1. The basic principle of thermocouple temperature measurement
Figure 1 : Seeback Effect
Figure 1
2. Classification& Feature
Criteria for thermocouple selection include cost, maximum and minimum operating temperatures, chemical stability, material compatibility, atmospheric protection, mechanical limitations, exposure time limits, sensor lifetime, sensitivity, and outputEMF.
2.1. Cu-CuNi (Constantan) Alloy Thermocouple (Type T)
T-type thermocouple is used at low temperatures. The temperature measurement range is -250~ 350°C. Copper will oxidize rapidly when it is higher than 400 °C. Therefore,we must avoid problems caused by copper electrodes with high thermal conductivity. Since one lead of this thermocouple is copper, no special compensating cable is required. T-type thermocouple has good sensitivity, high stability, fast response speed, and low price. The pair wire has good uniformity and can be made very slender which be bent arbitrarily according to the actual needs of measurement under complex conditions. It has high mechanical strength, good pressure resistance and good applicability in industrial measurement.
2.2. Fe-CuNi Alloy Thermocouple (Type J)
J-type thermocouple is popular forit’s high Seebeck coefficient and low price. The maximum temperature of continuous operation in oxidizing gas can reach 800 °C, and it can be well applied in reducing gas at 0-550°C, but it will degrade rapidly whenthe temperature is higher than 550°C. It is mostly used in oil refining and chemical industries that are resistant to hydrogen and carbon monoxide gas corrosion.
2.3. NiCr - CuNi (Constantan) Alloy Thermocouple (Type E)
E-type thermocouple provides high output at -250 ~ 900 °C. It combines the advantages of K-type and T-type, and has the advantages of large Seebeck coefficient, good sensitivity, low thermal conductivity, and strong corrosion resistance.
2.4. NiCr-NiSi alloy thermocouple (Type K)
K-type thermocouple is the most commonly used thermocouple and is designed for use in oxidizing environments. The maximum continuous use temperature is 1100 °C, although there is drift and loss of accuracy due to oxidation above 800 °C. K-type thermocouple has hysteresis instability in the range of 300 to 600 °C, which may lead to varying degrees of error.
2.5. NiCrSi-NiCrMg alloy (TypeN)
N-type thermocouple has strong oxidation resistance at high temperatures up to 1300 °C, good long-term stability of thermoelectromotive force and good reproducibility of short-term thermal cycles. It has good resistance to nuclear radiation and low temperature. Due to the improved linear response stability and the conversion algorithm between thermal electromotive force and temperature, the K-type instability is effectively solved.N-type has a slightly lower voltage-temperature curve than K-type.
2.6. Platinum Rhodium 30 - Platinum Rhodium 6 (Type B)
B-type thermocouple is suitable for continuous use up to 1600°C and intermittent use up to 1800°C. However, there is a local minimum value of its thermal electromotive force, which is specifically manifested in the double-valued ambiguity between 0~42°C. It can be used for long-term in oxidizing and neutral gas environment, and short-term in vacuum.
2.7. Platinum Rhodium 10 - Platinum (Type S)
S-type thermocouple has stable physical and chemical properties and high measurement accuracy. It is often used as a reference thermometer for precision measurement. The continuous temperature measurement in an oxidizing or inert gas environment can reach up to 1400 °C, and the short-term measurement can reach up to 1650°C.
2.8. Platinum Rhodium 13 - Platinum (R type)
R-type thermocouple gives similar performance to Type S, but with slightly higher output and improved stability.
3. Measuring circuit and cold junction compensation
Figure2 shows the circuit for the actual measurement of the thermal EMF, which is installed at the input of the measuring instrument at the position where the thermocouple is in direct contact with the temperature. The measuring instrument can be a digital voltmeter or a direct-reading analog/digital conversion device. The input impedance of the voltage measuring instrument is greater than the resistance of the thermocouple and its extension wire. Because in actual situations, it is necessary to prevent excessive current from flowing through the circuit, so as to avoid possible other thermoelectric effects or cause the IR drop of the entire thermocouple and its wires. The voltage generated by the metal thermocouple is not high, generally 10-80 μV/°C, and the digital voltmeter needs to have a high enough resolution when measuring the output of the thermocouple.
