Temperature Measurements with Thermocouples: How-To Guide
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Overview
This document is part of the “How-To Guide for Most Common Measurements” centralized resource portal.
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Temperature and Thermocouple Overview
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Temperature is a measure of the average kinetic energy of the particles in a sample of matter, expressed in units of degrees on a standard scale. You can measure temperature in many different ways that vary in cost of equipment and accuracy. Thermocouples are one of the most common sensors used to measure temperature because they are relatively inexpensive yet accurate sensors that can operate over a wide range of temperatures. |
View a 60-second video on how to take a Thermocouple Measurement |
A thermocouple is created whenever two dissimilar metals touch and the contact point produces a small open-circuit voltage as a function of temperature. This thermoelectric voltage is known as the Seebeck voltage, named after Thomas Seebeck, who discovered it in 1821. The voltage is nonlinear with respect to temperature. However, for small changes in temperature, the voltage is approximately linear, or
(1)
Where ΔV is the change in voltage, S is the Seebeck coefficient, and dT is the change in temperature.
Several types of thermocouples are available, and different types are designated by capital letters that indicate their composition according to American National Standards Institute (ANSI) conventions. For example, a J-type thermocouple has one iron conductor and one constantan (a copper-nickel alloy) conductor. Other types of thermocouples include B, E, K, N, R, S, and T.
How to Make a Thermocouple Measurement
Background Theory
To better understand how to make a thermocouple measurement, you need to first understand how a thermocouple works. The first part of this section explains the basic theory of thermocouples, while the later part describes how to actually connect the thermocouple to an instrument and how to make a temperature measurement.
To measure a thermocouple Seebeck voltage, you cannot simply connect the thermocouple to a voltmeter or other measurement system because connecting the thermocouple wires to the measurement system creates additional thermoelectric circuits.
Figure 1. J-Type Thermocouple
Consider the circuit illustrated in Figure 1, in which a J-type thermocouple is in a candle flame, the temperature of which you want to measure. The two thermocouple wires are connected to the copper leads of a data acquisition device. Notice that the circuit contains three dissimilar metal junctions – J1, J2, and J3. J1, the thermocouple junction, generates a Seebeck voltage proportional to the temperature of the candle flame. J2 and J3 each has its own Seebeck coefficient and generates its own thermoelectric voltage proportional to the temperature at the data acquisition terminals. To determine the voltage contribution from J1, you need to know the temperatures of junctions J2 and J3 as well as the voltage-to-temperature relationships for these junctions. You can then subtract the contributions of the parasitic junctions at J2 and J3 from the measured voltage at junction J1.
Thermocouples require some form of temperature reference to compensate for these unwanted parasitic “cold” junctions. The most common method is to measure the temperature at the reference junction with a direct-reading temperature sensor and subtract the parasitic junction voltage contributions. This process is called cold-junction compensation. You can simplify computing cold-junction compensation by taking advantage of some thermocouple characteristics.
By using the Thermocouple Law of Intermediate Metals and making some simple assumptions, you can see that the voltage a data acquisition system measures depends only on the thermocouple type, thermocouple voltage, and cold-junction temperature. The measured voltage is independent of the composition of the measurement leads and the cold junctions, J2 and J3.
According to the Thermocouple Law of Intermediate Metals, illustrated in Figure 2, inserting any type of wire into a thermocouple circuit has no effect on the output as long as both ends of that wire are the same temperature, or isothermal.
Figure 2. Thermocouple Law of Intermediate Metals
Consider the circuit in Figure 3. This circuit is similar to the previously described circuit in Figure 1, but a short length of constantan wire has been inserted just before junction J3 and the junctions are assumed to be held at identical temperatures. Assuming that junctions J3 and J4 are the same temperature, the Thermocouple Law of Intermediate Metals indicates that the circuit in Figure 3 is electrically equivalent to the circuit in Figure 1. Consequently, any result taken from the circuit in Figure 3 also applies to the circuit illustrated in Figure 1.
Figure 3. Inserting an Extra Lead in the Isothermal Region
In Figure 3, junctions J2 and J4 are the same type (copper-constantan); because both are in the isothermal region, J2 and J4 are also the same temperature. Because of the direction of the current through the circuit, J4 contributes a positive Seebeck voltage and J2 contributes an equal but opposite negative voltage. Therefore, the effects of the junctions cancel each other, and the total contribution to the measured voltage is zero. Junctions J1 and J3 are both iron-constantan junctions, but they may be at different temperatures because they do not share an isothermal region. Because they are at different temperatures, junctions J1 and J3 both produce a Seebeck voltage but with different magnitudes. To compensate for the cold junction J3, its temperature is measured and the contributed voltage is subtracted out of the thermocouple measurement.
