Resistance Temperature Detector, also known as RTD or resistance thermometer, is a type of temperature sensor. These sensors are composed of metal elements whose resistance change is a function of temperature. As the temperature increases, the resistance of the metal increases, and as the temperature decreases, its resistance decreases. An RTD is made of wires (of known resistance) connected to a transmitter. This sensor prepares the resistance signal output from the wires for the temperature measurement process.
History of RTD Sensor Use
Application of the increase in electrical resistance of conductors with increasing temperature was first explained by Sir William Siemens in 1871 before the British Royal Society. The principle was used between 1885 and 1900.
One of the applications of increasing resistance with increasing temperature was in the space shuttle. Resistant platinum thermometers were used in the space shuttle. But the STS-51F shuttle's main engine shut down due to multiple RTD failures. In this project, multiple thermal cycles caused the failure of RTDs and engine shutdown. Following this incident, the RTDs in the shuttle engine were replaced with thermocouples.
What is the Working Mechanism of the RTD Sensor?
Pay attention to this sentence:
“The resistance of metals changes with temperature.”
This sentence is the basic principle of RTD operation. According to this sentence, if we measure the resistance change of a metal, we can measure the amount of temperature change according to its temperature coefficient. The temperature coefficient of the RTD sensor is equal to the average resistance changes in the temperature range of 0 to 100 °C, which is represented by α.
In this equation, R0 is the resistance at 0°C, and R100 is the resistance at 100°C. So it can be said that the resistance at temperature t is equal to:
The last equation shows the direct relationship between temperature and resistance. The temperature can be calculated by calculating the resistance and using the above equations.
An Example of Temperature Calculation in an RTD Sensor
Let's assume that the PT100-type RTD sensor shows resistance values of 100 and 139.1 ohms at temperatures of 0 and 100 °C, respectively. We want to know if the sensor shows a resistance of 116 ohms, What the temperature will be.
In the first step, we must calculate the temperature coefficient. Using equation (1), we have:
Now using equation (2):
Therefore, the desired temperature was 40.92°C.
Types of RTDs
RTD sensors can be divided into two categories based on internal resistance and the number of wires used. In this section, we will examine these two important classifications of these sensors.
RTD sensor classification based on internal resistance
PT100 and PT1000 are two of the most famous RTD sensors.
"PT" in the name pt100 indicates that the sensor is made with a platinum element, and 100 is its resistance factor. The PT100 sensor is one of the most accurate temperature-measuring tools with a resistance coefficient of 100 ohms. There are several versions of the pt100 sensor that have different temperature coefficients and are denoted by the letter alpha or α.
The most common alpha-type sensor is "385". The number 385 means the temperature coefficient of this sensor is equal to 0.00385/°C. Typically, if the temperature coefficient is 0.00385/°C, it is considered to be saved next to the sensor.
Figure 1- PT100
PT1000 RTD Sensors
PT1000 is the second most used resistance sensor. Its resistance coefficient is 1000 ohms and is mainly used as two wires. As it turns out, a pt1000 has more resistance than a pt100.
Pt1000 has a higher resistance value and requires less current. They are suitable for systems that consume less energy. Since their electricity consumption is low, they generate less heat and have fewer errors due to heat increase (self-heating).
Figure 2- PT1000
Classification of RTD Sensors Based on the Number of Wires
RTDs are divided into two, three, and four wire categories based on the number of wires.
As the name of this category suggests, these RTDs have two wires in their structure. This type of RTD is usually used in applications where the approximate temperature value is necessary. Increasing the resistance of wires in this category reduces the accuracy of RTDs.
This configuration is the simplest, least expensive, and at the same time the most limited RTD. Using two-wire RTDs, when the receiving device is directly connected to the sensor without the presence of connecting wires, they measure the temperature with reasonable accuracy. The inherent resistance caused by the use of connecting wires when using a two-wire configuration cannot be compensated, and using them increases the measurement error.
