Thermocouples and RTDs are both contact temperature measurements, they are closely linked to the temperature transmitter. And although they both serve the same purpose of measuring the temperature of an object, their principles and characteristics are different.
1. Thermocouple
The thermocouple is the most widely used in temperature measurement. Its main features are a wide temperature range, relatively stable performance, a simple structure, and good dynamic response. It can also transmit 4-20mA electrical signals remotely, and it is also convenient for automatic and centralized control.
1.1 Thermocouple temperature measurement principle
The thermocouple principle of temperature measurement is based on the thermoelectric effect.
It connects two different conductors or semiconductors into a closed loop. When the temperatures at the two contacts are different, a thermal potential is generated in the loop, a phenomenon known as the thermoelectric and the Seebeck effect. The thermal potential developed in closed-loop consists of two types of potential; the temperature difference potential and the contact potential.
1.2 Temperature difference potential
The temperature difference potential is the potential generated at the two ends of the same conductor due to the temperature difference. Different conductors have different electron densities, so they produce other possibilities.
Contact potential is when two different conductors come into contact because their electron densities are different, producing a certain amount of electron diffusion. The magnitude of the contact potential depends on the material properties of the two other conductors and the temperature at the point of contact.
1.3 International Standard Specifications
Internationally applied thermocouples have a standard specification, which stipulates that thermocouples are divided into eight different divisions. Namely, B, R, S, K, N, E, J, and T. They can measure temperatures as low as minus 270°C and as high as 1800°C, with B, R, and S belonging to the platinum series of thermocouples. As platinum is a precious metal, they are also known as precious metal thermocouples, while the remaining ones are cheap metal thermocouples.
1.4 The structure of a thermocouple
There are two types of thermocouple structure, the standard type, and the armored type.
Standard thermocouples generally consist of a thermal electrode, an insulating tube, a protective sleeve, and a junction box.
On the other hand, the armored thermocouple is a solid combination of thermocouple wire, insulating material, and metal protection sleeve assembled and then stretched and processed. However, the electrical signal of the thermocouple requires a special wire for transmission, which we call the compensation wire.
1.5 Thermocouple compensation wire
Different thermocouples require different compensation wires. Its primary function is to be connected to the thermocouple. It then keeps the reference end of the thermocouple away from the power supply to stabilize the temperature at the reference end.
Compensation wire is divided into two types of compensation and extension, an extension of the chemical composition of the wire and the thermocouple being compensated for the same. Still, in practice, the extension of the wire is not the same material as the thermocouple metal. Therefore, we generally use the thermocouple with the same electron density wire.
The wire connection to the thermocouple is wide. Most of the compensation wires are made of copper-nickel alloy.
2. RTD
RTDs are not only widely used for industrial temperature measurement. But are also used as standard reference instruments. However, the range of temperature measurement limits its application. The temperature measurement principle of the RTD is based on the property that the resistance of a conductor or semiconductor varies with temperature.
2.1 Advantages of RTDs
2.1.1. Remote transmission of electrical signals.
2.1.2. High sensitivity.
2.1.3. High stability.
2.1.4. Interchangeability and accuracy are good.
2.1.5. But it needs power supply excitation, not instantaneously measuring the temperature change.
Industrial RTDs, we generally use Pt100, Pt10, Cu50, Cu100. Platinum RTDs typically measure temperatures from -200 to 800°C and copper RTDs from -40 to 140°C. The RTD and thermocouple are the same distinguishing type, but he does not need compensation wire and is cheaper than the thermocouple.
2.2 Platinum RTD installation
There are many forms of platinum RTD installation, including fixed thread installation, movable thread installation, fixed flange installation, movable flange installation, portable pipe joint installation, in-line pipe joint installation, etc.
3. Temperature range of RTD and thermocouple
The most significant difference between RTD and thermocouples is the choice of the temperature range. RTD is a temperature sensor that measures low temperature, generally measuring temperature in -200~800℃, while thermocouple is a temperature sensor that measures medium and high temperature, usually measuring temperature in 400~1800℃.
