Thermocouples are one of the most commonly used temperature detection devices in the industry. Its advantages are:
1 high measurement accuracy. Because the thermocouple is directly in contact with the measured object, it is not affected by the intermediate medium.
2 wide measurement range. Common thermocouples can be measured from -50 to +1600°C. Some special thermocouples can measure -269°C (such as gold, iron, nickel-chromium) and up to +2,800°C (such as tungsten-germanium).
3 simple structure, easy to use. Thermocouples are usually composed of two different types of wire, and are not limited by the size and the beginning. They have a protective sleeve and are very convenient to use.
1. Thermocouple Temperature Measurement Fundamentals
Two different material conductors or semiconductors A and B are welded together to form a closed loop, as shown in Figure 2-1-1.
Show. When there is a temperature difference between the two sticking points 1 and 2 of 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 the thermoelectric effect. Thermocouples use this effect to work
Made for.
2. Thermocouple type and structure formation
(1) Types of thermocouples
Commonly used thermocouples can be divided into standard thermocouples and non-standard thermocouples. The standard thermocouple called refers to the country
The standard stipulates the thermocouple with its relationship between the thermoelectric potential and the temperature, the allowable error, and a unified standard indexing table.
With its supporting display instrument available. Non-standardized thermocouples are less than standard in terms of use range or magnitude
Thermocouples generally do not have a uniform indexing table and are mainly used for measurements in certain special situations.
Standardized Thermocouples From January 1, 1988, thermocouples and thermal resistances were all produced according to IEC international standards, and seven standardized thermocouples designated as S, B, E, K, R, J, and T were designed for unified design in China. Thermocouples.
(2) The structure of the thermocouple To ensure reliable and stable operation of the thermocouple, its structural requirements are as follows:
1 The welding of the two hot electrodes forming the thermocouple must be firm;
2 The two hot electrodes should be well insulated from each other to prevent short circuit;
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 fully isolated from harmful media.
3. Thermocouple cold junction temperature compensation
Because thermocouple materials are generally more expensive (especially when precious metals are used), and the distance from the temperature measurement point to the instrument is very long. In order to save the thermocouple material and reduce the cost, the cold end of the thermocouple is usually used to compensate the wire (free The end) extends into a relatively stable temperature control room and is connected to the meter terminals. It must be pointed out that the role of thermocouple compensation wire only extends the thermode, so that the cold junction of the thermocouple moves to the instrument terminal of the control room. It does not eliminate the influence of the temperature change of the cold junction on the temperature measurement and does not compensate. Therefore, other correction methods need to be used to compensate for the effect of temperature at the cold junction temperature t0≠0°C.
When using a thermocouple to compensate for the wire, it must be noted that the model matches, and the polarity cannot be connected wrongly. The temperature of the connecting wire of the compensation wire and the thermocouple cannot exceed 100°C.
Thermocouple welding method
1, arc welding
Arc welding can be divided into branch welding and AC welding.
In DC welding, the thermocouple is connected to the positive pole of the power source, and the carbon rod (spectrum) is connected to the negative pole of the power source. The carbon rod is used to contact the top of the thermoelectrode to start arcing. After the measuring end is melted into a sphere, it rapidly leaves the carbon rod. This welding method is simple, easy to operate, easy to stain the measuring end, and used in the welding of precious metal thermocouples.
AC welding is suitable for welding cheap metal thermocouples. Before welding, the 25-30mm section of oxide at the measurement end should be carefully removed, then the two electrodes should be aligned at the top and twisted into a twist. During welding, the flux is applied to the top of the hot electrode and melted in the arc flame for 3-5 seconds. After being spheroidized, the flux can be quickly removed and the residue on the solder joint can be clearly removed. This method is simple and easy to operate, but the thermocouple solder joints and adjacent electrodes are carburized.
2, argon arc welding
The argon arc welding device consists of five parts: a DC welding power source, a high-frequency oscillator, a welding torch, a welding power source, and a fixture. During the welding, a tantalum-tungsten wire protruding from the nozzle of the welding torch is used as a negative electrode, and the welded thermocouple is fixed on the jig as a positive electrode. When the two poles pass through high frequency and high voltage, they will ignite the arc and use the thyristor to adjust the pressure to control the intensity of the arc. Under the protection of the argon gas, the arc discharge will be generated between the helium-tungsten and the welded thermocouple, and the arc will be generated. The high temperature melts the end face of the thermocouple wire into a spherical shape. In order to facilitate the alignment of the thermocouple and the electrode, the work clamp and the welding torch can move horizontally and vertically in space. The welding torch is equipped with 1mm and 1.5mm diameter tantalum-tungsten electrodes for welding thermocouples of different diameters.
3, gas welding
When gas welding is used, the top of the hot electrode should be heated and fluxed (such as nickel-chromium-nickel-silicon solder is a mixture of tetraborate sodium and quartz sand), and then the hot electrode is placed on acetylene or oxyhydrogen flame In the middle, after being melted into a ball shape, it is quickly taken out and immediately placed in hot water to remove the residue from the solder joint. This method is simple and widely used. Suitable for welding of cheap metal thermocouples.
4, toner welding
The carbon powder welding device is similar to arc welding, except that the pole of the power source is not connected to the carbon rod, but is connected to the graphite crucible containing the carbon powder, and the other pole is connected to the thermocouple to be welded. When welding, insert the hot electrode into the graphite powder and solder it in a few seconds. This welding method is more convenient than arc welding, but it can easily cause brittle fracture of the hot electrode. This method is suitable for the welding of cheap metal thermocouples.
5, brine welding
The beaker was charged with sodium chloride solution, platinum wire was placed in the aqueous solution as an electrode, and the hot electrode was used as the other electrode. When soldering, place the top of the thermocouple slightly in contact with the solution and turn on the power. After the arc starts, turn off the power quickly. This welding method is suitable for welding 0.03mm-0.3mm thermocouples with finer diameters.
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