What are the welding methods for thermocouple hot electrodes

Everyone on Earth knows that a thermocouple is a temperature measuring element that welds two conductors, commonly known as thermoelectric electrodes, together. It is also the most commonly used instrument in temperature instruments. So, in the manufacturing process of thermocouples, how is the welding of thermoelectric electrodes stopped?

1. Gas welding method

This method is easy to manipulate and widely used. When using gas welding, the top of the thermoelectric electrode should be heated and dipped in solder flux (such as nickel chromium nickel silicon thermocouple solder flux mixed with half sodium tetraborate and half quartz sand), and then the thermoelectric electrode should be placed in an acetylene or hydrogen oxygen flame. After melting into a spherical shape, it should be quickly removed and immediately put into hot water to wash away the residue on the welding point. This welding method is practical for the manufacture of low-cost metal thermocouples.

2. Arc welding method

The arc welding method is divided into direct current arc welding and exchange arc welding.

DC arc welding is the process of connecting a thermocouple to the positive pole of a power source and a carbon rod (spectral) to the negative pole of the power source. The carbon rod and the top end of the thermoelectric electrode are used to start an arc, and the carbon rod is quickly separated after the measuring end melts into a spherical shape. This welding method is simple, easy to operate, and the measuring end is not easily contaminated, making it suitable for welding precious metal thermocouples.

Exchange arc welding is used for welding low-cost metal thermocouples. Before welding, the oxide in 25-30mm section of the measuring end shall be carefully cleaned, and then the top ends of the two electrodes shall be aligned and twisted into Fried Dough Twists shape. When welding, dip the top end of the thermoelectric electrode in solder flux and melt it in an arc flame for 3-5 seconds. After it forms a ball shape, quickly remove it and understand the residue on the solder joint. This method has simple equipment and easy operation, but the thermocouple welding points and adjacent electrodes are prone to carburization and contamination.

3. Saltwater welding method

This welding method is practical for welding thermocouples with a diameter of 0.03mm-0.3mm. Fill a beaker with sodium chloride solution, place platinum wire as one electrode in the aqueous solution, and use the hot electrode as the other electrode. When welding, slightly engage the top of the thermocouple with the solution, turn on the power, and quickly disconnect the power after the arc starts.

4. Argon arc welding method

The argon arc welding installation consists of five departments: DC welding power supply, high-frequency oscillator, welding gun, butt welding power supply, and fixtures. When welding, the cerium tungsten wire extending from the welding gun nozzle should be used as the negative electrode, and the thermocouple to be welded should be firmly attached to the fixture as the positive electrode. When the process between the north and south poles passes through high frequency and high voltage, the ignition of the arc will be affected. The controllable silicon voltage is used to control the arc intensity, and under the maintenance of argon gas, arc discharge will occur between the cerium tungsten and the welded thermocouple. The low temperature generated by the arc will melt the end face of the thermocouple wire into a spherical shape. In order to facilitate the alignment of thermocouples and electrodes, the task fixture and welding gun can be moved vertically in space. The welding gun is equipped with cerium tungsten electrodes with diameters of 1mm and 1.5mm for thermocouple welding applications with different diameters.

5. Carbon powder welding method

This method is practical for welding low-cost metal thermocouples. The installation of carbon powder welding is similar to arc welding, except that one pole of the power supply is not connected to a carbon rod, but to a graphite crucible containing carbon powder, and the other pole is connected to the thermocouple being welded. When welding, remove the thermoelectric electrode from the graphite powder and wait for a few seconds to complete the welding. This welding method is more convenient than arc welding, but it is prone to causing brittle fracture of the hot electrode.