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Abstract: In temperature measurement applications, thermocouples are widely used for their ruggedness, reliability, and fast response speed. This application note discusses the basic working principle of a thermocouple, including the definition and function of the reference (cold) junction. This article also gives considerations for selecting a cold-junction temperature measurement device according to the specific application, and gives three design examples.

Overview There are many types of transmitters in temperature measurement applications. Thermocouples are the most commonly used and can be widely used in automotive, home and other fields. Compared with RTD, thermistor, and temperature detection integrated circuit (IC), thermocouples can detect a wider temperature range and have a higher cost performance. In addition, the ruggedness, reliability, and fast response time of thermocouples make them the first choice in a variety of work environments.

Of course, thermocouples also have some drawbacks in temperature measurement, such as poor linearity. Although they can measure a wider temperature range than RTD and temperature sensor ICs, their linearity is greatly reduced. In addition, RTD and temperature sensor ICs can provide higher sensitivity and accuracy, making them ideal for precise measurement systems. Thermocouple signal levels are low and often require amplification or high-resolution data converters for processing.

If the above problems are eliminated, the low price, easy use, and wide temperature range of the thermocouple make it widely used.

Thermocouple Basics A thermocouple is a differential temperature measurement device that consists of two different metal / alloy wires, one for the positive terminal and the other for the negative terminal. Table 1 lists the four most commonly used thermocouple types, the metals used, and the corresponding temperature measurement ranges. Each type of thermocouple has unique thermoelectric characteristics within its specified temperature range.

Table 1. Common thermocouple types

Type Positive metal / alloy Negative metal / alloy Temperature range (° C)

T copper nickel copper alloy -200 to +350

J Iron Nickel Copper 0 to +750

K Nichrome Nickel-based thermocouple alloy -200 to +1250

E Nichrome Nickel-copper alloy -200 to +900

 

Two different types of metal are joined (welded) to form two nodes. As shown in Figure 1a, the loop voltage is a function of the temperature difference between the two nodes. This phenomenon is called the Seebeck effect and is used to explain the process of converting thermal energy into electrical energy. Seebeck effect Compared to the Peltier effect, the Peltier effect is used to explain the process of converting electrical energy into thermal energy. Typical applications are electrothermal refrigerators. As shown in Figure 1a, the measured voltage VOUT is the difference between the junction voltage at the detection (hot) junction and the junction voltage at the reference (cold junction). Because VH and VC are generated by the temperature difference between the two junctions, VOUT is also a function of the temperature difference. The scaling factor, α, corresponds to the ratio of the voltage difference to the temperature difference, and is called the Seebeck coefficient.

The simplest case is when the cold junction temperature is 0 ° C (freezing point). If TC = 0 ° C, VOUT = VH. In this case, the measured voltage at the hot end is a direct conversion of the junction temperature. The National Bureau of Standards (NBS) provides lookup tables for the correspondence between voltage characteristic data and temperature for various types of thermocouples. All data are based on 0 ° C cold junction temperature. Using the freezing point as a reference point, the hot-end temperature can be determined by looking up the VH in the appropriate table.

In the early days of thermocouple applications, the freezing point was used as the standard reference point for thermocouples, but in most applications it is not practical to obtain a freezing point reference temperature. If the cold junction temperature is not 0 ° C, then the cold junction temperature must be known in order to determine the actual hot junction temperature. Considering the voltage at the non-zero cold junction temperature, it is necessary to compensate the thermocouple output voltage, the so-called cold junction compensation.

Selecting a Cold-Junction Temperature Measurement Device As mentioned above, in order to achieve cold-junction compensation, the cold-junction temperature must be determined, which can be achieved with any type of temperature detection device. In general-purpose temperature sensor ICs, thermistors, and RTDs, different types of devices have different advantages and disadvantages, which need to be selected according to specific applications.

 

For devices with very high accuracy requirements, a calibrated platinum RTD can maintain high accuracy over a wide temperature range, but its cost is high.

When the accuracy requirement is not very high, the thermistor and silicon temperature sensor IC can provide higher cost performance. The thermistor has a wider temperature range than the silicon IC, and the sensor IC has higher linearity, so the performance index is more a little better. Correcting the non-linearity of the thermistor will take up more microcontroller resources. The temperature sensor IC has excellent linearity, but the temperature measurement range is narrow.

In short, the cold junction temperature measurement device must be selected according to the actual needs of the system, and precision, temperature range, cost, and linearity indicators need to be carefully considered in order to obtain the best cost performance.

Considerations Once the cold junction compensation method is established, the compensated output voltage must be converted to the corresponding temperature. A simple method is to use a lookup table provided by NBS. The software needs a memory to implement the lookup table, but the lookup table provides a fast and accurate measurement scheme for continuous repeated queries. The other two schemes for converting thermocouple voltage to temperature value are more complicated than look-up tables. These two methods are: 1) linear approximation using polynomial coefficients, and 2) analog linearization of thermocouple output signals.

Software linear approximation only needs to determine polynomial coefficients in advance, and does not need to be stored, so it is a more general scheme. The disadvantage is that it takes a long time to solve the multi-order polynomial. The higher the polynomial order, the longer the processing time, especially in the case of a wide temperature range. When the polynomial order is high, the lookup table provides a more accurate and effective temperature measurement scheme.

Before the emergence of software testing schemes, analog linearization was often used to convert measured voltages into temperature values ​​(except for manual lookup table retrieval). This hardware-based method uses analog circuits to correct the nonlinearity of the thermocouple response. Its accuracy depends on the order of the modified approximation polynomial, which is still used in current multimeters capable of testing thermocouple signals.

Application Circuits Three typical applications for cold junction compensation using silicon sensor ICs are discussed below. All three circuits are designed to address a narrow temperature range (0 ° C to + 70 ° C and -40 ° C to + 85 ° C). The cold junction temperature is compensated with accuracy within a few degrees Celsius. The second circuit contains a remote diode temperature detector, which is provided with a test signal by a diode-connected transistor. The analog-to-digital converter (ADC) in the third circuit has built-in cold junction compensation. All three circuits use K-type thermocouples (composed of Nichrome and Ni-based thermocouple alloys) for temperature measurement.