What is the difference between the signal output of thermocouple and thermal resistor?

What is the difference between the signal output of thermocouples and thermal resistors? Both thermocouples and thermal resistors are sensors for detecting temperature, but thermocouples and thermal resistors are two different detection elements. The difference between these two monitoring elements is very large.

1. The nature of the signal. The thermal resistor itself is a resistor. The change in temperature causes the thermal resistor to produce a positive or negative resistance change; while the thermocouple produces a change in induced voltage, which changes with the change in temperature.

2. The temperature ranges detected by the two sensors are different. The thermal resistor generally detects a temperature range of 0-150 degrees, and the higher measurement range can reach about 600 degrees (of course, negative temperatures can be detected). Thermocouples can detect a temperature range of 0-1000 degrees (or even higher). Therefore, the former is a low-temperature detection, and the latter is a high-temperature detection. What is the difference between the signal output of thermocouples and thermal resistors?

3. From the material point of view, the thermal resistor is a metal material with temperature-sensitive changes. The thermocouple is a bimetallic material, that is, two different metals. Due to the change in temperature, an electric potential difference is generated at both ends of two different metal wires.

4. The input modules of thermal resistors and thermocouples corresponding to PLC are also different. This sentence is fine, but generally PLCs are directly connected to 4~20ma signals, while thermal resistors and thermocouples are generally connected to PLCs with transmitters. If you connect to DCS, you don't need to use a transmitter! Thermal resistors are RTD signals, and thermocouples are TC signals!

5. PLCs also have thermal resistor modules and thermocouple modules, which can directly input resistance and thermocouple signals.

6. Thermocouples have models such as J, T, N, K, and S. Some are more expensive than resistors, and some are cheaper than resistors, but including compensation wires, the comprehensive cost of thermocouples is high.

When installing thermocouples and thermal resistors, attention should be paid to accurate temperature measurement, safety, reliability, and easy maintenance, and not affecting equipment operation and production operations. To meet the above requirements, pay attention to the following points when selecting the installation location and insertion depth of thermocouples and thermal resistors:

1. In order to ensure sufficient heat exchange between the measuring end of the thermocouple and thermal resistor and the measured medium, the measuring point position should be reasonably selected, and try to avoid installing thermocouples or thermal resistors near valves, elbows, and dead corners of pipelines and equipment.

2. Thermocouples and 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 thermocouples that measure the temperature of pipeline fluids,

the measuring end should generally be inserted into the pipeline (vertically or tilted). If the pipeline diameter of the measured fluid is 200 mm, the insertion depth of the thermocouple or 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 heat-shrouded thermocouple can be used. The shallow insertion thermocouple protective sleeve should be inserted into the main steam pipeline to a depth of not less than 75 mm; the standard insertion depth of the heat-shrouded thermocouple is 100 mm;

(3) If the temperature of the flue gas in the flue needs to be measured, even if the flue diameter is 4 m, the insertion depth of the thermocouple or thermal resistor is 1 m;

(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 protective sleeve should be installed.

Correct use

The correct use of thermocouples can not only accurately obtain the temperature value and ensure the product quality, but also save the material consumption of thermocouples, which can save money and ensure product quality. Improper installation, thermal conductivity and time lag errors are the main errors in the use of thermocouples.

1. Errors introduced by improper installation

For example, the installation position and insertion depth of the thermocouple cannot reflect the actual temperature of the furnace. In other words, the thermocouple should not be installed too close to the door or the heating place, and the insertion depth should be at least 8 to 10 times the diameter of the protection tube; the gap between the thermocouple protection tube and the wall is not filled with insulating materials, which causes heat overflow or cold air intrusion in the furnace. Therefore, the gap between the thermocouple protection tube and the furnace wall hole should be blocked with insulating materials such as refractory mud or asbestos rope to prevent hot and cold air convection from affecting the accuracy of temperature measurement; the cold end of the thermocouple is too close to the furnace body so that the temperature exceeds 100℃; the installation of the thermocouple should avoid strong magnetic fields and strong electric fields as much as possible, so the thermocouple and the power cable should not be installed in the same conduit to avoid interference and errors; the thermocouple cannot be installed in the area where the measured medium rarely flows. When using a thermocouple to measure the gas temperature in the tube, the thermocouple must be installed against the flow direction and fully in contact with the gas.

2. Errors introduced by insulation degradation

If the thermocouple is insulated, the protection tube and the wire drawing board are too dirty or salty, resulting in poor insulation between the thermocouple poles and the furnace wall, which is more serious at high temperatures. This will not only cause the loss of thermoelectric potential but also introduce interference. The errors caused by this can sometimes reach hundreds of degrees.

3. Errors introduced by thermal inertia

Due to the thermal inertia of the thermocouple, the indicated value of the instrument lags behind the change of the measured temperature.

This effect is particularly prominent when performing rapid measurements. Therefore, thermocouples with thinner thermocouples and smaller protection tube diameters should be used as much as possible. When the temperature measurement environment permits, the protection tube can even be removed. Due to the measurement lag, the amplitude of the temperature fluctuation detected by the thermocouple is smaller than the amplitude of the furnace temperature fluctuation. The greater the measurement lag, the smaller the amplitude of the thermocouple fluctuation, and the greater the difference with the actual furnace temperature. When using a thermocouple with a large time constant to measure or control temperature, although the temperature displayed by the instrument fluctuates very little, the actual furnace temperature may fluctuate greatly. In order to accurately measure the temperature, a thermocouple with a small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient and is proportional to the diameter of the hot end of the thermocouple, the density of the material and the specific heat. If you want to reduce the time constant, in addition to increasing the heat transfer coefficient, the effective way is to minimize the size of the hot end. In use, materials with good thermal conductivity, thin walls and small inner diameter protective sleeves are usually used. In more precise temperature measurements, bare wire thermocouples without protective sleeves are used, but thermocouples are easily damaged and should be calibrated and replaced in time.

4. Thermal resistance error

At high temperatures, if there is a layer of coal ash on the protective tube and dust is attached to it, the thermal resistance increases, hindering the conduction of heat. At this time, the temperature indication is lower than the true value of the measured temperature. Therefore, the outside of the thermocouple protective tube should be kept clean to reduce the error.