Proper use of thermocouples

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The thermocouples available in China can operate continuously at 1600 degrees Celsius in the R type

Proper use of thermocouples & f& h. o$ F2 _Using thermocouples correctly not only enables accurate measurement of temperature levels, ensuring that products meet quality standards, but it also helps to reduce the amount of material needed for thermocouples; this saves costs while still maintaining product quality. Improper installation, as well as errors such as thermal conductivity and time lag, are the main errors that occur when using thermocouples. 1 Errors caused by improper installation, such as the location and insertion depth of the thermocouple not reflecting the actual temperature inside the furnace. In other words, the thermocouple should not be installed too close to the door or the heating elements, and its insertion depth should be at least 8–10 times the diameter of the protective tube ； The gap between the protective sleeve of the thermocouple and the furnace wall is not filled with insulating material, which allows heat to escape from the furnace or cold air to enter. Therefore, the space between the thermocouple’s protective sleeve and the holes in the furnace wall should be filled with insulating materials such as refractory mortar or asbestos rope, in order to prevent convection of hot and cold air from affecting the accuracy of temperature measurement ； The cold end of the thermocouple is too close to the furnace, causing the temperature to exceed 100℃ ； The installation of thermocouples should avoid strong magnetic and electric fields as much as possible; therefore, thermocouples and power cables should not be placed in the same conduit to prevent interference that could cause errors ； Thermocouples cannot be installed in areas where the flow of the medium being measured is very slow. When using a thermocouple to measure the temperature of gas inside a tube, it must be installed in the direction opposite to the flow velocity, and it must have full contact with the gas. 1 G9 j7 h$ v＊ B2 – Errors caused by poor insulation. If the thermocouple’s insulation is compromised, or if the protective tube and wire harnesses are covered with excessive dirt or salt deposits, this leads to poor insulation between the thermocouple elements and the furnace walls. This problem becomes more severe at high temperatures; it not only results in a loss of thermoelectric potential but also introduces interference. The errors resulting from this can sometimes reach several hundred units. 3 Errors introduced by thermal inertia: l4 P; W1 `＊ z% U） O3 C$ H. Due to the thermal inertia of the thermocouple, the reading displayed by the instrument lags behind changes in the actual temperature, and this effect is particularly pronounced during rapid measurements. Therefore, thermocouples with thinner thermoelectrodes and a smaller diameter protective tube should be used as much as possible. If the temperature measurement environment permits, the protective tube can even be removed. Due to the measurement lag, the amplitude of the temperature fluctuations detected by the thermocouple is smaller than that of the furnace temperature fluctuations. The greater the measurement lag, the smaller the amplitude of the thermocouple fluctuations, and the larger the discrepancy from the actual furnace temperature. When using thermocouples with a large time constant for temperature measurement or control, although the temperature displayed by the instrument fluctuates very little, the actual temperature of the furnace can fluctuate significantly. To measure temperature accurately, a thermocouple with a small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient, and directly proportional to the diameter of the hot end of the thermocouple, as well as to the density and specific heat of the material. To reduce the time constant, in addition to increasing the heat transfer coefficient, the most effective approach is to minimize the size of the hot end. In use, protective sleeves with good thermal conductivity, thin walls, and a small inner diameter are typically employed. In more precise temperature measurements, bare wire thermocouples without protective sheaths are used, but these thermocouples are prone to damage and should be calibrated and replaced promptly. + ]% c) J; D4 y3 U5 f+ U2 |4 Thermal resistance error + m- R7 l; T% y: When the temperature is high, if there is a layer of soot or dust on the protective tube, the thermal resistance increases, hindering heat conduction; as a result, the temperature reading is lower than the true value of the temperature being measured. Therefore, the outside of the thermocouple protection tube should be kept clean to reduce errors. A brief discussion on a few minor issues related to thermocouples: 1. What is the measurement principle of thermocouples? 