Thermocouple is a temperature sensing element, is an instrument, it directly measure temperature, and the temperature signals into a thermoelectric emfs, through electrical instrument ( Secondary instrument) Convert the temperature of the measured medium. But the thermocouple in the process of measurement, and easily affected by environmental factors such as error and failure occurs. So in order to choose and buy to qualified thermocouple products, thermocouple when the choose and buy, on the spot test or a multimeter is used to inspect. Multimeter test how good the thermocouple method 1, before the multimeter test, we first by macroscopic observation, protecting tube corrosion penetration, whether is leaking, etc. Then in expanding the multimeter measurement on and off, prefabricated thermocouple resistance is generally not more than 2 ohm, cables are rarely more than 50 ohm resistor. Generally greater than 1 k can determine is broken. 2, the resistance value is measured with a multimeter, resistance of more than 100 k is bad. 3, measured with a multimeter ohms method to measuring thermocouple, adjusted the amount of resistance, connected to both ends, with a lighter under a little bit hot, if the multimeter pointer significantly larger or smaller suggesting that this is good, pointer that have gone bad. Can use a multimeter millivolt voltage on both ends of the gear measurement, such as no voltage is bad. Note 1, this is just a simple thermocouple and explosion-proof thermocouple judgment, long-term work in high temperature environment, prone to temperature drift, such as k type thermocouple work for a long time in more than 1000 degrees; E type thermocouple work for a long time in more than 800 degrees; S type thermocouple work for a long time in more than 1250 degrees, drift occurs, 2-5 degrees 1000 degrees drift are common.