Using the notation VJx(Ty) to indicate the voltage generated by the junction Jx at temperature Ty, the general thermocouple problem is reduced to the following equation:
VMEAS = VJ1(TTC ) + VJ3(Tref ) (2)
where VMEAS is the voltage the data acquisition device measures, TTC is the temperature of the thermocouple at J1, and Tref is the temperature of the reference junction.
Notice that in Equation 2, VJx(Ty) is a voltage generated at temperature Ty with respect to some reference temperature. As long as both VJ1 and VJ3 are functions of temperature relative to the same reference temperature, Equation 2 is valid. As stated earlier, for example, NIST thermocouple reference tables are generated with the reference junction held at 0 °C.
Because junction J3 is the same type as J1 but contributes an opposite voltage, VJ3(Tref ) = -VJ1(Tref ). Because VJ1 is the voltage that the thermocouple type undergoing testing generates, you can rename this voltage VTC . Therefore, Equation 2 is rewritten as follows:
VMEAS = VTC (TTC ) - VTC (Tref ) (3)
Therefore, by measuring VMEAS and Tref , and knowing the voltage-to-temperature relationship of the thermocouple, you can determine the temperature at the hot junction of the thermocouple.
There are two techniques for implementing cold-junction compensation – hardware compensation and software compensation. Both techniques require that the temperature at the reference junction be sensed with a direct-reading sensor. A direct-reading sensor has an output that depends only on the temperature of the measurement point. Semiconductor sensors, thermistors, and RTDs are commonly used to measure the reference-junction temperature.
With hardware compensation, a variable voltage source is inserted into the circuit to cancel the parasitic thermoelectric voltages. The variable voltage source generates a compensation voltage according to the ambient temperature, and thus adds the correct voltage to cancel the unwanted thermoelectric signals. When these parasitic signals are canceled, the only signal a data acquisition system measures is the voltage from the thermocouple junction.
With hardware compensation, the temperature at the data acquisition system terminals is irrelevant because the parasitic thermocouple voltages have been canceled. The major disadvantage of hardware compensation is that each thermocouple type must have a separate compensation circuit that can add the correct compensation voltage; this fact makes the circuit fairly expensive. Hardware compensation is also generally less accurate than software compensation.
Alternatively, you can use software for cold-junction compensation. After a direct-reading sensor measures the reference-junction temperature, software can add the appropriate voltage value to the measured voltage to eliminate the parasitic thermocouple effects. Recall Equation 3, which states that the measured voltage, VMEAS, is equal to the difference between the voltages at the hot junction (thermocouple) and cold junction.
Thermocouple output voltages are highly nonlinear. The Seebeck coefficient can vary by a factor of three or more over the operating temperature range of some thermocouples. For this reason, you must either approximate the thermocouple voltage-versus-temperature curve using polynomials or use a look-up table.
Connecting a Thermocouple to an Instrument
For this section, consider an example using an NI cDAQ-9172 chassis and an NI 9211 C Series thermocouple module. Similar procedures apply for connecting a thermocouple to different instruments (see figure 4).
Required equipment includes the following:
- cDAQ-9172 eight-slot Hi-Speed USB chassis for NI CompactDAQ
- NI 9211 four-channel, 14 S/s, 24-bit, ±80 mV thermocouple input module
- J-type thermocouple
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Figure 4. NI CompactDAQ System with NI 9211 Thermocouple Module
The NI 9211 has a 10-terminal, detachable screw-terminal connector that provides connections for four thermocouple input channels. Each channel has a terminal to which you can connect the positive lead of the thermocouple, TC+, and a terminal to which you can connect the negative lead of the thermocouple, TC–. The NI 9211 also has a common terminal, COM, which is internally connected to the isolated ground reference of the module. Refer to Figure 5 for the terminal assignments for each channel and Figure 6 for a connection schematic.
Figure 5. Terminal Assignments
Figure 6. Connection Schematic
Getting to See Your Measurement: NI LabVIEW
Now that you have connected your thermocouple to the measurement device, you can use LabVIEW graphical programming software to transfer the data into the computer for visualization and analysis.
Figure 7 shows an example of displaying measured temperature data inside the LabVIEW programming environment.
[+] Enlarge Image
Figure 7. LabVIEW Front Panel Showing Temperature Data
Recommended Hardware and Software
Example Thermocouple Measurement System
NI CompactDAQ: 3-minute “out of the box” video
Take a Virtual Tour of NI CompactDAQ
Learn about and test-drive LabVIEW software for free
Thermocouple Webcasts, Tutorials, and Other How-To Resources
Taking Thermocouple Temperature Measurements
Performing High-Accuracy Temperature Measurements Using an NI Digital Multimeter and Switch
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