3 t r (3 g7 D8. How to choose the right heat installation sleeve? - L; U7 L0 N0 Y$ V+ M6 W. The shape of the heat installation sleeve is determined primarily by the temperature, pressure, density, and flow rate of the medium, as well as the required insertion length. ASME/ANSI PTC19.3 provides comprehensive regulations on this; sleeve strength analysis software can be used to determine whether the sleeve design meets the process requirements. For heat sleeves installed on-site, it is necessary to calculate their strength. The main factors that affect the strength of these sleeves are as follows: 1) Vibrations caused by flow ； The fluid passing through the sheath tube generates vortices at a certain frequency, known as the vortex zone frequency, and this frequency is directly proportional to the flow velocity. If this frequency is close to or identical to the natural frequency of the heat sleeve, resonance occurs, resulting in the absorption of a large amount of thermal energy. This leads to high stresses that can damage the heat sleeve as well as the sensors inside it. ASME technical standards require that the ratio of the vortex region frequency to the natural frequency of the thermal sleeve should be less than 0.8. 5 i x0 c2 m) I& w2). Stress induced by flow ； Fluid flow varies with flow velocity and density, and a force is exerted by the heat sleeve; the pressure resulting from this flow can be calculated. , h（ y4 A% g3 W9 U. K: |" G3）. Process pressure ； The maximum static pressure that a heat sleeve can withstand can be calculated. "The common connection methods for heat-mounted sleeves are threaded connection, flanged connection, and welding. 0 J/ m b- k7 T: @9. How to choose the right bimetallic thermometer? 7 e d+ s" e7 e3 a3 l8 n5 u$ L When installed horizontally, choose an axial or universal bimetallic thermometer ； When installed vertically, choose a radial or universal bimetallic thermometer ； % Z% g＊ L7 L. y" @2 \6 q7 _# K, m, O& J When installed at an inclined angle, select an axial, radial, or universal bimetallic thermometer according to actual needs ； If it is necessary to set upper and lower limit alarm controls for the measurement points, an electric contact bimetallic thermometer can be selected. 10. What are the advantages and disadvantages of bimetallic thermometers? The advantages of bimetallic thermometers are their relatively low cost and intuitive readings, while the disadvantages are their limited temperature measurement range and relatively low accuracy. It is usually used as an on-site measuring and display instrument. 11. What are the characteristics of temperature transmitters? 0 j9 Q5 _4 T9 L% t; r9 E0 xThe temperature transmitter is characterized by $ U0 h） J4 Y2 `0 a4 M2 c- low static power consumption, safety and reliability, no need for maintenance, and a long service life. 1 Q3 I X- X6 \3 I$ u- has a small volume, allowing it to be integrated with thermocouples and thermal resistors; this not only facilitates installation but also saves on the costs associated with installing temperature sensors. -The transmission signal is a 4-20mA standard signal, which not only has strong interference resistance and a long transmission distance, but also helps to save on the cost of more expensive compensation wires. -It is available in formats that comply with the HART protocol as well as FF and PROFBUS bus communication protocols. f- M A9 p$ h12. What is the principle of operation of a pressure-type thermometer? According to the law of liquid expansion, for a given mass of liquid, at constant volume, the pressure of the liquid is linearly related to its temperature. The pressure and temperature of gases and vapors are also related by a certain functional relationship; therefore, the scale of a pressure thermometer should be evenly divided. A pressure-type thermometer consists of a bulb filled with a temperature-sensitive medium, a pressure-transmitting element (capillary), and a pressure-sensitive element (bimetallic spring). 13. What is the measurement principle of an infrared thermometer? - The f＊ H+ U5 c6 g. ~ q infrared thermometer consists of an optical system, a photodetector, a signal amplifier, a signal processing unit, and a display/output module. The optical system focuses the infrared radiation energy from the target within its field of view; this infrared energy is concentrated on the photodetector and converted into a corresponding electrical signal, which is then transformed into the temperature value of the target being measured. 14. How to choose the appropriate compensation wire or cable? 1 w% R2 Q (N& E4 x: \- b: R$ The compensation wires and cables for thermocouples are primarily used to extend the thermoelectromotive force of the thermocouple to secondary instruments or control rooms. There are mainly two types of compensation wires: extended type and compensatory type. The extended type uses the same material as the thermal electrode, thus offering higher precision ； The compensation type uses materials with thermoelectric potential characteristics matching those of the thermal electrode, so its accuracy is not as high as that of the extended type.

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