On 2018-08-08 14:48:54
Requirements for thermocouples for experimental furnaces
The thermocouple used in the experimental electric furnace should be selected reasonably according to the requirements. The commonly used thermocouples are as follows:
1. Platinum-rhodium/platinum thermocouple -- the dividing number is S, the positive electrode is 90% platinum and 10% rhodium alloy, and the negative extremely pure platinum wire.
The advantages of this thermocouple are that it is easy to prepare platinum-rhodium alloy with very high purity, so it is easy to replicate, and the temperature measurement accuracy is high, which can be used as the reference thermocouple in the range of 630.74 -- 1064.43℃ in the international practical temperature scale. Its high physicochemical stability, suitable for use in oxidation and neutral atmosphere; It has a higher melting point, so the upper limit of temperature measurement is also high. In industrial measurement, it is generally used to measure the temperature above 1000℃, can be used continuously for a long time below 1300℃, and can be used for short-term temperature measurement up to 1600℃.
Platinum rhodium/platinum thermocouple disadvantage is that the price is expensive, thermoelectric power is small, the reducing gas, metal, metal oxide, and silicon oxide and sulfur oxide atmosphere use will soon be contamination and deterioration, so he must use it in the atmosphere and protect casing, in addition, the thermocouple thermoelectric performance of nonlinear is larger, the hot electrode can sublimate at high temperatures, make rhodium molecules penetrate into the platinum extremely contamination, it lead to thermoelectric potential instability.
Two, platinum-rhodium 30/ platinum-rhodium 6 thermocouple, referred to as double platinum-rhodium thermocouple. The positive and negative poles of the thermocouple are platinum-rhodium alloy, only the proportion of alloy content is different, the positive pole contains rhodium 30%, the negative pole contains rhodium for 6%, double platinum-rhodium thermocouple's anti-fouling ability is strong, in the temperature measurement 1800℃ temperature still has good stability. Its temperature measurement accuracy is high, suitable for oxidation, neutral medium, can be long-term continuous measurement of 1400-1600℃ high temperature, short-term measurement up to 1800℃.
Double platinum rhodium thermocouple sensitivity is low, use should be equipped with a high sensitivity display instrument. At room temperature, the temperature has little effect on the thermoelectric potential, so it is not necessary to make temperature compensation.
Nickel-chromium/nickel-chromium thermocouple -- it is labeled K, the positive component is 9-10% chromium, 0.4% silicon, the rest is nickel, the negative component is 2.5-3% silicon, <0.6% chromium, the rest is nickel.
The advantages of this kind of thermocouple are strong oxidation resistance and corrosion resistance, good other academic stability, large thermoelectric potential, thermoelectric potential and temperature linear relationship is good, the price of the thermal electrode material is cheap, can be used in 1000℃ below the long-term continuous, short-term temperature measurement up to 1300℃.
The disadvantage of nickel-chrome/nickel-silicon thermocouples is that they can be easily corroded at temperatures above 500 ° c and in reductive media, as well as in sulphide atmospheres. Therefore, protective casing must be added when working in these atmospheres. In addition, the temperature measurement accuracy of these thermocouples is also lower than that of platinum-rhodium/platinum-platinum thermocouples.
The commonly used thermocouple materials of the experimental furnace are required as follows:
1. High temperature resistance. The temperature measuring range of the thermocouple mainly depends on the high temperature performance of the hot electrode.
2. Good redominance -- the thermocouple with the same two kinds of hot electrode materials is required to have the same electrothermal performance and stability, so that the thermocouple can be produced in batches with good interchangeability;
3. High sensitivity and good linearity -- it is required that the thermoelectric potential generated by the electric couple is large enough and has a linear relationship with the temperature;
In addition to the above requirements, thermoelectric materials are expected to meet the resistance coefficient and resistance temperature coefficient as small as possible, and its price is cheap, adequate supply.