When accuracy is not essential, the two-wire RTD is the least expensive. Wire colors are defined in IEC 60751-2008 standard wire colors are shown in Figure 8.
Figure 3 - Two-wire RTD
This category is the most popular type of RTD in various industries. In this type of RTD, the increase in cable resistance is prevented and the calculation error is reduced.
In the three-wire sensor, the resistance of the wires is reduced from the measured resistance to having better accuracy in temperature measurement. Therefore, to minimize the effects of line resistance and temperature increase in the wires, it is common to use a three-wire sensor. This sensor has an additional wire that leads to the formation of two measurement circuits. One of these circuits is used as a reference.
Figure 4 - Three-wire RTD
The most accurate type of RTD is the four-wire type. In this category, the resistance of the wires is calculated and their effect on the temperature measurement is removed. For this reason, in cases where we need to determine the temperature with high accuracy, this type of RTD will be the right choice.
In addition to reducing the resistance of the wires from the measured resistance value, the 4-wire sensors also eliminate the effect of the resistance of the contact point. For this reason, while using a 4-wire sensor, corrosion and environmental conditions are less of a concern. On the other hand, the 4-wire configuration reduces the problems caused by the length of the connecting wire.
Figure 5 - Four-wire RTD
RTD Sensor Installation Method
Usually, it is recommended to use a thermowell when installing the sensor. Thermocouples are cylindrical fittings used to protect temperature sensors installed in industrial processes. A thermowell is a closed tube at one end, and an RTD is placed at the open end. In this way, heat is transferred to the sensor through the thermowell wall. Figure 6 is an example of correct RTD and thermowell installation.
Figure 6 - RTD and thermowell installation
The structure of a thermowell and temperature sensor in a process line is shown in the figure below:
Figure 7 - thermowell
Most RTD sensors have a protective sheath made of Inconel to protect the sensor from the environment and mechanical shocks. Inconel is oxidation-resistant material that is suitable for use in pressure and heat environments. When Inconel is heated, it forms a thick, stable oxide layer that protects its surface. Therefore, it maintains its strength in a wide temperature range and is suitable for high-temperature applications, which cannot be tolerated by aluminum and steel. Figure 8 shows a pod.
Figure 8 - Protective sheath
RTD Sensor Maintenance and Calibration
Although most sensors have a protective sheath or are installed in a thermowell, it is necessary to check their health. Part of this maintenance process includes examining damages caused by:
- Strong tremors, etc.
In case of damage, it is necessary to replace or repair the sensors as soon as possible. One of the steps of these sensors upkeep proper is calibrating them at specific periods. RTD calibration periods depend on temperature cycling, vibrations, and shocks to the sensor. This calibration is done by comparing the sensor resistance with a standard.
Structure and Principal Components of RTDs
The six primary components of RTD sensors are a metal element, wire, protective tube, connectors, outer diameter, and cold end.
The Metal Element of the Sensor
All metals create resistance when the temperature changes. The amount of their resistance change depends on the temperature coefficient of the metal. In RTDs, the temperature coefficient means the rate of change of the resistance value with a change of one Kelvin in temperature.
For RTD sensors, copper, nickel, platinum, and Balco are used as metal elements.
The change in copper resistance has a valid linear relationship with temperature change. Oxidation of copper limits its use to temperatures below 150°C or 302°F. For this reason, copper RTDs are used to measure the winding temperature of motors, generators, and turbines. In addition, RTD with copper elements can be used in environments that do not have an oxidizing atmosphere.
In the first RTD sensor, copper was used as the metallic element. But in the end, it was found that platinum has better performance and more accurate reading.
The metal element in the platinum RTD is made of pure platinum wire and has a positive temperature coefficient. The linear stability of the platinum element makes the platinum RTD a very accurate sensor for industrial applications.
