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Principle and construction of thermocouples and thermal resistors
Dec 24, 2018

Principle and construction of thermocouples and thermal resistors

Temperature measuring instruments can be divided into two types according to the temperature measurement method: contact type and non-contact type. Generally speaking, the contact type temperature measuring instrument is relatively simple and reliable, and the measuring precision is high. Generally, there are bimetals, thermocouples, and thermal resistors used in the industry. The bimetal is generally on-site indication. The galvanic couple and resistor are generally used for remote display of distributed control such as DCS and PLC. Today Huaheng Instrument introduces you to the thermocouple and the thermal resistance.

Principle and construction of thermocouples and thermal resistors

First, the principle and structure of thermocouple

Thermocouples are one of the most commonly used temperature sensing components in the industry. The advantages are:

1 Measurement accuracy is high. Because the thermocouple is in direct contact with the object to be tested, it is not affected by the intermediate medium.

2 Wide measurement range. Commonly used thermocouples can be continuously measured from -50 to +1600 °C. Some special thermocouples can measure -269 °C (such as gold-iron nickel-chromium), up to +2800 °C (such as tungsten-bismuth).

3 simple structure, easy to use. Thermocouples are usually made up of two different wires and are not limited by size and opening. They have a protective sleeve and are very convenient to use.

1. Basic principle of thermocouple temperature measurement

The conductors or semiconductors A and B of two different materials are welded together to form a closed loop. When there is a temperature difference between the two attachment points 1 and 2 of the conductors A and B, an electromotive force is generated between the two, and thus a current of a magnitude is formed in the loop. This phenomenon is called a thermoelectric effect. Thermocouples use this effect to work.

2. Thermocouple type and structure formation

(1) Type of thermocouple

Commonly used thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The standard thermocouple that is called refers to the thermocouple that the national standard specifies the relationship between the thermoelectric potential and the temperature, the allowable error, and the uniform standard index table. It has the display instrument that is compatible with it. Non-standardized thermocouples are not as good as standardized thermocouples in terms of use or order of magnitude. Generally, there is no uniform indexing table, which is mainly used for measurement in some special occasions. Standardized thermocouple

Since January 1, 1988, China's thermocouples and thermal resistors have all been produced in accordance with IEC international standards, and seven standardized thermocouples of S, B, E, K, R, J, and T (or indexing numbers) have been designated as China. Unified design thermocouple.

(2) Structure of thermocouple In order to ensure reliable and stable operation of the thermocouple, its structural requirements are as follows:

1 The welding of the two hot electrodes constituting the thermocouple must be firm;

2 The two hot electrodes should be well insulated from each other to prevent short circuits;

3 The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable;

4 The protective sleeve should ensure that the hot electrode is sufficiently isolated from harmful media.

Principle and construction of thermocouples and thermal resistors

3. Temperature compensation of the cold junction of the thermocouple

Because thermocouple materials are generally more expensive (especially when using precious metals), and the temperature measurement point is far away from the meter, in order to save thermocouple materials and reduce costs, the compensation wire is usually used to cool the cold end of the thermocouple (free The end extends to a relatively stable temperature control room and is connected to the meter terminals. It must be pointed out that the function of the thermocouple compensation wire only extends the hot electrode, so that the cold end of the thermocouple moves to the instrument terminal of the control room, which itself does not eliminate the influence of the temperature change of the cold end on the temperature measurement, and does not compensate. Therefore, other correction methods are needed to compensate for the influence of the cold junction temperature t0 ≠ 0 ° C on the temperature measurement.

When using the thermocouple compensation wire, it must be noted that the model is matched, the polarity cannot be connected incorrectly, and the temperature between the compensation wire and the thermocouple connection terminal cannot exceed 100 °C.

For example: (S-type thermocouple) platinum-iridium 10-platinum thermocouple

Platinum rhodium 10-platinum thermocouples (S-type thermocouples) are noble metal thermocouples. The diameter of the filament is specified to be 0.5 mm, the allowable deviation is -0.02 mm, and the nominal chemical composition of the positive electrode (SP) is platinum-rhodium alloy, in which yttrium is 10%, platinum is 90%, and the negative electrode (SN) is pure platinum. Commonly known as a single platinum rhodium thermocouple. The thermocouple has a long-term maximum operating temperature of 1300 ° C and a short-term maximum operating temperature of 1600 ° C.

S-type thermocouples have the highest accuracy, best stability, wide temperature range and long service life in the thermocouple series. It has good physical and chemical properties, good thermoelectric potential stability and high oxidation resistance at high temperatures. It is suitable for oxidizing and inert atmospheres. Because the S-type thermocouple has excellent comprehensive performance and conforms to the internationally used S-type thermocouple with temperature scale, it has long been used as an interpolation instrument for international temperature scales. Although the "ITS-90" stipulates that it will no longer be used as an internal inspection instrument for international temperature scales. However, the International Temperature Advisory Committee (CCT) believes that S-type thermocouples can still be used to approximate the international temperature scale.

