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
• Why Use thermocouples To Measure Temperature?
• They are inexpensive.
• They are rugged and reliable.
• They can be used over a wide temperature range.
• What Does A Thermocouple Look Like?
• Here it is.  Note the two wires (of two different metals) joined in the junction.
• What does a thermocouple do?  How does it work?
• The junction of two dissimilar metals produces a temperature dependent voltage.
• How Do You Use A Thermocouple?
• You measure the voltage the thermocouple produces, and convert that voltage to a temperature reading.
• It may be best to do the conversion digitally because the conversion can be fairly nonlinear.
• Things You Need To Know About Thermocouples
• A junction between two dissimilar metals produces a voltage.
• In the thermocouple, the sensing junction - produces a voltage that depends upon temperature.
• Where the thermocouple connects to instrumentation - copper wires? - you have two more junctions and they also produce a temperature dependent voltage.  Those junctions are shown inside the yellow oval.
• When you use a thermocouple, you need to ensure that the connections are at some standard temperature, or you need to use an electronically compensated system that takes those voltages into account.  If your thermocouple is connected to a data acquisition system, then chances are good that you have an electronically compensated system.
• Once we obtain a reading from a voltmeter, the measured voltage has to be converted to temperature.  The temperature is usually expressed as a polynomial function of the measured voltage.  Sometimes it is possible to get a decent linear approximation over a limited temperature range.
• There are two ways to convert the measured voltage to a temperature reading.
• Measure the voltage and let the operator do the calculations.
• Use the measured voltage as an input to a conversion circuit - either analog or digital.
Let us look at some other types of base-metal thermocouples.  Type T thermocouples are widely used as are type K and Type N.
• Type K (Ni-Cr/Ni-Al) thermocouples are also widely used in the industry. It has high thermopower and good resistance to oxidation. The operating temperature range of a Type K thermocouple is from -269 oC to +1260 oC. However, this thermocouple performs rather poorly in reducing atmospheres.
• Type T (Cu/Cu-Ni) thermocouples can be used in oxidizing of inert atmospheres over the temperature range of -250 oC to +850 oC. In reducing or mildly oxidizing environments, it is possible to use the thermocouple up to nearly +1000 oC.
• Type N (Nicrosil/Nisil) thermocouples are designed to be used in industrial environments of temperatures up to +1200 oC.
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 n 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
• A variety of thermocouples today cover a range of temperature from -250 oC to +3000 oC. The different types of thermocouple are given letter designations: B, E, J, K, R, S, T and N
• Types R,S and B are noble metal thermocouples that are used to measure high temperature. Within their temperature range, they can operate for a longer period of time under an oxidizing environment.
• Type S and type R thermocouples are made up of platinum (Pt) and rhodium (Rh) mixed in different ratios. A specific Pt/Rh ratio is used because it leads to more stable and reproducible measurements. Types S and R have an upper temperature limit of +1200 oC in oxidizing atmospheres, assuming a wire diameter of 0.5mm.
• Type S and type R thermocouples are made up of platinum (Pt) and rhodium (Rh) mixed in different ratios. A specific Pt/Rh ratio is used because it leads to more stable and reproducible measurements. Types S and R have an upper temperature limit of +1200 oC in oxidizing atmospheres, assuming a wire diameter of 0.5mm.
• Type B thermocouples have a different Pt/Rh ratio than Type S and R. It has an upper temperature limit of +1750 oC in oxidizing atmospheres. Due to an increased amount of rhodium content, type B thermocouples are no quite so stable as either the Type R or Type S.
• Types E, J, K, T, and N are base-metal thermocouples that are used for sensing lower temperatures. They cannot be used for sensing high temperatures because of their relatively low melting point and slower failure due to oxidation.
• Type B thermocouples have a different Pt/Rh ratio than Type S and R. It has an upper temperature limit of +1750  oC in oxidizing atmospheres. Due to an increased amount of rhodium content, type B thermocouples are no quite so stable as either the Type R or Type S. we will look into some differences between different base-metal thermocouples.
• Type E (Ni-Cr/Cu-Ni)  thermocouples  have  an  operating  temperature  range from -250  oC to +800 oC. Their use is less widespread than other base-metal thermocouples due to its low operating temperature. However, measurements made by a Type E have a smaller margin of error. 1000 hours of operation in air of a Type E thermocouple at +760  oC, having 3mm wires, shold not lead to a change in EMF equivalent to more than +1 oC.
• Type J (Fe/Cu-Ni) thermocouples are widely used in industry due to their high thermopower and low cost. This type of thermocouple has an operating temperature range from 0 oC to +760  oC.