Thermocouple protection tube is to protect the casing of thermocouple core. 1, the thermocouple ( 热电偶) Is commonly used in temperature measuring instrument of temperature measuring element, and it is directly measured temperature, and the temperature signals into a thermoelectric emfs, through electrical instrument ( Secondary instrument) Convert the temperature of the measured medium. 2, the appearance of all kinds of thermocouple is often due to the need to vary, but they are roughly the same basic structure, usually by the hot electrode, insulation sleeve to protect the main parts such as pipe and connection box, and usually display meter, recording meter and electronic regulator. Thermocouple cold junction compensation principle of thermocouple measuring temperature to the cold end ( Measure the end for the hot end, by the end of the lead connected to the measurement circuit is called cold junction) Temperature remains constant, the size of thermoelectric potential and measured temperature in certain proportion relationship. When measuring, the cold end ( Environment) Changes in temperature, will be affected the accuracy of measurement. Take action at the cold end compensation due to the impact of the cold end temperature change is called the thermocouple cold junction compensation. Thermocouple cold junction compensation is usually used in a composed of thermocouple cold end series bridge. The three bridge arms for standard resistance, in addition, there are a bridge arm by ( Copper) Thermocouple. When the cold end temperature change ( Rise, for example) Thermocouple thermoelectric potential will change (to produce Reduce) The thermocouple resistance in series, while the bridge will be change and make the change at the ends of the bridge voltage also ( Higher) 。 If the parameter is a good choice and wiring is correct, produced by the bridge voltage as it happens and thermoelectric potential changes over temperature equal to the amount, the thermocouple measurement circuit, the total output voltage ( Potential) Exactly reflect the measured temperature value. This is the thermocouple cold junction compensation principle. Thermocouple is don't need to cold junction compensation, because of the so-called 'cold junction compensation' refers to the thermocouple thermoelectric potential is zero for the standard measurement, it does not need to drive source. And instrument at room temperature, so for thermocouple, it is not at 0 degrees as a standard for measuring, thus the uncertainty. So in the circuit of instrument, is typically a cold junction compensation circuit. Thermocouple and thermocouple temperature measuring principle is different, it is to rely on their own resistance with the temperature change and change the principle of temperature measurement, we give platinum resistance for a current incentives, direct reading voltage on both ends, with almost no instrument in temperature. If the four wire measurement, the distance of the instrument and sensor can also be further. There are many types in the application of temperature measurement transducer, thermocouple is a common one kind of temperature sensor, it can be widely used in industrial production of various temperature control areas. It and RTD, thermistor, temperature detection integrated circuits ( IC) Compared to the thermocouple sensors that can detect a wider temperature range, high cost performance. In addition, the firm, reliability and rapid response time of thermocouple made it under various working conditions. Thermocouple, of course, there are some defects in the temperature measurement, for example, poor linear characteristic. Although they are more than RTD, temperature sensor IC can be measured with wide temperature range, but the linearity is compromised. In addition, RTD and temperature sensor IC can provide higher sensitivity and accuracy, is accurate temperature measurement system, an ideal choice. Thermocouple signal level is very low, often need to enlarge or high resolution data converter for processing. If the above problems, thermocouple and low price, easy to use, wide temperature range makes it widely used.