The shortcomings of the S-type thermocouple are the thermoelectric potential, the thermoelectric potential rate is small, the sensitive reading is low, the mechanical strength is lowered at high temperature, the pollution is very sensitive, and the precious metal material is expensive, so the one-time investment is large.

Second, the principle and structure of the thermal resistance

Thermal resistance is the most commonly used temperature detector in the mid-low temperature zone. Its main features are high measurement accuracy and stable performance. Among them, platinum thermal resistance has the highest measurement accuracy, and it is not only widely used in industrial temperature measurement, but also made into a standard reference instrument.

Principle and construction of thermocouples and thermal resistors

1. Thermal resistance temperature measurement principle and material

Thermistor temperature measurement is based on the fact that the resistance value of a metal conductor increases as the temperature increases. Most of the thermal resistance is made of pure metal materials. At present, platinum and copper are the most widely used. In addition, thermal resistances have been fabricated using materials such as dian, nickel, manganese and tantalum.

2. Thermal resistance structure

(1) Mastery type thermal resistance

From the temperature measurement principle of the thermal resistance, it is known that the change in the measured temperature is directly measured by the change in the resistance of the thermal resistance. Therefore, variations in the resistance of various wires such as the lead wires of the thermal resistor body may affect the temperature measurement. In order to eliminate the influence of lead resistance, a three-wire system or a four-wire system is generally used.

(Two-wire system: two wires and transmission power supply transmit signals, that is, the load and power supply of the sensor output are connected in series, the power supply is externally introduced, and the load is connected in series to drive the load. Three-wire system: three-wire system The sensor is the positive terminal of the power supply and the positive terminal of the signal output, but they share a COM terminal. Four-wire system: two wires for the power supply and two wires for the signal. The power supply and the signal work separately.)

(2) armored thermal resistance

The armored thermal resistance is a solid body composed of a temperature sensing element (resistor), a lead wire, an insulating material, and a stainless steel sleeve. Its outer diameter is generally φ2 to φ8 mm. Compared with ordinary type thermal resistance, it has the following advantages: 1 small volume, no air gap inside, thermal inertia, small measurement lag; 2 good mechanical properties, vibration resistance, impact resistance; 3 can bend, easy to install 4 long service life .

(3) End face thermal resistance

The end face thermal resistance temperature sensing element is wound by a specially processed resistance wire and is closely attached to the end face of the thermometer. Compared with the general axial thermal resistance, it can reflect the actual temperature of the measured end face more correctly and quickly, and is suitable for measuring the bearing bush. And the end face temperature of other parts.

(4) Flameproof thermal resistance

Explosion-proof thermal resistance is limited to the explosion caused by sparks or arcs in the explosion-mixed gas inside the casing through a special-structure junction box, and the production site will not lead to an explosion. Explosion-proof thermal resistance can be used for temperature measurement in explosion-prone areas in the Bla~B3c class.

Principle and construction of thermocouples and thermal resistors

3. The composition of the thermal resistance temperature measurement system

The thermal resistance temperature measuring system generally consists of a thermal resistor, a connecting wire and a display instrument. The following two points must be noted:

1The thermal resistance and the index number of the display instrument must be the same

2 In order to eliminate the influence of the change of the resistance of the connecting wire, the three-wire connection method must be adopted.

As the name suggests, the resistance of a resistor varies with temperature, for example, a platinum wire or a copper wire resistor. Industrial thermal resistance is generally Pt100, Pt10, Pt1000, Cu50, Cu100, the range of temperature measurement of platinum thermal resistance is generally minus 200-800 degrees Celsius, copper thermal resistance is minus 40 to 140 degrees Celsius.

For example, a thermal resistance made of platinum wire is called Pt100. That is to say, its resistance is 100 ohms at 0 degrees, 18.52 ohms at 200 degrees, 175.86 ohms at 200 degrees, and 375.70 ohms at 800 degrees.

For example, a thermal resistance made of copper wire, index number Cu50. It has a resistance of 50 ohms at 0 degrees and 71.400 ohms at 100 degrees.

The thermal resistance formula is Rt=Ro(1+A*t+B*t*t); Rt=Ro[1+A*t+B*t*t+C(t-100)*t*t*t In the form of t, the temperature is Celsius, Ro is the resistance at zero degrees Celsius, and A, B, and C are all specified coefficients. For Pt100, Ro is equal to 100.

Index number definition: Represents the temperature range, and represents the thermocouple or thermistor of each index number, how many volts or millivolts of voltage is output.