation used to convert thermocouple voltage to temperature (oC) over a wide range of temperatures.  We can write the polynomial as:

The coefficients,  an are tabulated in many places.  Here are  the NBS polynomial coefficients for a type K thermocouple.  (Source: T. J. Quinn, Temperature , Academic Press Inc.,1990)
 
 

Type K
Polynomial Coefficients
 
an
0
0.226584602
1
24152.10900
2
67233.4248
3
2210340.682
4
-860963914.9
5
4.83506x1010
6
-1.18452x1012
7
1.38690x1013
8
-6.33708x1013


What If The Surrounding Temperature Exceeds Limits?

        There are really no thermocouples that can withstand oxidizing atmospheres for temperatures above the upper limit of the platinum-rhodium type thermocouples. We cannot, therefore, measure temperature in such high temperature conditions.

        Other options for measuring extremely high temperatures are radiation or the noise pyrometer. For non-oxidizing atmospheres, tungsten-rhenium based thermocouples shows good performance up to +2750 oC. They can be used, for a short period, in temperatures up to +3000 oC.

        The selection of the types of thermocouple used for low temperature sensing is primarily based on materials of a thermocouple. In addition, thermopower at low temperatue is rather low, so measurement of EMF will be proportionally small as well.


More Facts On Various Thermocouple Types
Links to Related Lessons
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