RTD vs Thermocouple- What is the difference? What are they used for?

Both RTDs and thermocouples are sensors used to measure heat such as Fahrenheit and Kelvin. Both thermocouple and thermal resistance belong to contact temperature measurement in temperature measurement. Although its role is to measure the temperature of the object the same. But their working principles and characteristics are different. Thermocouple is the most widely used temperature device in temperature measurement. Thermal resistance does not require compensation wires and is cheaper than thermocouples.

RTD vs Thermocouple

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.

RTDs stands for ‘Resistance Temperature Detectors’

Thermal resistance: no compensation wire is needed, the price is cheaper

The principle of temperature measurement of thermal resistance: based on the characteristics of the resistance of conductors or semiconductors that change with temperature.

Thermal resistance is one of the most commonly used temperature detectors in medium and low-temperature areas. The thermal resistance temperature measurement is based on the characteristic that the resistance value of the metal conductor increases with the increase in temperature for temperature measurement.

The advantage of thermal resistance: It can also transmit electrical signals remotely. High sensitivity and strong stability. Interchangeability and accuracy are better. However, power supply excitation is required. The temperature change cannot be measured instantaneously.

Disadvantages of thermal resistance: Although thermal resistance is widely used in industry. But because of his temperature measurement range, his application has been limited.

Industrial thermal resistance generally uses Pt100, Pt10, Cu50, Cu100, the temperature range of platinum thermal resistance is generally minus 200-800 degrees Celsius, and copper thermal resistance is minus 40 to 140 degrees Celsius.

Thermal resistors are mostly made of pure metal materials. At present, platinum and copper are the most widely used. In addition, materials such as nickel, manganese, and rhodium have now been used to make thermal resistors.

Thermal resistance does not require compensation wires and is cheaper than thermocouples.

Thermocouple: can transmit 4-20mA electrical signal far away

Thermocouple temperature measurement principle:

The principle of thermocouple temperature measurement is based on the thermoelectric effect. Connect two different conductors or semiconductors into a closed loop. When the temperature at the two junctions is different. The thermoelectric potential will be generated in the loop. This phenomenon is called the thermoelectric effect, also known as the Seebeck effect.

The thermoelectric potential generated in the closed-loop is composed of two kinds of electric potential. Thermoelectric potential and contact potential.

Thermoelectric potential refers to the electric potential generated by the temperature difference between the two ends of the same conductor.

Different conductors have different electron densities, so they generate different electric potentials.

The contact potential, as the name implies, refers to when two different conductors are in contact. Because their electron densities are different, a certain amount of electron diffusion occurs. The electric potential formed when they reach a certain equilibrium. The magnitude of the contact potential depends on the material properties of the two different conductors and the temperature of their contact points.

The advantages of thermocouples:
Wide temperature measurement range. The performance is relatively stable. At the same time, the structure is simple. The dynamic response is good. It can transmit 4-20mA electrical signals remotely. It is convenient for automatic control and centralized control.

Thermocouples currently used internationally have a standard specification. Internationally, thermocouples are divided into eight different divisions, namely B, R, S, K, N, E, J, and T. The lowest temperature can be measured at minus 270 degrees Celsius and the highest can reach 1800 degrees Celsius.

Among them, B, R, and S belong to the platinum series of thermocouples. Because platinum is a precious metal. So they are also called precious metal thermocouples and the remaining ones are called cheap metal thermocouples.

There are two types of thermocouples, common type, and armored type.

Ordinary thermocouples are generally composed of hot electrodes, insulating tubes, protective sleeves, and junction boxes. The armored thermocouple is a combination of thermocouple wire, insulating material, and a metal protective sleeve. A solid combination is formed by stretching.

But the electrical signal of the thermocouple needs a special wire to transmit, this kind of wire is called compensation wire.

Different thermocouples require different compensating wires, and their main function is to connect with the thermocouple to keep the reference end of the thermocouple away from the power supply so that the temperature of the reference end is stable.

Compensation wires are divided into two types: compensation type and extension type. The chemical composition of the extension wire is the same as that of the thermocouple being compensated. However, in practice, the extended wire is not made of the same metal as the thermocouple. Generally, a wire with the same electron density as the thermocouple is used instead.

The connection between the compensation wire and the thermocouple is generally very clear. The positive pole of the thermocouple is connected to the red wire of the compensation lead, and the negative pole is connected to the remaining color. Most of the general compensation wires are made of copper-nickel alloy.

  1. Thermocouple English Thermocouple, abbreviated as TC, works on the principle of outputting a linear millivolt signal as the temperature changes. The instrument amplifies the signal and converts it into a temperature signal.
  2. Thermal resistance English Resistance abbreviation RTD working principle is: the resistance value changes linearly with temperature changes.
  3. The temperature transmitter can convert the thermocouple mV voltage signal or the resistance value signal of the thermal resistance into a 4-20mA standard signal for automation system control.
  4. Generally speaking, thermal resistance is cheaper than thermocouple.

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.

  1. 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.

  1. 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.

  1. 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!

  1. 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.

  1. 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.
  1. 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.

  1. 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.

  1. 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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