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3 JAN 2016
When you need to measure temperature over extended ranges, for instance a few hundred degrees C, thermocouples are often the preferred temperature sensor. They are rugged, inexpensive, and some varieties can measure temperatures over ranges exceeding 1000 C.
A thermocouple is constructed by connecting two wires of dis-
Type 'K' Thermocouple (One of the most common) & Terminator Block
Contrary to popular belief, the voltage developed by a thermocouple is not generated by the junction itself, but by the difference in temperature between the junction point, and the other end of the wire by which is connected to the measuring instrument. For this reason it is possible to make a thermocouple junction in a number of ways -
Thermocouples are based on the Seebeck effect -
Creating a temperature gradient along a conductor also creates a voltage gradient
Demonstrating the Seebeck effect with a single metal is difficult. For example, if you take a wire, heat it in the middle, and measure the voltage at each end, you will note the voltage to be zero, as the voltage drops along each end of the wire will equal each other and cancel out. To be able to measure the Seebeck voltage requires the use of two conductors, each with a different coefficient, as shown below. The amount of voltage you see at the ends of the wires is proportional to beoth the difference in temperature along their lengths, and the difference in the two materials' Seebeck coefficients.
A thermocouple's voltage is generated by differing Seebeck voltages developed along each of the differing wires -
The Seebeck coefficients for most materials, especially metals are typically not very large. The following table lists the coeffcients for a few materials. You can see that for the case of metals, pairing chromium with nickel gives the largest difference, which is only 26uV/K (+18uV/k -
Material |
Seebeck coefficient (uV/K) |
Chromium |
18.8 |
Gold |
1.79 |
Copper |
1.70 |
Aluminum |
- |
Platinum |
- |
Nickel |
- |
P- |
300 to 1000 |
N- |
- |
After Herwaarden & Meijer in [1] |
There are a wide range of thermocouple types available for different types of applications. The output voltage vs temperature curves for a few common types of thermocouples are shown in the graph below.
While more voltage (higher sensitivity) is a desirable feature, it is far from the only factor to be considered in selecting a thermocouple. Some other key factors that should be considered are:
Service temperature -
Linearity -
Mechanical -
Chemical -
Cost -
You may have noticed that all of the voltage vs. temperature curves in the chart above show 0mV output at 0 degrees C. One way of accomplishing this measurement would be to bring both the end-
'Ice-
One downside, however, of using an ice point reference is that stopping off at the local 7-
A Measurement of Termination Block Temperature can be used to compensate the thermocouple without an ice-
Since thermocouples are common sensors, you can buy integrated circuits (ICs) that implement the key signal processing functions. For example, the Analog Devices AD8495 provides an on-
References:
[2] Transducer Interfacing Handbook, Daniel H. Sheingold, 1981, Analog Devices, Norwood MA