The structure, working principle and detecting method of good or bad of thermocouple

Thermocouple is a kind of temperature measuring element, which can convert different temperatures into different electrical signals of different sizes. It is widely used in some temperature measuring fields, such as temperature measuring instruments and temperature measuring systems in metallurgy, petrochemical, thermal power station, textile and paper making industries. The common shape of thermocouple is shown in Figure 14-22.

Figure 14-22 Common thermocouple shapes

Thermoelectric Effect and Thermocouple Measurement Principle

(1) Thermoelectric effect

When the two different conductor (or semiconductor) connected on both ends, as shown in figure 14 to 23, if the temperature of the node 1 T 1 is greater than the temperature of the node 2 T 2, then the loop will be electromotive force (often called thermoelectric potential), as the two conductor connection form closed loops, and loop currents, the phenomenon known as the seebeck effect, namely thermoelectric effect. The larger the temperature difference between the two nodes, the higher the electromotive force generated by the loop, and the greater the current in the loop.

Fig. 14-23 Illustration of thermoelectric effect

(2) Use thermocouple to measure temperature

As shown in Fig. 14-23, if the temperature T 2 of node 2 is fixed (such as 0℃), the EMF generated by the loop changes with the temperature T 1 of node 1. As long as the EMF or current value of the loop is measured, the temperature T 1 of node 1 can be determined. The wiring of measuring temperature with thermocouple is shown in figure 14-24. Figure 14-24 Two ways of connecting thermocouple measuring temperature

Fig. 14-24(a) In the wiring, conductor B is divided into two parts, and conductor C(conductor and ammeter) is connected in the middle. As long as the temperature at point 3 and 4 is the same, the electromotive force of the circuit is the same as that of direct connection at point 3 and 4. In the wiring of Figure 14-24(b), node 2 is eliminated, but the electromotive force in the loop is the same as that of node 2 as long as the temperature at point 3 and 4 is the same. When measuring temperature with thermocouple, the wiring mode as shown in Figure 14-24(b) is generally used. In this mode, 3 and 4 are called cold end (or free end), and node 1 is called hot end. When measuring temperature, node 1 is contacted with the tested object.

In the wiring of Figure 14-24(b), if the measurement is to be as accurate as possible, the meter should be directly connected to the 3 and 4 ends without wires. However, since the measuring object is often far away from the measuring instrument, compensation wires are often used to connect the thermocouple and the measuring instrument in general measurement. There are two kinds of compensating conductors: one is made of elongated conductors of the same material as the thermocouple, and the other is made of alloy conductors with similar thermoelectric potential characteristics as the thermocouple.

(3) Cold end temperature compensation

In the use of thermocouple temperature measurement, the instrument according to the size of the electromotive force generated by the thermocouple to determine the measured temperature value, and the size of the electromotive force and the temperature difference between the hot and cold end, the greater the temperature difference, the greater the electromotive force generated by the thermocouple, in order to make the value of the electromotive force and the temperature value of one to one corresponding, usually let the cold end is 0℃. In actual measurement, the cold end temperature usually agree with environmental temperature, such as 25 ℃ or so, if the hot end of the cold end of 0 ℃, 40 ℃ when the thermocouple electromotive force as E40, the instrument displays temperature should be 40 ℃, so in the hot end of the cold end is 25 ℃, 40 ℃ when the thermocouple electromotive force must be less than E40, instrument display temperature value will be less than 40 ℃, appear large deviation measurement. In order to make the measurement accurate, it is necessary to compensate the cold end temperature of the thermocouple.

① Ice bath compensation method Ice bath compensation method refers to the cold end of the thermocouple is placed in the ice water mixture, so that the cold end temperature is constant to 0℃ compensation method. Ice bath compensation method as shown in figure 14-25, compensation conductor end through the junction box connected to the hot end of thermocouple, the other end connected to the copper wire form the contact, the contact as the cold end, it is placed in a mixture of ice at 0 ℃, and the other end of the copper wire millivoltmeter, used for measuring thermocouple electromotive force, if the millivoltmeter marked as temperature scale according to certain rules, the device is the temperature measuring device.

Fig. 14-25 Ice bath compensation method

The ice bath compensation method is usually used in the laboratory for temperature measurement because the ice melts quickly and the cold end cannot be kept at 0℃ for a long time.