Figure 2
In actual measurement, it is not only necessary to measure the thermoelectric potential generated by the thermocouple due to temperature, but also the temperature of the cold junction of the thermocouple must be known, so that the temperature of the measurement junction can be finally obtained. There are two ways to deal with the temperature of the cold junction, the first is the freezing point method,put the cold junction of the thermocouple into an ice-water mixture or a zero-degree thermostat, so that the temperature of the cold junction can be guaranteed to be 0°C. The advantage of this method is that the error is small and the disadvantage is the ice-water mixture is cumbersome to make, and it cannot be guaranteed to be absolutely 0°C the zero-degree thermostat is limited by the diameter of the couple wire; the second is the cold junction compensation method, the cold junction of the couple is directly connected to the measuring instrument, and the temperature of the cold junction is measured , convert the temperature value of the cold end into a certain thermoelectric potential and add it to the measurement data, so as to obtain the temperature of the measurement end. Its advantage is that it is simple, without intermediate links and additional equipment, and can carry out large-scale data collection. The disadvantage is that the cold junction temperature measurement will introduce errors, which will affect the uncertainty of the measurement results.
4. PhysicalIsolation& ErrorElimination
Due to the factors of the environment and the measured object, many tests need to electrically or chemically isolate the thermocouple wire to prolong the service life of the electrode and improve the measurement accuracy. Insulation materials generally need to meet air tightness, mechanical strength, chemical stability, thermal conductivity, corrosion resistance and other conditions, the commonly used insulating bushing materials are mainly divided into three categories: metal, non-metal and composite materials. Metal material protective sheaths include titanium alloy for corrosive environment below 250°C; low carbon steel for oxidizing environment at 600°C; stainless steel for acid-resistant, corrosion-resistant and high-strength environment in the range of 800-1000°C; made of platinumprecious metal casings such as alloys and tungsten-rhenium alloys can be used for high-temperature measurements in reducing environments ranging from 1400 to 2000°C, but they are expensive. Non-metallic casings mainly include quartz tubes, which are generally used below 1100°C, and have good corrosion resistance, thermal shock resistance, and air tightness; Porcelain tubes can be used up to 1500°C, and can be used for a long time below 1200°C. It has good wear resistance, corrosion resistance, high temperature strength and electrical insulation performance, but poor air tightness and thermal shock resistance; magnesium oxide can be used up to 1800 °C, and has good high temperature insulation performance, high thermal conductivity, and strong corrosion resistance. The disadvantage is poor thermal shock resistance; graphite can be used up to 2000°C. Its thermal conductivity and thermal shock resistance are good, and it is corrosion resistant, but its mechanical strength is not high. Carbon free ions will affect the thermoelectric characteristics of the thermode at high temperature. Composite protective tubes are mainly made of different proportions of ceramics and metals. The insulation temperature ranges from 1200 to 1800°C, and the maximum temperature can reach 2200°C. It has good thermal conductivity, thermal shock resistance, heat resistance and wear resistance. It is generally used for temperature measurement and insulation of liquid metals, such as molten iron, molten steel, and non-ferrous metal melts.
Although the metal casing has good heat resistance, its high temperature wear resistance is poor. The non-metal casing has good corrosion resistance, heat resistance and wear resistance, but its thermal shock resistance is poor and fragile. Therefore, a special composite coating protective sheath has been produced by using material surface treatment technology in the insulating sheath. At present, the wear-resistant surface carburizing and boronizing process has been applied, and the application of polymer materials such as chlorinated vinyl resin, polyethylene, etc. PTFE, which can be coated on the surface of the metal casing to obtain a good corrosion-resistant coating, is made of other new materials and new processes, such as intermetallic compound coatings, gradient functional coating materials, nano-ceramic coatings, etc. It is also used in different special working conditions.
Since the thermal electromotive force generated by the thermocouple is only at the millivolt level, precision measuring equipment is required to measure small voltages and be able to distinguish small voltage changes, but at the same time, the external noise effect of the electric field and magnetic field in the measurement environment has a great influence on the measurement of low electromotive force. The noise sources affecting thermocouple measurement mainly include common-mode noise generated by ground loops, normal-mode noise generated by electromagnetic fields, and static noise generated by rotating equipment. The best way to prevent ground loops is to use isolated thermocouple that avoid grounding the thermocouple junction, and to use a measurement system with high impedance to ground. The method to reduce normal mode noise is to use twisted pair cables with small gaps for the cables of the measurement circuit, and reduce the order of magnitude of common mode noise in the circuit by connecting filter elements, or avoid parallel arrangement with nearby high current lines as much as possible. noise disturbance. Static noise can be effectively suppressed by using shielded cables, and can also be reduced by using a high-impedance measurement system to ground and effectively grounding.
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