Error of thermal resistance

Attention should be paid to the installation of thermistor, which is conducive to accurate temperature measurement, safe and easy maintenance, and does not affect equipment operation and production operation To meet the above requirements, the following points should be noted when selecting the installation location and insertion depth of the thermistor:
1. In order to ensure sufficient heat exchange between the measuring end of the thermistor and the measured medium, the position of the measuring point should be selected reasonably, and the installation of the thermistor near the dead corners of valves, elbows, pipelines, and equipment should be avoided as much as possible.
2. Thermistors with protective sleeves suffer from heat transfer and dissipation losses. In order to reduce measurement errors, the thermistor should have sufficient insertion depth:
(1) For thermocouples used to measure the fluid temperature at the center of pipelines,
Generally, the measuring end should be inserted into the center of the pipeline (vertically or obliquely installed) If the diameter of the tested fluid pipeline is 200 millimeters, the insertion depth of the thermistor should be selected as 100 millimeters;
(2) For temperature measurement of high temperature, high pressure, and high-speed fluids (such as main steam temperature), in order to reduce the resistance of the protective sleeve to the fluid and prevent the protective sleeve from breaking under the action of the fluid, shallow insertion of the protective tube or the use of a hot sleeve type thermal resistor can be adopted. The shallow insertion type thermal resistor protective sleeve should be inserted into the main steam pipeline to a depth of not less than 75mm; The standard insertion depth for hot sleeve thermal resistors is 100mm;
(3) If it is necessary to measure the temperature of the flue gas in the flue, although the diameter of the flue is 4m, the insertion depth of the thermistor is 1m;
(4) When the insertion depth of the measuring element exceeds 1m, it should be installed vertically as much as possible, or support frames and protective sleeves should be added.
Proper use of thermal resistors can not only accurately obtain temperature values and ensure product quality, but also save material consumption of thermal resistors, which not only saves money but also ensures product quality. Incorrect installation, thermal conductivity, and time lag are the main errors in the use of thermocouples.
1. Errors caused by improper installation:
If the installation position and insertion depth of the thermistor cannot reflect the true temperature of the furnace, in other words, the thermistor should not be installed too close to the door and heating area, and the 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 wall is not filled with insulation material, which may cause hot overflow or cold air to enter the furnace. Therefore, the gap between the thermal resistance protective tube and the furnace wall hole should be blocked with insulation materials such as refractory mud or asbestos rope to prevent the convection of cold and hot 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 ℃; The installation of thermocouples should avoid strong magnetic and electric fields as much as possible, so the thermistor and power cable should not be installed in the same conduit to avoid introducing interference and causing errors; Thermistors cannot be installed in areas where the measured medium flows very little. When measuring the gas temperature inside a tube using a thermistor, the thermocouple must be installed against the flow velocity direction and fully in contact with the gas.
2. Errors introduced by insulation deterioration:
If the thermistor is insulated, excessive dirt or salt residue on the protective tube and wire plate can cause poor insulation between the electrodes of the thermistor and the furnace wall, which is more severe at high temperatures. This not only causes loss of thermoelectric potential but also introduces interference, resulting in errors that can sometimes reach up to 100 degrees.
3. Error introduced by thermal inertia:
Due to the thermal inertia of the thermistor, the indicated value of the instrument lags behind the changes in the measured temperature, which is particularly prominent during rapid measurements. Therefore, thermal resistors with thinner thermoelectric electrodes and smaller protective tube diameters should be used as much as possible. When the temperature measurement environment permits, the protective tube can even be removed. Due to measurement lag, the amplitude of temperature fluctuations detected by thermal resistance is smaller than that of furnace temperature fluctuations. The larger the measurement lag, the smaller the amplitude of the thermal resistance fluctuation, and the greater the difference from the actual furnace temperature. When using a thermistor with a large time constant for temperature measurement or control, although the temperature displayed on the instrument fluctuates very little, the actual furnace temperature may fluctuate greatly. In order to accurately measure temperature, a thermistor 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 thermal end of the thermal resistance, 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 way is to minimize the size of the thermal end as much as possible. In use, materials with good thermal conductivity and protective sleeves with thin pipe walls and small inner diameters are usually used. In more precise temperature measurements, bare wire thermistors without protective sleeves are used, but thermistors are prone to damage and should be calibrated and replaced in a timely manner.