RTDs with nickel metal elements have a limited temperature range and become non-linear at temperatures above 300°C or 572°F. These sensors are less expensive than platinum RTDs and are resistant to corrosion, but they wear out quickly and lose their accuracy. Their temperature range is -80 to 260°C or -112 to 500°F.
Balco is an alloy consisting of 70% nickel and 30% iron. Balco's thermal conductivity is similar to nickel but with twice the resistance. RTD sensors with bulk metal such as copper are low cost, with a high coefficient of resistance, exceptional linearity, strong mechanical properties, and corrosion resistance.
Copper, nickel, and platinum are usually replaced by balco in the production of modern RTD sensors due to their lower cost.
RTD Protective Tubes
316 stainless steel is used to make the RTD tube, which is usually suitable for assemblies up to 500°F. If we want to use RTDs at temperatures above 500°F, it is recommended to use Inconel 600 instead of steel.
The outer diameter of RTD sensors comes in a wide range of sizes but is typically considered to be between 0.063 inches (1.6 mm) and 0.5 inches (12.7 mm).
RTDs are available in 2-, 3-, and 4-wire configurations. The 3-wire formatting is the most common type for industrial applications. The insulating materials of the wires are Teflon and fiberglass. Teflon is moisture-resistant and can be used up to 400°F, while fiberglass can withstand temperatures up to 1000°F.
Two common metals used for supports and fittings are brass and stainless steel. Brass is chosen for its corrosion resistance, while stainless steel is resistant to corrosion and chemicals. The connectors are designed for easy installation and secure the wires so they do not twist or wrinkle.
RTD Cold-end Terminal
The RTD sensor is connected to the monitoring device or other equipment such; as PLCs through the terminal.
Figure 9 - RTD structure
Accuracy of the RTD Sensor
The practical accuracy of the RTD (pt100 sensor) depends on the tolerance of the RTD, the measurement temperature, the accuracy of the mating tool, and the effects of the connection wire, and installation. But when we talk about the accuracy of the RTD sensor, we usually refer to the degree of temperature deviation or the degree of its toleration because its "real" tolerance depends on the temperature. Several international standards determine the tolerance and accuracy of RTDs. The most common standard used for rating platinum RTDs (pt100 sensor) is (IEC 751 1995). IEC 751 defines two performance classes for 100Ω platinum RTDs (pt100 sensors) with an alpha of 0.00385, class A and class B:
Advantages of RTDs
High sensitivity, linearity in a wide operating range, and proper operation in large temperature ranges are among the most essential advantages of RTDs. In addition to these advantages, the use of RTD will bring high accuracy, repeatability, and high stability compared to other types of temperature sensors.
Disadvantages of RTDs
Large dimensions, fragility, and susceptibility to electrical noise are the biggest disadvantages of RTDs. Undesirability in high temperatures is another disadvantage of this equipment. RTDs are rarely used in industry at temperatures above 660°C. Because at this temperature, it becomes extremely hard to prevent platinum from being contaminated with impurities.
How to Choose the Correct RTD Sensor?
To choose the right type of RTD, one should answer the following questions:
- Which type of RTD metal element is suitable for the desired application?
- What are the details of the application in which we want to use RTDs? (For example: What kind of substance do we want to measure? Solid? Liquid? Or gas?)
- What conditions govern the RTD installation environment? (Conditions such as required IP range, level of resistance to chemicals, etc.)
- What temperature range do we want to measure?
- What equipment receives the output of the sensor to continue the control process?
Answering questions like this can help us choose a better RTD.
RTDs are one of the most popular temperature measurement sensors after thermocouples.
The performance of this equipment is based on the relationship between resistance change and temperature change.
These sensors can be classified by the number of wires or the internal resistance amount. From the point of view of the number of wires, RTD is divided into three categories: two, three, and four wires.
The three-wire type of this equipment is widely used in the industry.
The four-wire type is the most accurate, and the two-wire type is the cheapest type of RTD.
PT100 and PT1000 are the most vital RTD categories in terms of internal resistance value.