The correct use of thermocouples not only ensures accurate temperature readings and product quality, but also saves on thermocouple material consumption, thus saving money and guaranteeing product quality. Incorrect installation, thermal conductivity, and time lag are the main sources of error in thermocouple use.
1. Errors introduced by improper installation
For example, the position and insertion depth of the thermocouple may not reflect the true temperature of the furnace. In other words, the thermocouple should not be installed too close to the door or heating elements, and the insertion depth should be at least 8-10 times the diameter of the protective tube; the gap between the thermocouple's protective sheath and the wall is not filled with insulating material, causing heat to escape from the furnace or cold air to enter. Therefore, the gap between the thermocouple protective tube and the furnace wall hole should be sealed with refractory clay or asbestos rope 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 body, causing the temperature to exceed 100°C; the thermocouple should be installed as far away from strong magnetic and electric fields as possible, so the thermocouple and power cables should not be installed in the same conduit to avoid interference and errors; the thermocouple cannot be installed in an area where the measured medium has little flow. When using a thermocouple to measure the temperature of gas in a pipe, the thermocouple must be installed against the direction of flow and in full contact with the gas.
2. Errors introduced by poor insulation
For example, if the thermocouple is poorly insulated, or if there is excessive dirt or salt residue on the protective tube and terminal block, resulting in poor insulation between the thermocouple poles and the furnace wall, this is even more serious at high temperatures. This will not only cause loss of thermoelectric potential but also introduce interference, and the resulting error can sometimes reach hundreds of degrees.
3. Errors introduced by thermal inertia
Due to the thermal inertia of the thermocouple, the instrument's indicated value lags behind the change in the measured temperature. This effect is particularly prominent during rapid measurements. Therefore, thermocouples with thinner thermoelements and smaller protective tube diameters should be used whenever possible. If the temperature measurement environment permits, the protective tube can even be removed. Due to measurement lag, the amplitude of temperature fluctuations detected by the thermocouple is smaller than the amplitude of furnace temperature fluctuations. The greater the measurement lag, the smaller the amplitude of the thermocouple fluctuations, and the greater the difference from the actual furnace temperature. When using a thermocouple with a large time constant for temperature measurement or control, although the temperature displayed by the instrument fluctuates very little, the actual furnace temperature may fluctuate significantly. To accurately measure temperature, 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 thermocouple's hot junction, the density of the material, and the specific heat. To reduce the time constant, in addition to increasing the heat transfer coefficient, the most effective method is to minimize the size of the hot junction. In practice, materials with good thermal conductivity are usually used, along with thin-walled protective tubes with small inner diameters. In more precise temperature measurements, bare-wire thermocouples without protective tubes are used, but these thermocouples are easily damaged and should be calibrated and replaced promptly.