When measuring, if the temperature of the cold end of the thermocouple is not 0℃, the error correction method can be used to compensate. If hot end of thermocouple temperature measurement for T, the cold end temperature of T 1, instrument measurement value of 1 E, E T - 1 T 1 T - T temperature difference electromotive force value, and the electromotive force generated by the T 1-0 temperature difference value for E T 1-0 (the value by thermocouple indexing table to look up the corresponding materials), then the instrument measured values E T - 1 and revised E T T 1-0, E the electromotive force T - 0 on the instrument by the corresponding value is the actual temperature value. Deviation correction method has two methods: one is manual correction, the other is automatic correction. Manual correction method is as shown in Figure 14-26. If the ambient temperature (air temperature) is 40 ° C, the mechanical zero adjustment knob can be adjusted to adjust the meter hand to 40 ° C for cold end temperature correction. Some digital temperature measuring instruments usually adopt automatic correction mode, that is, automatically add the cold end temperature value to the measured value and display.

Figure 14-26 Manual correction method

Structure that

Thermocouples come in a variety of shapes, but the basic structure is the same. Figure 14-27 shows a typical thermocouple composition.

Figure 14-27 Typical thermocouple composition

Use thermocouple with digital multimeter to measure the temperature of electric iron

Some digital multimeter with temperature measurement function, VC9208 digital multimeter has this function, it uses K type thermocouple and temperature measurement block with the temperature measurement of - 40 ~ 1000℃ temperature. The K-type thermocouple (Ni-Cr - Ni-Si) supporting the VC9208 digital mulimeter is shown in Fig. 14-28, which is composed of hot end (temperature measuring end), compensation conductor and cold end.

Figure 14-28 Thermocouple for a digital multimeter

Using thermocouple measuring the temperature of the electric operation process as shown in figure 14 to 29, insert the thermocouple of the black plug (cold end), red "mA" hole plug (cold end) inserted into the ".com "hole, then will block a selection switch on" ℃ "side, the hot end of thermocouple WenDuan (measured) electric contact, and then observe the screen shows the values for" 244 ", explain the solder iron temperature is 244 ℃.

Figure 14-29 Operation of measuring the temperature of an electric soldering iron with a thermocouple

Good or bad detection

A thermocouple is composed of two different conductors welded together, one end of which is welded together, and the other end of which is connected to a measuring instrument through a compensating conductor. Test thermocouple quality can be carried out in the following two steps.

Step 1: Measure the resistance of the thermocouple. Multimeter to R x 1 Ω block, red, black pens and two compensation of thermocouple wire, respectively, if the thermocouple and compensation conductor should be normal, measured resistance smaller (several to dozens of Europe), if the value is infinite, for the thermocouple or compensation conductor should be open.

The second step: measure the thermoelectric conversion effect of thermocouple. Multimeter to dc voltage is the smallest block, red, black pens and two compensation of thermocouple wire, respectively, and then will contact with high temperature of the hot junction of the thermocouple objects (e.g., heat wok), if the thermocouple is normal, multimeter whose hands will indicate certain voltage value, as the hot end temperature rise, the clock indicating voltage will gradually increase, measured with a digital multimeter, voltage change is more obvious, if the voltage value is 0, thermocouple to thermoelectric conversion, thermocouple damage or failure.

Flexible use of multiple thermocouple connections

Thermocouple can not only be used alone, but also can be connected together to achieve a variety of flexible temperature measurement functions.

(1) Measure the temperature difference between two points

Measured by thermocouple temperature difference between two points of connection as shown in figure 14 to 30, the two thermocouple homogenous B together, two very instrument the two input respectively, if A contact T thermocouple temperature of voltage to U T 1, another thermocouple contact T temperature of voltage to U T 2, then U T 1-1-2 is T T U T 2 temperature difference of the voltage, it drives the instrument shows that temperature difference value.

Figure 14-30 wiring for measuring temperature difference between two points using a thermocouple

(2) Measure the average temperature of multiple points

Using the average temperature thermocouple measuring multi-point connection as shown in figure 14 and 31, the thermocouple B all together, and then received A instrument input, each A pole respectively through A resistance of A resistance R received another input, from the instrument to the thermocouple parallel to instrument, instrument display for each point temperature averages.

Figure 14-31 wiring for measuring average temperature at multiple points using a thermocouple

(3) measure the sum of multi-point temperatures

The wiring for measuring the sum of multi-point temperatures with thermocouples is shown in Fig. 14-32. In fact, it connects each thermocouple in series and sends the voltage generated by each thermocouple to the instrument by superposition.

Figure 14-32 wiring for measuring the sum of multi-point temperatures using a thermocouple

(4) multiple thermocouples share one instrument

(5) The wiring of multiple thermocouples sharing one instrument is shown in Figure 14-33. When the switch is switched to different positions, the corresponding thermocouples are connected with the instrument.

Figure 14-33 wiring for multiple thermocouples sharing one instrument