Knowledge || Fault Analysis of Temperature Meters

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1 Thermal resistance thermometers: The common faults of industrial thermal resistors are open circuits and short circuits. Breakage is more common in general, due to the thinness of the thermal resistance wire. Open circuits and short circuits are easy to identify; one can use the “×1Ω” setting on a multimeter. If the measured resistance is less than R0, there may be a short circuit somewhere ； If the multimeter reads infinity, it can be determined that the resistor is open-circuited. A short circuit in a resistor is generally easy to resolve; as long as it does not affect the length and thickness of the resistance wire, the shorted area can be dried out and the insulation can be strengthened. To repair a broken resistor, it is necessary to change the length of the resistance wire, which in turn affects the resistance value; therefore, it is better to replace it with a new resistor. If welding is used for repair, the resistor must be tested to ensure it functions properly before it can be used. The common faults that occur in thermal resistance temperature measurement systems during operation and their corresponding solutions are listed in the table below. 2 Proper use of thermocouples: Using thermocouples not only allows for accurate determination of temperature values, ensuring that products meet quality standards, but it also helps to reduce the amount of material needed for these thermocouples; this saves costs while still ensuring product quality. In addition to the common errors caused by reversed connection of compensation wires, incorrect use, and loose wiring (solutions: use compensation wires properly and tighten the wiring terminals), errors such as improper installation, thermal conductivity, and time lag are the main errors that occur during the use of thermocouples. 2.1. Errors resulting from 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 fireclay 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. 2.2. Errors caused by degraded insulation include situations where the thermocouple’s insulation is compromised, or where excessive dirt or salt deposits on the protective tube and pull wires result in poor insulation between the thermocouples and the furnace walls; this problem becomes more severe at high temperatures. It not only leads to a loss of thermoelectromotive force but also introduces interference, with the resulting errors sometimes reaching several hundred degrees. 2.3. Errors introduced by thermal inertia: Due to the thermal inertia of thermocouples, the reading indicated by the instrument lags behind changes in the temperature being measured; 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 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. 2.4. Thermal resistance error: At high temperatures, if there is a layer of coal ash 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. Common faults of industrial thermocouples and their solutions: 3. Bimetallic thermometers. The working principle of a bimetallic thermometer relies on two metals with different coefficients of thermal expansion; one end of these metals is welded to a fixed point, while the other end bends as the temperature changes, which in turn converts this deformation into an angle of deflection of the pointer, thereby indicating the temperature. If linear error occurs during use, the issue of inaccurate temperature indication can be addressed by adjusting the knob behind the thermometer; the thermometer can only be used after it has been calibrated and found to be accurate. 4. Pressure-type thermometer: A pressure-type thermometer measures temperature by utilizing the expansion and contraction of liquids. A sealed system composed of a bulb, a capillary tube, and a spring tube is filled with a fluid that serves as the temperature-measuring medium. When the bulb detects a change in temperature, the pressure within the sealed system changes due to the variation in the volume of the fluid; this causes a change in the curvature of the spring tube, resulting in movement at its free end. Through connecting rods and transmission mechanisms, this movement drives the pointer to rotate, thereby indicating the measured temperature on the dial. This instrument features a linear scale, a small thermowell volume, fast response time, high sensitivity, and intuitive readings. The common faults of standard pressure thermometers include a stationary pointer and large indication errors. In the case of a stuck pointer, it can be pulled out using a pointer remover; after re-setting the pointer and verifying that its function is correct, the thermometer can be used again. 4.1. Common faults: Temperature control instruments work by using thermal resistors or thermocouples to regulate the object being measured. The main common faults include the following: · Improper installation location, which prevents adequate heat exchange between the medium and the measuring element, resulting in low readings ； · It is due to poor insulation at the temperature measurement point, which results in rapid local heat dissipation and causes the temperature measured there to be lower than the system temperature ； · It is the loose wiring and poor contact that cause inaccurate readings. This results in a high value for the thermal resistor and a low value for the thermocouple ； · It’s a short-circuit fault. This results in a low or minimum value for the thermal resistor, as well as a low value or malfunction of the thermocouple ； · It is a circuit breakage (open circuit) fault. This results in the thermal resistor indicating the maximum value, while the thermocouple shows no indication or indicates the minimum value. Furthermore, when analyzing faults in temperature control instrument systems, it should be noted that the majority of instruments in such systems use electric instruments for measurement, indication, and control, which results in significant measurement lag. 4.2. Common fault analysis methods: 1) First, check the readings of the temperature instrument system. If these readings shift to their maximum or minimum values, it can be determined that there is a fault in the instrument system. The reason for this is that the temperature instrument system generally has a significant lag in its measurements, so sudden changes do not occur. Failures in temperature control instruments occur either due to broken thermocouples, thermal resistors, or compensation wires, or as a result of malfunctions in their transmitter amplifiers. 2) Check whether the indication value of the temperature control instrument system is oscillating rapidly continuously; this phenomenon is usually caused by improper adjustment of the PID control parameters. 3) Check whether the indicated values of the temperature control instrument system show large, slow fluctuations; this phenomenon is generally caused by changes in process operations. If there are no such changes in process operations, it can be determined that there is a fault within the instrument control system itself. 4) After identifying a fault in the temperature control system itself, first check the input signal to the control valve of the instrument to see if there are any changes. If the input signal remains unchanged while the control valve is still operating, it can be concluded that there is a leakage issue with the diaphragm of the control valve ； Check the input signal to the control valve actuator; if the input signal does not change while the output signal does, it indicates that there is a fault with the actuator of the instrument ； Check the input signal of the instrument locator and the output signal of the instrument’s regulator; if the regulator’s input signal remains unchanged while the output signal changes, it can be determined that there is a fault with the regulator itself.