Thermocouple and RTD-Select the right temperature sensor
Thermocouple (TC) and resistor temperature detector (RTD) are the most widely used temperature sensors in automation and process control. They are embedded in motors, valves, turbines, bearings and many other devices. Most smart instruments, such as flow meter, pressure transmitter, and liquid level transmitter, also have an embedded temperature sensor -used to correct the main measurement variables or for process control.
When used alone, the temperature sensor is usually installed in a thermocouple jacket in a tank, container and pipeline. Thermal suite protection sensor is exempted from environmental impact, but it will slow down the response time and reduce accuracy. Installing sensors in thermocouple sleeve is a different theme, which will not be introduced here. On the contrary, this article discusses directly immersed in the sensor; that is, the TC temperature sensor and RTD temperature sensor that is directly inserted and exposed to the process without heat suite protection.
Theory
The thermocouple is composed of two different metals, and the two ends are connected -reference point (cold end) and the measuring point connection point (thermal end) outside the process. The metal's response to temperature changes is different, and the electromotive force (EMF) voltage is generated according to the temperature difference between the knots (the Syberg effect). The resistance temperature detector is based on the principle of increased the temperature in the wire with the temperature.
In these two cases, the sensors are connected to the calibrated sensor or signal regulator to receive the input voltage or resistance, calculate the correct temperature, and output it as 4-20 mA, MV or digital signal to the automation system.
The above content is very basic, but it proposes the first problem to consider when choosing a sensor: how to connect the sensor to the sensor, signal regulator or automation system? Such equipment and systems are essentially electronics. They need to be installed in a reasonable and safe position to stay away from high temperature.
The TC must use the thermocouple extension line wiring, which is the same as the wires used in TC. For example, the K-type TC temperature sensor uses a nickel chromium line, which is connected to a nickel-aluminum nickel wire at the sensor node. The extended line must be the same ingredient, that is, a nickel chromium wire and a nickel aluminum wire. Generally, the longer extension line is not encouraged, because these extension lines act as antenna, making measurement more vulnerable to electromagnetic and radio frequency interference. When processing the long extension line, the cost may also be a problem, especially those extension lines that use special materials (such as R -type TC). In some cases, compensation cables made of cheaper materials with similar EMF features are available.
Wind RTD temperature sensor components are fragile, so they are placed in a protective cover.
On the other hand, the RTD temperature sensor can use the standard cable to connect a farther distance; however, they are usually limited by the problem of self -heating errors. In these two cases, the extension line must be shielded to prevent electrical noise in the device. Please note that the TC extension line is more likely to be affected by noise than the RTD temperature sensor cable.
Harsh environment
Temperature sensing is usually performed in a "unfriendly" environment, such as corrosive, oxidative or restored atmosphere, usually accompanied by severe vibration and electrical noise. When choosing RTD or TC, the environment must be considered. If the risk of sensor failure caused by the process environment is high, the heat suite made of materials made of the process environment should be considered.
Vibration-winding RTD is most vulnerable to vibration. In high vibration applications, line around RTD may fail due to mechanical stress, so it should not be used. The film RTD has higher tolerance to vibration, but it is not as good as TC, and the latter has the highest vibration resistance.
Electric noise-As mentioned above, the extension of RTD and TC is easily affected by electrical noise. In a high noise environment, the extension line should be carried, shielded, grounded as possible as possible. RTD is a better choice in a high noise environment.
His harsh environment-When dealing with harsh environments, check whether RTD or TC sensor manufacturers provide protection for adverse conditions. Protecting the winding RTD in the shell is very strong and is not affected by most environmental problems. In order to provide additional protection, RTD can be applied to a full -fluoride (PFA) polytrafluoroethylene for electroplating baths, high -pressure systems or similar applications. The RTD extension can be protected with polyvinyl chloride, PFA or glass fiber insulation materials.
TC, especially those TCs with metal shells, are much stronger than RTD, and can better deal with corrosive or oxidized environments. When using the exposed TC, be careful in the harsh environment.
TC is classified by types, namely E, J, K, N, T, S, R, and B. Each type is suitable for a specific temperature range from -201 ° C to 1700 ° C. The structure of each type makes it suitable for various environments:
● Type E: suitable for vacuum, inertia, mild oxidation or restoring atmosphere
● Type J: It can be used, exposed or not exposed, when lacking free oxygen
● type K: usually require metal or ceramic protection
● Type N: Antioxidant
● T -type: can be used for oxidation or restore atmosphere
● S, R, B: always use ceramic tube, porcelain auxiliary tube and silicon carbide or metal external tube to protect according to conditions
Other precautions
Measurement range : RTD can measure the temperature of up to 1000 ° C, but it is difficult to obtain an accurate measurement value from RTD at a temperature above 400 ° C. The measuring temperature of TC can reach 1700 ° C. The general acceptance rules are: for temperatures below 850 ° C, use RTD to ensure accuracy; for temperatures above 850 ° C, use TC. Industrial measurement is usually 200 ° C to 400 ° C, so RTD may be the best choice within this range.
Response time : Although both sensors can quickly respond to temperature changes, TC is faster. However, in some cases, the manufacturing process allows the production response time to shorten the film RTD.
Size : Both sensors are quite small, with a diameter of about 0.5 mm. If there is a problem with the space of the installation sensor, please contact the supplier to understand the size and the installation option.
Vibration : Generally, TC is more sensitive to vibration than RTD. However, in certain manufacturing processes, a thin film RTD with higher vibration resistance can be produced than standard RTDs.
Self -heating : RTD is made of very thin lines or very fine coatings, and the voltage from the power supply -TC does not require power. Although the required power is only 1 mA to 10 mA, it will cause the platinum elements in RTD to "warmed", which affects the accuracy of measurement. If the long extension line is used, more power may be required to overcome the resistance in the wire, thereby increasing the problem of self -heating.
Stability : The long -term stability of RTD is very good, which means that its reading will have severe repetitiveness and stability for a long time. On the other hand, TC tends to drift. Due to changes in the oxidation, corrosion and other metallurgical characteristics of sensor elements, EMF generated by TC will change over time. TC drift is irreversible and requires some measures to detect it, such as software or testing ring resistance.
accuracy : Generally speaking, RTD is more accurate than TC. RTD can generate 0.1C accuracy, while TC usually can only be accurate to 1C.
Although it is not a technical issue, TC is much cheaper than RTD, mainly because of the low production cost. The number of sensors required for specific applications may be a major factor.
Careful choosing the correct temperature sensing technology is essential to ensure the best performance, reliability and cost benefits. When determining which type of sensor is determined, the suppliers selling TC and RTDs are usually good sources of information, just like the experience of using different types of sensors in various applications before.
