What is the difference between a resistance temperature detector (RTD) and a thermocouple? Both RTDs and thermocouples are sensors used to measure heat such as Fahrenheit and Kelvin. These devices are used in a wide range of applications and settings, often presenting people with the dilemma of choosing between RTDs or thermocouples. Each temperature sensor has its own advantages and disadvantages that make it suitable for certain conditions and environments.
What is RTD?
RTDs are made of metal wires, usually copper or platinum, that offer resistance to the flow of electricity. The RTD’s resistance changes when its temperature changes, allowing it to be used as a gauge for measuring heat. RTDs are considered to be more accurate than thermocouples as they have a linear relationship between resistance and temperature. RTDs are also less affected by electromagnetic fields than thermocouples.
RTD Working Principle
The full English name of RTD is “Resistance Temperature Detector”, so to be precise, it should be translated as “Resistance Temperature Detector”.
RTD is a special kind of resistor whose resistance value increases as the temperature increases and decreases as the temperature decreases. In industry, this feature is used for temperature measurement, so RTD is also commonly known as “thermal resistance”.
Not all metals are suitable for making RTDs. Materials that meet this characteristic need to meet the following requirements:
- The resistance value of the metal has a linear relationship with the temperature change energy;
- The metal is more sensitive to temperature changes, that is, the resistance change (temperature coefficient) caused by unit temperature changes is relatively large;
- The metal can resist fatigue caused by temperature changes and has good durability;
There are not many metals that meet this requirement. Common RTD materials are: platinum (Pt), nickel (Ni), and copper (Cu).
Take platinum thermal resistance as an example. According to the different resistance values, it can be divided into Pt50, Pt100, Pt200, Pt500 and Pt1000.
The numerical value in the name indicates the resistance value of the thermal resistance at 0°C.
For example: Pt100, indicating that the resistance value of the sensor at 0°C is 100Ω.
And Pt1000, it means that the resistance value of the sensor at 0 ℃ is 1000Ω.
The resistance value of RTD thermal resistance at different temperatures can be approximated by the formula: R=R0(1+αT).
1) R0 represents the resistance value of RTD at 0℃;
2) a is called the temperature coefficient, which represents the change value of the resistance at unit temperature;
3) T represents the measurement temperature, the unit is °C;
According to the number of lead wires of RTD thermal resistance, RTD can be divided into two-wire, three-wire and four-wire.
The lead of the two-wire RTD is to directly lead out two wires at both ends of the resistor to the temperature measurement module. The temperature measurement module adopts the principle of bridge balance, and RTD is used as one arm of the bridge to measure.
A three-wire RTD can largely eliminate the influence of the sensor leads themselves on the measurement results. The detection accuracy is greatly improved compared to the two-wire system.
Resistance Temperature Detector advantages
No compensation line is required, and the price is cheap;
It can transmit electrical signals over long distances;
High sensitivity and strong stability;
Good interchangeability and high precision.
Disadvantages of thermal resistance:
Although thermal resistance is widely used in industry. But it requires power excitation.
Temperature changes cannot be measured instantaneously.
The temperature measurement range is limited and the application is limited.
What is Thermocouple?
Thermocouples, on the other hand, are made of two different types of metals that are joined together at the sensor end. The junction between these two metals produces a voltage that is proportional to the temperature difference between the junction and the measuring point. Thermocouples are less expensive than RTDs and can measure a wider range of temperatures. They are also faster at responding to changes in temperature.
Thermocouple Working Principle
A thermocouple is a temperature sensing element. It converts the temperature signal into a thermoelectromotive force signal and converts it into the temperature of the measured medium through an electrical instrument.
The basic principle of thermocouple temperature measurement is that two homogeneous conductors of different compositions form a closed loop. When there is a temperature gradient at both ends, a current will flow through the loop. At this time, there is a seebeck electromotive force – thermal electromotive force between the two ends. This is called the Seebeck effect.
The two homogeneous conductors with different compositions are the hot electrodes, and the end with the higher temperature is the working end. The end with the lower temperature is the free end. The free end is usually at some constant temperature.
According to the functional relationship between thermoelectromotive force and temperature, a thermocouple indexing table is made. The index table is obtained under the condition that the free end temperature is at 0°C. Different thermocouples have different scales.
When a third metal material is inserted into the thermocouple loop. As long as the temperature of both junctions of the material is the same. The thermoelectric potential generated by the thermocouple will remain constant. That is, it is not affected by the access of the third metal into the loop.
Therefore, when measuring the temperature of the thermocouple, the measuring instrument can be connected. After measuring the thermoelectromotive force, the temperature of the measured medium can be known.
- High measurement accuracy: The thermocouple is in direct contact with the measured object and is not affected by the intermediate medium.
- Fast thermal response time: Thermocouples are sensitive to temperature changes.
- Large measurement range: thermocouples can measure temperature continuously from -40 to +1600 °C.
- Reliable performance and good mechanical strength.
- Long service life and easy installation.
Types and structures of thermocouples
Types of thermocouples Thermocouples include k type (nickel-chromium-nickel-silicon), n-type (nickel-chromium-silicon-nickel-silicon-magnesium), e-type (nickel-chromium-copper-nickel), j-type (iron-copper-nickel) , t-type (copper-copper-nickel), s-type (platinum-rhodium 10-platinum), r-type (platinum-rhodium 13-platinum), b-type (platinum-rhodium 30-platinum-rhodium 6) and so on.
Structural form of thermocouple: The basic structure of a thermocouple is a thermal electrode, an insulating material and a protective tube. Display instrument, recording instrument or computer and other supporting use. In field use, thermocouples suitable for various environments are developed according to various factors such as the environment and the measured medium.
Conclusions, which one should you use?
It really depends on the specific application and what is more important: accuracy or speed. If you need to measure very high or very low temperatures, a thermocouple is the better choice. If you need more accuracy, then an RTD is the way to go.
Read more about: What Is 0-10V Signal Output?
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Wu Peng, born in 1980, is a highly respected and accomplished male engineer with extensive experience in the field of automation. With over 20 years of industry experience, Wu has made significant contributions to both academia and engineering projects.
Throughout his career, Wu Peng has participated in numerous national and international engineering projects. Some of his most notable projects include the development of an intelligent control system for oil refineries, the design of a cutting-edge distributed control system for petrochemical plants, and the optimization of control algorithms for natural gas pipelines.