Introduction to Thermocouple Measurement Methods

Temperature and Thermocouple Overview

Temperature is a measure of the average kinetic energy of particles in a sample of an object, and its standard unit is "degrees". Temperature can be measured in different ways, with different costs and accuracy. Thermocouples are one of the most common sensors for measuring temperature because they are relatively cheap, accurate, and have a relatively wide measurement range.

Whenever two dissimilar metals come into contact, the contact point will produce a low no-load voltage that is a function of temperature. This is the thermoelectric effect. This temperature difference voltage is the Seebeck voltage, named after the physicist Thomas Seebeck who discovered the phenomenon in 1821. This voltage is nonlinear with respect to temperature, but for a small range of temperature changes, it can be approximated as linear, or:

(1)

Where ∆V is the voltage change, S is the Seebeck coefficient, and ∆T is the temperature change.

There are many types of thermocouples, all of which are identified by capital letters according to the American National Standards Institute (ANSI) convention. For example, a J-type thermocouple consists of an iron conductor and a copper-nickel alloy conductor. Other types of thermocouples include B, E, K, N, R, S, and T.

How to Measure Thermocouples

Background

In order to better understand how to perform thermocouple measurements, you must first understand the working principle of thermocouples. The first part of this document will explain the basic principles of thermocouples, and the subsequent parts will explain how to connect thermocouples to instruments and how to perform temperature measurements.

Thermocouple Seebeck voltage will generate an additional temperature difference circuit if connected directly to the measurement system, so it cannot be measured by simply connecting it to a voltmeter or other measurement system.

Figure 1. J-type thermocouple

As shown in Figure 1, the circuit uses a J-type thermocouple to measure the temperature of the candle flame. The two thermocouple lines are connected to the copper terminals of the data acquisition device. Note that there are three metal connections in the circuit - J1, J2 and J3. J1 is the thermocouple measurement point, which produces a Seebeck voltage proportional to the candle flame temperature. In addition, J2 and J3 each have their own Seebeck coefficients and produce a temperature difference voltage proportional to the temperature at the data acquisition terminal, called the cold junction voltage. In order to determine the voltage component of J1, it is necessary to know the temperature of the J2 and J3 junctions and the relationship between the junction voltage and temperature. In this way, the voltage of the J1 junction can be obtained by subtracting the parasitic junction voltage components of J2 and J3 from the measured voltage.

Thermocouples require a specific temperature reference to compensate for the error caused by the cold junction. The most commonly used method is to use a directly readable temperature sensor to measure the reference junction temperature and subtract the parasitic junction voltage component. This process is called cold junction compensation and can use the characteristics of certain thermocouples to simplify the calculation of cold junction compensation.

By using the thermocouple law of the metal transition layer and other assumptions, we know that the voltage data acquisition system measurement depends only on the thermocouple type, the voltage at the measuring end and the temperature at the cold end. The measured voltage is independent of the voltage components of the measuring wire and the cold ends J2 and J3.

Figure 2. Metal transition layer thermocouple law

Consider the circuit in Figure 3. This circuit is similar to the circuit described in Figure 1 above, but a short length of copper-nickel alloy wire is inserted before the J3 junction. All junctions are at the same temperature. Assuming that the J3 and J4 junctions are at the same temperature, the metal transition layer thermocouple law states that the circuit in Figure 3 is electrically the same as the circuit in Figure 1. Therefore, any results measured with the circuit in Figure 3 are applicable to the circuit in Figure 1.

Figure 3. Inserting an additional wire in an isothermal environment [page]

In Figure 3, the J2 and J4 junctions are of the same type (copper-nickel alloy); because they are in isothermal environments, J2 and J4 are also at the same temperature. Because of the direction of current in the circuit, a positive Seebeck voltage is generated at J4 and a negative Seebeck voltage is generated at J2. Therefore, the junctions cancel each other out and the total amount of voltage measured is zero. The J1 and J3 junctions are both iron-copper-nickel alloy junctions. However, their temperatures may be different because they may not be in isothermal environments. Because they are in different temperature environments, both J1 and J3 junctions generate Seebeck voltages, but of different magnitudes. To compensate for the cold junction J3, its temperature is measured and the voltage applied to it is subtracted from the thermocouple measurement.

