Both advantages of thermocouple temperature measurement related research methods2

Thermocouple measuring temperature difference, rather than temperature. Only know that at the end of the thermocouple temperature, can use thermoelectric potential to find out at the other end of the temperature. It is also different from PN junction, thermistor temperature sensor. These sensors of a certain voltage or resistance is corresponding to a definite temperature. And when using a thermocouple temperature measurement, must be determined with the aid of a temperature standard cold end temperature, to measure the temperature on thermal. To this end, the most simple and straightforward method is to use an ordinary thermometer to measure the thermocouple cold end temperature, and added to the hot and cold end of thermocouple to measure temperature difference, is the real temperature on heat. Although protean all kinds of cold junction compensation circuit, complex, are based on the simple way to contain basic principle.

figure 1 is the most simple of thermocouple temperature measurement circuit, with I - CL7106 to cold junction compensation and its function of the temperature measurement. The circuit used in the thermocouple type K, the temperature difference electromotive force is 407 mu V / ℃, so in the W2 IC1 set the reference voltage is 407 mv, IC1 became full voltage of 814 mv voltmeter, equivalent to 2000 ℃ temperature range. Cold junction compensation diode D1, it measured the temperature coefficient of 238 mv / ℃, and then use pressure to compress the temperature coefficient of resistance points to 407 mu V / ℃, consistent with thermocouple. Mv / 238 ℃ present 407 mu V / ℃ material 5848, choose the ratio of the resistors R1 and R2 for 5748, R1 = 33 k Ω, R2 = 574 Ω. Will decrease when the temperature rises, D1 voltage, the voltage rise on the R2, and keep strictly proportional relationship with D1 temperature coefficient of mu V / 407 ℃, of course, choose the D1, R1, R2, the resistance ratio should also be changed.

thermocouple temperature difference electromotive force produced by superposition voltage on R2, input to the IC1 IN + side again. At the same time, the use of W1, R3 and R4 will IC1 COM side partial pressure, with the benchmark voltage at the power supply is used to adjust balance, make the voltmeter indicates zero when 0 ℃. The voltage across the IC1 the IN - side. So, when not measuring of the measuring junction of the thermocouple, the temperature difference electromotive force is zero. Electric meter indicates the environment temperature is D1 measure. During the measurement, the measured pressure of thermoelectric potential on D1 point, is hot end temperature meter indication.

this calibration method is, in a certain environment temperature ( It is best to 0 ℃) , make the digital table shows temperature adjustable W1. Then put thermocouple at 100 ℃ water, adjustable W2 according to 100 ℃. Can also be used in higher temperature calibration. The above calibration is the premise of R1, R2, and the temperature coefficient of D1 match, otherwise, shall be calibrated in advance R2.

temperature measurement circuit of figure 2 is DT838 digital multimeter. The cold junction compensation principle of the circuit and the circuit is similar, but use an op-amp LM358. The low potential end of thermocouple into the land of total ( COM) , and easy to use multimeter originally jack into the thermocouple. The temperature of the voltmeter is still set to full 814 mv. Characterized by voltage, the temperature of the cold end compensation diode by LM358 and R1, R2 of inverting follower into COM as the reference point of phase voltage, and through the resistor R3 and R4 partial pressure has turned 407 mu V / ℃ plus the IN - side IN the IC1, R9 thermocouple temperature difference electromotive force produced by resistance to IC1 IN + side. So, regardless of the environment temperature rising or thermocouple measuring the temperature rise, all increase the voltmeter reading, completed the sum function. The circuit is 0 ℃ balance function is similar to figure 1 circuit, but its voltage is added to the operational phase. The calibration method is also the same as shown in figure 1.

figure 3 is independence from the ICL7106 temperature measurement circuit, can be used in digital form can also be used for pointer table. Its cold junction compensation principle and temperature measurement principle and the circuit is the same, but the temperature difference electromotive force with op-amp tries to enlarge, so as to adapt to a wider range of applications. The circuit according to graph element numerical value, the output voltage of 1 mv / ℃. 0 ℃ when the output voltage is zero, 0 ℃ below the output load voltage, when using a pointer table must change the pointer to 0 point location. If the temperature range is small, also can change the output to 10 mv / ℃, as long as the figure of R5, W2 Ω respectively in 160 k, 20 k Ω can.

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