Temperature sensors are commonly used devices to measure temperature. Temperature sensors are usually categorized into two types: RTDs and thermocouples. Generally, RTDs and thermocouples play a crucial role in various industrial settings.
In this post, we will focus on the difference between RTDs and thermocouples. Hopefully, after reading this post, you will be able to understand the differences and select the best temperature sensor for your needs.
Different Definition
An RTD is a sensor that measures temperature by utilizing the physical property that the resistance of a material changes with temperature. It consists of a wire, an insulating material, and a resistor body.
A thermocouple is a kind of sensor that utilizes the voltage difference generated at the contact point of two different metal conductors to measure the temperature. It consists of two different metal conductors, insulating materials and a junction box.
Read More about: Temperature Sensor vs Temperature Transmitter
Different Operation Principle
RTD Working Principle:
An RTD is a temperature measurement based on the thermal effect of resistance. The property that the resistance of a resistive body changes with temperature. Therefore, the temperature can be measured by measuring the change in resistance value of the temperature-sensitive RTD. RTD is one of the most commonly used temperature detectors in low and medium temperature applications.
The resistance value and temperature of a metal RTD can generally be expressed by the following approximate relational equation:
Rt=Rt0[1+α(t-t0)],
Rt is the resistance value at temperature t;
Rt0 is the corresponding resistance value at temperature t0 (usually t0=0°C);
α is the temperature coefficient.
Thermocouple Working Principle:
The working principle of a thermocouple is based on the thermoelectric effect. That is, when two different metals A and metal B, form a closed loop, if the temperature of the two joints is different, an electric potential will be generated in the loop. This electric potential is known as the thermoelectric potential. And its magnitude is related to the material of the conductors and the temperature difference between the two joints.
Specifically, a thermocouple consists of two conductors or semiconductors A and B of different materials welded together to form a closed loop. When there is a temperature difference between the two junctions of conductors A and B (the working and hot ends, the free end or the cold end), an electric potential is generated between the two, resulting in the formation of a current in the loop.
The magnitude of this current is proportional to the temperature difference, independent of the material and structure of the conductors. Therefore, by measuring the thermal potential generated by a thermocouple, it is possible to determine the temperature difference between the two joints.
Read More about: How does a Temperature Transmitter Work?
Different Applications
RTDs and thermocouples have their own range of applicability in different applications. Resistors are widely used in environmental temperature measurement, thermal science research, food processing, etc. They are capable of measuring small temperature changes. They are capable of measuring small temperature changes.
A thermocouple is suitable for heat treatment, high-temperature furnaces, gas-fired thermal power stations and other occasions where high-temperature interval needs to be measured. Because it is not easily disturbed by other factors. Its response time and stability are suitable for these high-temperature application scenarios.
Sino-Inst has many successful customized cases for its temperature sensors. Below are the customized parameters we provided for the customer according to his requirements when he contacted us. You can refer to it.
- K-type thermocouple
- Measured medium: Steam mixture
- Process temperature: 650-700°C
- Flange diameter: approximately 127mm
- Probe length: 550 mm
- Thermocouple probe diameter: 15 mm
- Shell outer diameter: 19 mm, inner
- Diameter: 16 mm
Sino-Inst Featured RTD and Thermocouple
Different Structure
The structure of an RTD usually consists of a temperature sensing element, a protective shell and a terminal block. The temperature sensing element is the core of the RTD.
An ordinary thermocouple generally consists of a heat electrode, an insulated tube, a protective casing a terminal box and other parts. The armored thermocouple is the thermocouple wire, insulation materials and a metal protective casing. After the combination of the three, a temperature sensor is used. But the thermocouple electrical signals need a special wire to be transmitted, which we call the compensation wire.
Different Types
RTDs can be divided into 4 categories according to different structures. The following is the detailed introduction.
Ordinary type RTD:
From the principle of temperature measurement of RTD, it can be seen that the change of measured temperature is directly measured by the change of resistance value of RTD. Therefore, changes in the resistance of various wires, such as the lead wire of the RTD body, will have an effect on the temperature measurement.
Armored RTD:
Armored RTD is a combination of a temperature-sensing element (resistance body), leads, insulating material, and stainless steel casing. Compared with ordinary type RTD, it has the following advantages:
- small size, no air gap inside, thermal inertia, measurement hysteresis is small;
- good mechanical properties, vibration resistance, shock resistance;
- can be bent, easy to install;
- long service life.
End-face RTD:
The end-face RTD sensing element is wound with specially treated resistance wire material. It is tightly attached to the end face of the thermometer. Compared with general axial RTD, it can reflect the actual temperature of the measured end face more correctly and quickly. It is suitable for measuring the temperature of the end face of the axial tile and other machine parts.
Explosion-proof RTD:
An explosion-proof RTD is confined to the junction box through the special structure of the junction box. The explosion of the explosive gas mixture inside the enclosure due to the influence of sparks or arcs, and so on. Therefore, it is mainly used in some flammable and explosive places.
Thermocouple models are mainly S, R, B, N, K, E, J, T, and so on several kinds. Among them, S, R, B belong to a precious metal thermocouple, N, K, E, J, T belong to a cheap metal thermocouple.
S-type is characterized by strong oxidation resistance. It is suitable for continuous use in oxidizing and inert atmospheres. Long-term use temperature 1400℃, short-term 1600℃. Among all thermocouples, type S has the highest accuracy level and is usually used as the standard thermocouple.
Type-R is almost identical to Type S except that the thermoelectric potential is about 15% greater.
Type-B has a very small electromotive force at room temperature, so it is usually measured without a compensating wire. Its long-term use temperature is 1600 ℃, and its short-term use temperature is 1800 ℃. It can be used in oxidizing or neutral atmosphere, and can also be used under vacuum conditions for a short period of time.