The symbol VJx(Ty) is used to represent the voltage generated at the Jx junction at the Ty temperature. The general thermocouple problem is simplified to the following formula:

VMEAS = VJ1(TTC ) + VJ3(Tref ) (2)

Where VMEAS is the voltage value measured by the data acquisition system, TTC is the temperature of the thermocouple at the J1 junction, and Tref is the temperature of the reference end.

Note that in equation (2), VJx(Ty) represents the voltage generated in the Ty temperature environment with respect to a certain reference temperature. As long as VJ1 and VJ3 are temperature functions related to the same reference temperature, equation 2 holds true. For example, the NIST thermocouple reference table mentioned above was generated with the reference end kept at 0 degrees Celsius.

Because J3 and J1 are of the same type, but generate relative voltages, VJ3(Tref) = -VJ1(Tref). And because VJ1 is the voltage generated in the thermocouple type test state, this voltage can be renamed VTC. Therefore, Equation 2 can be rewritten as follows:

VMEAS = VTC (TTC ) - VTC (Tref ) (3)

Therefore, by measuring VMEAS and Tref and knowing the relationship between the thermocouple voltage and temperature, the temperature of the thermocouple measuring end can be determined.

There are two techniques for cold junction compensation – hardware compensation and software compensation. Both techniques require the use of a direct reading sensor to determine the reference junction temperature. A direct reading sensor has an input that is determined only by the temperature at the point of measurement. Semiconductor sensors, thermistors, and RTDs are commonly used instruments to measure the reference junction temperature.

With hardware compensation, a variable voltage source is inserted into the circuit to cancel parasitic temperature differential voltages. The variable voltage source generates a compensation voltage based on the ambient temperature, which is added to the correction voltage to cancel the unwanted temperature differential signal. When these parasitic signals are removed, the only signal measured by the data acquisition system is the voltage measured at the thermocouple measurement end. With hardware compensation, the temperature at the data acquisition system terminal is irrelevant because the parasitic thermocouple voltage has been cancelled. The main disadvantage of hardware compensation is that each thermocouple must have a separate compensation circuit that can add the corrective compensation voltage, which greatly increases the cost of the circuit. In general, hardware compensation is not as accurate as software compensation.

Alternatively, you may choose to use software to perform cold junction compensation. After measuring the reference junction temperature using a directly readable sensor, the software can add an appropriate voltage value to the measured voltage to eliminate the effect of the cold junction voltage. Recall that equation (3) specifies that the measured voltage VMEAS is equal to the voltage difference between the (thermocouple) measurement terminal and the cold junction.

Thermocouple output voltage is highly nonlinear. The Seebeck coefficient can vary by a factor of three or more over the operating temperature range of some thermocouples. Therefore, you must use a polynomial to model the voltage vs. temperature curve in the thermocouple or use a lookup table.

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Connecting the Thermocouple to the Instrument

This section uses the NI cDAQ-9172 chassis and the NI 9211 C Series thermocouple module as an example. Similar procedures apply for connecting thermocouples to different instruments (see Figure 4).

Equipment needed:

• DAQ-9172 Eight-Slot High-Speed ​​USB Backplane for NI CompactDAQ
• NI 9211 4-channel, 14 Sa/s, 24-bit, ∓80 mV thermocouple input module
• J-type thermocouple

Figure 4. NI CompactDAQ System

The NI 9211 has a 10-point, detachable screw-post connector that provides connections for four thermocouple input channels. Each channel has a connection point for the positive thermocouple terminal, TC+, and a connection point for the negative terminal, TC-. The NI 9211 also has a common connection point, COM. This port is usually connected internally to the module's ground reference. Figure 5 shows the wiring assignments for each channel, and Figure 6 shows the wiring diagram.

Figure 5. Terminal allocation

Figure 6. Connection diagram

View Your Measurements: NI LabVIEW

Now that the thermocouples are connected to the test equipment, you can use LabVIEW graphical programming software to transfer the data to a computer for visualization and analysis.

FIG. 7 shows an example of displaying measured temperature data in the LabVIEW programming environment.

Figure 7. LabVIEW front panel showing temperature data.