When we choose, if the measurement temperature is around 200℃, we should select RTD measurement. If the measurement temperature is 600℃, we should select K type thermocouple. Finally, if the measurement temperature is 1200~1600℃, we choose the S type or the B-type thermocouple.
4. Thermocouple and RTD
4.1 RTDs have the following characteristics compared to thermocouples.
4.1.1 the output signal is more significant at the same temperature. It is easy to measure.
4.1.2 when measuring resistance, we must use the external power supply.
4.1.3 the RTD temperature-sensing part of the larger size, while the working end of the thermocouple is a tiny solder joint, so the RTD temperature measurement response rate than the thermocouple is slow.
4.1.4 similar materials made of RTD are not as high as the upper limit of thermocouple temperature measurement.
4.2 Difference
4.2.1 The nature of the signal.
The RTD itself is resistive. Therefore, a temperature change will cause a positive or negative change in resistance. On the other hand, Thermocouples produce a change in induced voltage, which changes with temperature.
Although both contact temperature measuring instruments have different temperature measurement ranges, thermocouples are used in higher temperature environments.
Such as platinum rhodium 30 – platinum rhodium 6 (B type) measurement range of 300 degrees ~ ~ 1600 degrees, short-term can be measured 1800 degrees. S type measurement a 20 ~ ~ 1300 (short-term 1600), K type measurement a 50 ~ ~ 1000, short-term 1200). XK type a 50 ~ ~ 600 (800), E type a 40 ~ ~ 800 (900), and J type, T type, etc..
When the low-temperature zone, it outputs a shallow thermal potential. When the potential is small, the anti-interference measures and secondary meter and requirements are high. Otherwise, the measurement is not allowed. When in the lower temperature region, the relative error caused by changes in the cold end temperature and changes in the ambient temperature becomes prominent and is not easily fully compensated.
This time in the low temperature, the general use of RTD temperature measurement range of 200 ~ ~ 500, and even lower temperature measurement (such as carbon resistance can be measured to about 1K low temperature). We use platinum RTD Pt100 (also Pt50, 100, and 50 represent the resistance value of the RTD at 0 degrees. In the old index number with BA1, BA2 to represent, BA1 in 0 degrees when the resistance value of 46 ohms, in the industry also good copper resistance, index number CU50 and CU100, but the temperature measurement range is smaller, in a 50 ~ ~ 150 between, in some special occasions and indium resistance, manganese resistance, etc.).
4.2.2 The work of the site judgment
Thermocouples have positive and negative poles. Compensation wires also have positive and negative. First, we have to ensure that the connection is configured correctly. Instrument in operation, we often encounter short circuits, broken circuits, poor contact (with a multimeter can be judged), and deterioration (according to the surface color to identify).
When we check, we must make the thermocouple separate from the secondary meter and then short the compensation line on the secondary meter with a tool. The table indicates the room temperature and then short the thermocouple terminals. The table approves the ambient temperature where the thermocouple is located (not, the compensation line is faulty) and then uses the multimeter mv file to roughly estimate the thermal potential of the thermocouple (such as usual, please check the process).
In operation, RTD short circuit and disconnection with a multimeter can be judged suspected of a short course. We need to remove the resistance end to see the display meter. To ensure stable connection and configuration. Such as to the maximum, RTD short circuit back to zero, the wire short circuit. The table value shows low or unstable, the possibility of protection tube into the water shows the maximum, RTD disconnection shows the minimum short circuit.
4.2.3 From the material point of view
A thermistor is a metallic material with a temperature-sensitive change in the metal material. Due to temperature changes, the thermocouple is a bimetallic material, two different metals, in the two ends of two separate metal wires to produce the potential difference.
4.2.4 Measuring temperature range
The temperature ranges detected by the two sensors are not the same. Thermal resistance generally detects the temperature range of 0-150 degrees (of course, you can catch the negative temperature), thermocouples can see the temperature range of 0-1000 degrees (or even higher). Hence, the former is low-temperature detection. The latter is high-temperature detection.
