First determine the appearance of the thermocouple is no problem, it is good or bad, have to go through testing to determine. Put thermocouple under test on thermocouple special porcelain sleeve, with standard platinum-rhodium thermocouple in the tube furnace, the hot side insert tube furnace in a porous soaking with metal in the cylinder made of nickel. Put their compensation the cold end of the wire in the ice water to keep zero degrees Celsius in the container. Hold the tube furnace at the thermocouple in the use of high temperature, and keep the stable range. This time use after testing qualified, hui sten potentiometer, measure standard thermocouple and under test thermocouple thermoelectric potential difference and record. Table according to the records of thermoelectric potential difference, check of their corresponding temperature, if the thermocouple out-of-tolerance, under test can be judged to be unqualified. This kind of tube furnace, not analytical chemistry with silicon carbon tube furnace. For thermocouple is unqualified, can cut off a little from the hot end, welding again. Welding method is simple, the ordinary nickel-chromium thermocouple, can use autotransformer to low voltage, with the two thermocouple wire and into a pole, the other with the carbon rods, direct ignition arc, two thermocouple wire will melt into a small ball in the head. The operation is easy, can adjust voltage, will soon be master. This welding using the autotransformer, must pay attention to the insulation to secure them. Welding of expensive platinum rhodium thermocouple is another way. Insert after regulating power as a sodium chloride aqueous solution, the other is screwed together of thermocouple, use clamps insulation thermocouple, surface, with the click of a solution on both ends of the thermocouple can fuse. These two kinds of welding methods, pay attention to safety and practice, easy to master. To welding thermocouple and can detect, determine the qualified or not.