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What is the difference between a thermocouple and a thermal resistor? - Case

What is the difference between a thermocouple and a thermal resistor?

509 Published by at October 14,2019

What is the difference between a thermocouple and a thermal resistor?

As two contact temperature sensors: thermocouple and thermal resistance, the two names are only one word and can be used as a sensor to measure the temperature of the object. For selecting a thermocouple or selecting a thermal resistance, it should be judged according to the measured object environment. Many people are not sure what to choose, so when choosing a temperature sensor, you need to fully understand the difference between the thermal resistance and the thermocouple temperature sensor.

How to distinguish between thermocouples and ntc thermistors

1, thermocouple English Thermocouple, referred to as TC, works as follows: output linear millivolt signal with temperature changes. The meter converts the signal to a temperature signal.

2, thermal resistance English Resistance short name ntc thermistor works: the resistance value changes linearly with temperature.

3. The temperature transmitter can convert the thermocouple mV voltage signal or the resistance value signal of the thermal resistor into a 4-20 mA standard signal for automation system control.
 
4. In general, thermal resistance is cheaper than thermocouples.

Thermocouple and thermal resistance which is better
The choice of thermocouple should be based on the temperature range of use, the required accuracy, the atmosphere of use, the performance of the object to be measured, response time and economic benefits.

1. Measurement accuracy and temperature measurement range selection

When the temperature is between 1300~1800°C and the accuracy is relatively high, the type B thermocouple is generally used; the precision is not high, and the atmosphere allows for the use of tungsten-rhodium thermocouples. Above 1800°C, tungsten-rhodium thermocouples are generally used; 1000~1300°C requires high precision S-type thermocouple and N-type thermocouple; K-type thermocouple and N-type thermocouple are generally used below 1000°C, E-type thermocouple is generally used below 400°C; 250°C And the negative temperature measurement generally uses a T-type galvanic couple, and the T-type thermocouple is stable and has high precision at low temperatures.

2, the choice of use atmosphere

S-type, B-type, and K-type thermocouples are suitable for use in strong oxidation and weak reducing atmospheres. J-type and T-type thermocouples are suitable for weak oxidation and reducing atmospheres. If a gas-tight protective tube is used, The requirements of the atmosphere are not too strict.

3. Choice of durability and thermal response

Thermocouples with large wire diameters have good durability, but the response is slower. For thermocouples with large heat capacity, the response is slow. When measuring the temperature with large gradient, the temperature control is poor under temperature control. Requires a fast response time and requires a certain degree of durability.

4, the nature and state of the measurement object for the choice of thermocouple

The temperature measurement requirements of moving objects, vibrating objects, and high-pressure vessels are high. The atmosphere with chemical pollution requires a protective tube. In the case of electrical interference, the insulation is required to be relatively high.

Selection process: model - indexing number - explosion-proof grade - accuracy grade - mounting and fixing form - protection tube material - length or insertion depth.

The difference between thermocouple and RTD signal output
1, the nature of the signal, the thermal resistance itself is the resistance, the temperature changes, so that the thermal resistance produces a positive or negative resistance change; and the thermocouple, is the change in the induced voltage, he changes with the temperature changes.

2. The temperature range detected by the two sensors is different. The thermal resistance generally detects the temperature range of 0-150 degrees, and the highest measurement range can reach about 600 degrees (of course, the negative temperature can be detected). Thermocouples can detect a temperature range of 0-1000 degrees (or even higher), so the Rosemount 3051 transmitter is a low temperature test and the latter is a high temperature test.

3. From the material, the thermal resistance is a metal material with a temperature-sensitive metal material. The thermocouple is a bimetallic material, two different metals, due to temperature changes, in two different wires. The potential difference is generated at the end.

4, plc corresponding thermal resistance and thermocouple input module is also different, this sentence is no problem, but generally plc directly access 4 ~ 20ma signal, and thermal resistance and thermocouple generally have a transmitter Only access plc. If you are connected to dcs, you don't have to use a transmitter! The thermal resistance is the rtd signal and the thermocouple is the tc signal!

5, plc also has a thermal resistance module and a thermocouple module, which can directly input resistance and galvanic signals.

6, thermocouples have j, t, n, k, s and other models, have higher than the resistance, but also cheaper than the resistance, but count the compensation wire, the comprehensive cost of the thermocouple is higher.

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