N-type is characterized by high temperature oxidation resistance at 1300℃. The long-term stability of the thermal electromotive force and the reproducibility of the short-term thermal cycle is good. The resistance to nuclear irradiation and low-temperature resistance is also good, which can partially replace the S-type thermocouple.
K-type is characterized by strong antioxidant performance. It is suitable for continuous use in an oxidizing, inert atmosphere, with long-term use at a temperature of 1000 ℃. And short-term use at 1200 ℃. It is the most widely used among all thermocouples.
E-type is characterized by the largest thermal electric potential, i.e., the highest sensitivity among the commonly used thermocouples. It is suitable for continuous use in oxidizing and inert atmospheres. And the use temperature is 0-800℃.
J-type is characterized by both oxidizing atmosphere (use temperature upper limit 750 ℃). It can also be used for reducing the atmosphere (use temperature limit 950 ℃). And gas corrosion resistance, which is mostly used in the oil refining and chemical industry.
The T-type is characterized by the highest accuracy level among all the cheap metal thermocouples. And it is usually used to measure temperatures below 300℃.
Different Measurement Temperature
RTDs are commonly used as temperature sensors in low and medium-temperature zones. And are divided into platinum RTDs, copper RTDs and so on, with different temperature measurement ranges. In general, RTDs can measure temperatures from -200 to 1000℃.
Compared with RTDs, thermocouples have a much wider temperature range. The largest range of B-type thermocouples can even measure temperatures from 0-1600℃. And ordinary K-type thermocouples can also measure temperatures from -40-1200℃. But because thermocouples are not very accurate in low-temperature measurements, it is still appropriate to use RTDs when measuring lower temperatures.
Different Material
RTDs are mainly composed of a metallic material with temperature-sensitive changes. Thermocouples are bimetallic materials. Two different metals, due to temperature changes, produce a potential difference at the ends of two different wires.
The most common RTD materials are single metal. The most widely used RTD materials are platinum and copper. Platinum resistors are highly accurate and stable, with a certain degree of non-linearity. The higher the temperature, the smaller the rate of change of resistance. Copper resistance in the temperature range of resistance. And the temperature is a linear relationship. The temperature line number is large, more than 150 degrees Celsius, making it easy to oxidize.
Thermocouples are made of two different types of conductors (called thermocouple wire material or heat electrodes) connected at both ends to form a loop. Common thermocouple materials include K-type (nichrome-nickel-silicon), T-type (copper-nickel), J-type (iron-copper-nickel) and other high-temperature alloys. Each material has its specific temperature range and application area. Therefore, we need to choose the right material according to the actual needs.
Differential Responsible Time
Thermocouples usually respond faster than RTDs. Thermocouples have a smaller structure and better thermal conductivity. They can respond faster to changes in temperature. RTDs have a slower response time and take longer to reach a stable measurement.
When to Use RTD and Thermocouples?
Thermocouples and RTDs are common temperature sensors. Our choice of thermocouples or RTDs should be considered on a case-by-case basis. There are mainly the following points to pay attention to:
Measuring temperature range: The Thermocouple has a wide measuring range, usually up to -200℃~1600℃. While the measurement range of RTD is narrower, usually -200℃~1000℃. Therefore, it is more appropriate to choose a thermocouple when measuring high temperatures. While in the measurement of room temperature or low temperature, the choice of RTD is more appropriate.
Accuracy requirements: thermocouples usually have higher accuracy within a certain temperature range. They provide accurate measurements over a wide range of temperatures. RTDs are usually more accurate over a narrower temperature range.
Environmental conditions: In harsh environments such as high temperatures, strong corrosion, vibration, magnetic fields and irradiation, thermocouples have better stability and immunity to interference. And in general environmental conditions, RTDs have a longer service life and are more economical.
Cost budget: The measurement range and accuracy of the thermocouple are better than RTD, so the cost of the thermocouple is higher than that of RTD.
Is PT100 an RTD or a Thermocouple?
PT100 is an RTD.
PT100 is a thermistor. Its main component is platinum, chemical symbol Pt, also known as the legendary white gold. It has good temperature characteristics, good stability and good resistance to acids and alkalis. It has a large number of applications in industry. Of course, it is not cheap.
PT100 has a standard resistance temperature correspondence table. At 0 degrees, its resistance value is 100 Ω; at room temperature 25 ℃, its resistance value is 109.73 Ω. We can measure the change in its resistance, which can be converted to temperature. The easiest way to do this is to measure its resistance with a multimeter.
What Type of Wire is Used to Make an RTD?
RTDs have a total of three wiring methods, which are described below.
The 4-wire system is to draw 4 wires from both ends of the RTD. And the current and voltage measurement circuits are wired independently of each other. It is more accurate and requires more wires.
The 3-wire system is to draw 3 wires. And the reference of the current loop and the voltage measurement loop are wired as a single line (i.e., the I-terminal and V-terminal of the testing equipment are shorted). The accuracy is slightly better.
The two-wire system is a two-wire system in which the current and voltage measurement circuits are wired as one (i.e., the I- and V- terminals of the testing equipment are shorted, and the I+ and V+ terminals are shorted). Poor measurement accuracy.
The resistance of the connecting wires and contact resistance can have an effect on the measurement RTD, which is effectively eliminated by the four-wire and three-wire systems. If you consider from the point of view of testing accuracy, in comparison, the four-wire method is a better choice.
In conclusion, RTD and thermocouple are both commonly used temperature sensors in industrial production. Thermocouples have a higher measurement range and accuracy. RTDs are mainly used for temperature measurement in low-temperature places. If you still don’t know how to choose after reading this post. Then please contact us immediately. Our engineers are available 24 hours a day to provide you with solutions.
