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Thermocouple cold junction compensation
Abstract: the temperature measurement applications, the thermocouple because of its robustness, reliability and faster response speed obtained the widespread application. This application note discusses the basic working principle of thermocouple, including reference end (cold end) and the definition of a function. This paper also gives a concrete application in accordance with choice of cold end temperature measurement device of the matters needing attention, and three design example is presented.
An overview of the
Temperature measurement applications there are several types of transducer and thermocouple is one of the most commonly used, which can be widely used in automobile, household and other fields. And RTD, thermistor, temperature detection of integrated circuit (IC) compared to the thermocouple is capable of detecting a wider temperature range, high cost performance. In addition, the thermocouple is strong, reliability and rapid response time make it become the primary choice of various operating conditions. In thermocouple temperature measurement, of course, there are some defects, for example, poor linear characteristic. Although they are more than RTD, temperature sensor IC can be measured with wide temperature range, but the linearity is compromised. In addition to this, and RTD temperature sensor IC can provide higher sensitivity and accuracy, can be ideal for precision measurement system. Thermocouple signal level is very low, often need to zoom in or high resolution data converter for processing. If the above problems, low price, easy to use, wide temperature range of the thermocouple is widely used.
Thermocouple based
Thermocouple temperature measuring device is difference, formed by two different metal/alloy wire, a is used as the positive side, the other a is used as the negative side. Table 1 lists the four most commonly used thermocouple type, metal, and the corresponding temperature measurement range. Each thermocouple in the specified temperature range has a unique thermoelectric properties. Table 1. The commonly used thermocouple type
Type is the metal/alloy metal/alloy negative end temperature range (° C)
T of copper nickel copper alloy - 200 to + 350
J iron nickel copper alloy 0 to + 750
K nickel-chromium alloy nickel-based thermocouple alloy - 200 to + 1250
E nickel-chromium alloy of nickel copper alloy - 200 to + 900
Two different types of metal (welding) together, after forming two node, as shown in figure
1 a, the loop voltage is a function of temperature difference between two nodes. This phenomenon is known as the Seebeck effect, used to explain the process of the heat energy is converted to electrical energy. Seebeck effect relative to the Peltier effect, the Peltier effect used to explain the process of the electrical energy is converted into heat energy, typical applications have electric refrigerator. As shown in figure 1 a, measuring voltage VOUT is to end (hot) junction voltage with the reference junction voltage difference (cold end). Because of the VH and VC was produced by the two junction temperature difference, VOUT is also a function of the temperature difference. Scaling factor, alpha, corresponds to the voltage difference and the ratio of the temperature difference, known as the Seebeck coefficient. Figure 1 a. The loop voltage generated by the thermocouple temperature difference between the two nodes, and is the result of the Seebeck effect.
Figure 1 b. Common thermocouple configuration consists of two lines connected to the end, each line constant and open a terminal and the copper wire connection. As shown in figure 1 b is one of the most common thermocouple applications. Introduced the configuration of the third metal metal (middle) and two additional nodes. In this case, each terminal and wire electrical connection, the attachment for the system adds two additional nodes, temperature is the same, as long as the two nodes between metal (copper) will not affect the output voltage. This configuration allows the thermocouple used under the condition of no independent reference node. VOUT is still a function of the difference between the hot end and cold end temperature, and the Seebeck coefficient. However, due to the thermocouple measuring temperature difference, in order to determine the actual temperature at the hot and cold junction temperature must be known. Cold end temperature of 0 ° C (freezing point) is one of the simplest case, if the TC = 0 ° C, the VOUT = VH. In this case, measure the voltage is the node temperature of hot end of direct conversion value. National bureau of standards (NBS) provides a variety of types of thermocouple voltage characteristics data of the corresponding relationship between temperature lookup table. All data are based on 0 ° C cold end temperature. Used as reference points, freezing by looking for the appropriate form of the VH hot end temperature can be determined. At the beginning of the thermocouple applications, freezing point is a standard thermocouple reference points, but in most applications to obtain a reference temperature freezing is not reality. If the cold end temperature is not 0 ° C, so, in order to determine the actual temperature must be known to the cold end temperature of hot end. Considering the nonzero voltage of cold end temperature, necessary to compensation of the thermocouple output voltage, is the so-called cold junction compensation.