Temperature plays an extremely important role in the control of industrial production processes. The temperature measurement system provides accurate data for monitoring and automatic control in the industrial production process, reflects the temperature at the measuring point, and ensures safe, economical and efficient operation of production. The temperature measurement system is mainly composed of temperature sensitive elements, conversion units and display or control units. Generally, the conversion unit and display or control unit are completed by electronic circuits, and their fault analysis follows the general circuit analysis rules. This article mainly makes a simple analysis of the common faults of thermal resistance temperature elements. Working principle of thermal resistance elements Commonly used thermal resistance elements are copper thermal resistance and platinum thermal resistance. Since the conductivity of these two metals changes linearly with temperature, the method of detecting the resistance of the element is adopted to achieve the purpose of detecting temperature. The resistance of the temperature element at a temperature of 0°C is called the element's graduation number. The most common types are Cu50 and Pt100. Classification of common faults of thermal resistor temperature measuring elements The thermal resistor temperature measuring system works relatively stably and reliably, and the average service life of thermal resistor elements can reach 2 years. Some common faults can be divided into the following 4 categories: 1. The thermal resistor element is broken. The fault phenomenon is generally manifested as positive overflow of displayed data. The reason may be that the temperature element is open, the connecting wire is in poor contact or disconnected. Generally, the vibration of the measuring point is large or the protective sleeve of the element is not fixed firmly, which may cause the element to be broken or the joint to be loosened when the system is working, causing this fault. 2. Short circuit of the temperature element mainly refers to the complete or partial short circuit of the temperature element. The fault phenomenon is generally manifested as a negative temperature display value. Poor quality of temperature elements, excessive vibration in the working environment, and short circuit of the connecting wire can cause this fault. 3. Breakthrough refers to the soft breakdown of the thermal resistor. The fault is manifested as the display is sometimes normal and sometimes abnormal. The abnormal phenomenon is jump or inaccurate indication, deviating from the correct value. 4. Penetration is more common in temperature measurement environments containing flowing solid particles. When the pressure is negative, the displayed value deviates from the true value and is close to the room temperature. The reason is that the protective sleeve of the component is worn through by the flowing solid medium, and the measured temperature is biased towards the room temperature due to air leakage. If the actual temperature is higher than the room temperature, the displayed temperature is low, and vice versa. When the pressure is positive, the detection medium will contaminate the component and affect the measurement. If the medium is conductive, a short circuit will occur; if the medium is not conductive, the phenomenon is not obvious. Phenomenon of soft breakdown of thermal resistors After the soft breakdown of thermal resistors, the phenomenon is rather strange: when the fault occurs, the maintenance cycle is only one week on average. Each maintenance can solve the problem by replacing the component, but the replaced component is completely normal after measurement. Even if the component is not removed, it is normal to measure the resistance value with a multimeter in the operating environment, and the temperature value after measuring the resistance conversion corresponds to the actual value. The entire system can return to normal when the power is re-supplied. When the fault occurs again later and the system is abnormal, it is sometimes displayed as overflow, sometimes unstable, and sometimes low. Characteristics of soft breakdown of thermal resistors Through summary, the soft breakdown of temperature elements has the following important characteristics: 1. This fault is prone to occur when the quality of platinum resistor elements is poor. According to statistics, this type of fault accounts for 86% of the total faults. It can be seen that the poor quality of platinum resistor elements is the main cause of platinum resistor soft breakdown. The poor quality of platinum resistor elements is manifested in low skeleton insulation, poor quality of platinum (copper) metal, and low manufacturing process level. 2. This fault is prone to occur when the operating temperature of the thermal resistor temperature element exceeds the upper limit of its working range or is near the upper limit. For example, if Pt100 often works in an environment of 500℃, the probability of failure is very high. 3. The thermal resistor element can return to normal after leaving the working environment, can work normally at low temperatures, and the soft breakdown fault disappears. This is an important feature of the soft breakdown of the temperature element. Electrical mechanism of soft breakdown of thermal resistors After the soft breakdown of the thermal resistor temperature element, it will lose its original electrical characteristics. The thermal resistor element should have the basic characteristics of ordinary resistors at any temperature within its normal working range. After the thermal resistor soft breakdown, its U and I curves are no longer linear. When the current reaches a certain value, the insulation decreases, the thermal resistor breaks down, the resistance decreases, and the resistance and temperature no longer have a corresponding relationship. The actual UI characteristic diagram of the thermal resistor soft breakdown element is shown in the figure below. After actual measurement, the breakdown temperature of the thermal resistor that has experienced soft breakdown failure is about 350℃, and the minimum breakdown current ranges from 0.6-0.8mA. The measurement current of the general measurement circuit is .06mA. Some currents are even more than 1mA to enhance sensitivity. Therefore, for temperature elements of poor quality, soft breakdown failure is prone to occur when the measurement circuit current is greater than .06mA under high temperature working conditions. When the loop current is less than 0.6mA, the soft breakdown phenomenon disappears. Because the multimeter resistance range measures the loop current of only about 0.1mA, it is difficult to detect the fault of the soft breakdown component with a multimeter. Solutions to thermal resistor temperature measurement faults According to the various fault principles of thermal resistor temperature measurement, corresponding treatment is carried out, see Table 1 Fault classification Solutions to thermal resistor temperature measurement faultsReinforce the thermal resistor protection sleeve, select the measurement point with small vibration, select anti-vibration components, tighten the signal line connector, and use good quality and long-life externally wound integrally sintered platinum resistor components to reduce vibration wear. Replace with good quality externally wound integrally sintered platinum resistor components, select the appropriate type of platinum resistor, use wear-resistant sleeves, change the installation angle, and install protective devices
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