Installation requirements for thermal resistance sensors

Thermal resistors measure temperature by using the characteristic that resistance increases with increasing temperature. Thermistors in thermal resistor sensors are a new type of semiconductor temperature measuring element. Carriers participate in the conduction of semiconductors. Since the number of carriers in semiconductors is much less than the number of free electrons in metals, its resistivity is large. As the temperature rises, more valence electrons in semiconductors are excited by heat and transition to higher energy levels to generate new electron-hole pairs. As a result, the number of carriers participating in the electricity increases, and the resistivity of the semiconductor decreases (conductivity increases). Because the number of carriers increases exponentially with the increase in temperature, the resistivity of semiconductors also decreases exponentially with the increase in temperature. Thermistors are temperature-sensitive elements made by using the characteristic of semiconductors that the number of carriers changes with temperature. When the temperature changes by 1°C, the resistance value of some semiconductor thermistors will change by (3~6)%. Under certain conditions, the change in temperature can be obtained by measuring the change in thermistor value. Installation requirements for thermal resistor sensors: The installation of thermal resistors should be conducive to accurate temperature measurement, safe and reliable, and easy to maintain, and should not affect equipment operation and production operations.

To meet the above requirements, the following points should be noted when selecting the installation location and insertion depth of the thermal resistor

1. In order to ensure sufficient heat exchange between the measuring end of the thermal resistor and the measured medium, the measurement point position should be reasonably selected, and the thermal resistor should be installed near valves, elbows, and dead corners of pipelines and equipment as much as possible.

2. Thermal resistors with protective sleeves have heat transfer and heat dissipation losses. In order to reduce measurement errors, thermocouples and thermal resistors should have sufficient insertion depth:

(1) For thermal resistors that measure the temperature of the fluid in the center of the pipeline, the measuring end should generally be inserted into the center of the pipeline (vertical installation or inclined installation). If the pipeline diameter of the measured fluid is 200 mm, the insertion depth of the thermal resistor should be 100 mm;

(2) For the 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, the protective tube can be shallowly inserted or a thermal sleeve thermal resistor can be used. The insertion depth of the shallow-insertion thermal resistor protection sleeve into the main steam pipe should be no less than 75mm; the standard insertion depth of the thermal sleeve thermal resistor is 100mm;
(3) If the temperature of the flue gas in the flue needs to be measured, even if the flue diameter is 4m, the insertion depth of the thermal resistor is 1m.
(4) When the insertion depth of the measuring element exceeds 1m, it should be installed vertically as much as possible, or a support frame and a protective sleeve weighing sensor should be installed.