The principle of thermocouple thermometer is to 'galvanometer - copper - wire and copper wire' series into a loop, where the ends of the wire and copper wire connection, can form two 'joint' ( 结) , if the two joint temperature is different, can produce a voltage between them, in micro ammeter to measure the weak current through the wire and copper wire. To be used for thermocouple thermometer, must first make the following calibration. Put a joint in the ice water, another joint in boiling water, write down the current strength, this is the difference in temperature of 100 ℃ when the current value. For two kinds of known metal wires, current value and is directly proportional to the temperature difference of the two joint, measurement range is very large, namely by the - 200 ℃ to 1700 ℃, the sensitivity is high. Mercury thermometer in the freezing point of mercury ( - 38. 87℃） With the boiling point ( 356. 7℃） Thermocouple are two different metal materials together, under the different temperature, its both ends will produce different current, called the thermoelectric potential. Other thermometer, such as glass thermometer is to rely on thermal fluid principle of heat bilges cold shrink; Platinum resistance thermometer is platinum resistance under different temperature change to guide, and semiconductor, bimetallic just different principle. Thermocouple wiring must be used when a cold junction compensation of thermocouple output value is the mV and the temperature of hot end and cold end temperature difference between the corresponding, if there is no cold junction compensation, thermocouple output mV values of the corresponding temperature with thermocouple cold end temperature, is the real temperature of the hot end. If you have a thermocouple is through compensation wires to control box, the resulting error is the environment temperature control box; If you are using ordinary copper wires to control box, the error is the thermocouple junction box environment temperature. Acquisition of temperature on the low side. There are many types in the application of temperature measurement transducer, thermocouple is a common, can be widely used in automobile, household and other fields. And RTD, thermistor, temperature detection integrated circuits ( IC) Compared to the thermocouple is capable of detecting a wider temperature range, high cost performance. In addition, the firm, reliability and rapid response time of thermocouple makes it the first choice of various kinds of work environment. 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, can be ideal for precision measurement system. Thermocouple signal level is very low, often need to enlarge or high resolution data converter for processing. If the above problems, low price, easy to use, wide temperature range of the thermocouple is widely used. Thermocouple based thermocouple temperature measuring device is difference, composed of two different metal/alloy wire, a is used as a side, another is used as the negative side. Table 1 lists the four kinds of commonly used thermocouple type, metal, and the corresponding temperature measurement range. Each thermocouple in the specified temperature range has a unique thermoelectric properties.