As two major contact temperature sensors: thermocouple and thermal resistance. Their names differ by only one word and both can be used as sensors for measuring object temperature.

The choice of thermocouple or thermal resistance should be judged according to the measured object environment. Many people are not sure what to choose. Therefore, it is necessary to fully understand the difference between thermal resistance and thermocouple temperature sensor when choosing a temperature sensor.

Extended reading: RTD vs. Thermocouple: What’s the Difference, and Which Should You Use?

Here, we list some temperature measurement ranges of thermal resistance and thermocouple sensors, and the thermoelectric potential and thermal resistance values of 100°C.

Thermocouple

• Platinum Guy 10-Platinum (Type S) (0-1300℃), T=100℃, E(100,0)=0.646mV.
• Platinum Guy 13-Platinum (R type) (0-1300℃), T=100℃, E(100,0)=0.647mV.
• Zhenming-Zhensi (K type) (0-1200℃) T=100℃E(100,0)=4.096mV.
• Zhenming-Constantan (Type E) (-200-760℃), T=100℃, E(100,0)=6.319mV

Thermal resistance

• Platinum thermal resistance (pt100) (-200-850℃), T=100℃, R=138.50Q
• Copper thermal resistance, (Cu50) (-50-150℃), T=100℃, R=71.4Q.

RTD vs Thermocouple difference

A thermocouple is a sensor that measures temperature. It is a temperature sensor like a thermal resistance. But the main difference between thermocouple and thermal resistance lies in:

1. The nature of the signal.

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

2. The temperature ranges detected by the two sensors are different.

The thermal resistance generally detects the temperature range of 0-150 degrees. The highest measurement range can reach about 600 degrees (of course, negative temperature can be detected).

The thermocouple can detect a temperature range of 0-1000 degrees (or even higher). Therefore, the former is low temperature detection, and the latter is high temperature detection.

3. Different materials

From the material point of view, thermal resistance is a metal material. Metal materials with temperature-sensitive changes, thermocouples are bimetallic materials. Both two different metals.

Due to the change in temperature, a potential difference is generated at the two ends of two different metal wires.

4. PLC modules are different

The thermal resistance corresponding to PLC and the input module of thermocouple are also different, this sentence is no problem. But generally PLC is directly connected to 4-20ma signal, and thermal resistance and thermocouple are generally equipped with transmitter before they are connected to PLC. If you connect to DCS, you don’t need to use a transmitter! The thermal resistance is the RTD signal, and the thermocouple is the TC signal!

5. PLC also has thermal resistance module and thermocouple module, which can directly input thermal resistance and thermocouple signals.

6. Different prices

Thermocouples are available in J, T, N, K, S and other models. Some are more expensive than resistors, and some are cheaper than resistors. However, if the compensation wire is included, the comprehensive cost of the thermocouple is higher. Thermal resistance is a resistance signal, and thermocouple is a voltage signal.

7. Different measurement principles

The principle of thermal resistance temperature measurement is based on the property of the resistance of the conductor (or semiconductor) that changes with temperature. The measurement range is minus 00 to 500 degrees. Commonly used are platinum resistance (Pt100, Pt10), copper resistance Cu50 (minus 50-150 degrees).

The principle of thermocouple temperature measurement is based on the thermoelectric effect to measure temperature. Commonly used are platinum rhodium-platinum (graduation number S, measuring range 0~1300 degrees), nickel chromium-nickel silicon (graduation number K, measuring range 0~900 degrees), nickel chromium-constantan (graduation number E, measuring range 0 to 600 degrees), platinum rhodium 30-platinum rhodium 6 (grading number B, measuring range 0 to 1600 degrees).

RTD vs Thermocouple, how to choose?

The selection of thermocouple should be based on comprehensive considerations such as the use temperature range, the required accuracy, the use atmosphere, the performance of the measurement object, the response time and the economic benefits.

1. Selection of measurement accuracy and temperature measurement range

• When the operating temperature is 1300~1800℃ and the accuracy is relatively high, the B-type thermocouple is generally used;
• The accuracy is not high, and the atmosphere allows the use of tungsten rhenium thermocouples.
• Tungsten rhenium thermocouple is generally used above 1800℃;
• The operating temperature is 1000~1300℃, the accuracy is required and the high accuracy is available. S-type thermocouple and N-type thermocouple are available;
• Generally use K-type thermocouple and N-type thermocouple below 1000℃;
• Generally use E-type thermocouple below 400℃;
• T-type thermocouples are generally used for measurement at 250°C and negative temperature. T-type thermocouples are stable and have high accuracy at low temperatures.

2. The choice of atmosphere

S-type, B-type, and K-type thermocouples are suitable for use in strong oxidizing and weak reducing atmospheres. J-type and T-type thermocouples are suitable for weak oxidizing and reducing atmospheres. If a protective tube with better airtightness is used, the requirements for the atmosphere are not too strict.

3. Choice of durability and thermal response

Thermocouples with larger wire diameters have better durability, but their response is slower. For thermocouples with large heat capacity, the response is slow. When measuring a temperature with a large gradient, in the case of temperature control, the temperature control is poor. It requires a fast response time and a certain degree of durability, so it is more appropriate to choose an armored couple.

4. The nature and state of the measuring object to choose the thermocouple

The temperature measurement of moving objects, vibrating objects, and high-pressure vessels requires high mechanical strength. A chemically polluted atmosphere requires a protective tube. In the case of electrical interference, higher insulation is required.

Selection process: model-index number-explosion-proof grade-precision grade-installation and fixed form-protective tube material-length or insertion depth.

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