RTD - Resistance temperature sensor
Jack
May 08, 2024
What is an RTD sensor?
The full English name of RTD is "Resistance Temperature Detector", so it should be translated as "resistance temperature detector". RTD is a special resistance whose resistance value increases as the temperature increases and decreases as the temperature decreases. This property is used in industry for temperature measurement, so RTD is also commonly known as "thermal resistance".
RTD is a passive device. It does not produce output alone. External electronics can be used to measure sensor resistance by passing a small current through the sensor to generate a voltage. Usually 1 mA or lower measurement current, up to 5 mA, with no risk of self-heating.
The RTD consists of a resistive element and an insulating platinum wire. Sometimes, RTDS can have three or even four wires to improve accuracy and thus eliminate the resistance error associated with connecting leads. The material of the resistance element is made of platinum, because the metal has a very long-term stability, and there is a certain linear relationship between temperature and resistance, with a wide temperature range of this characteristic, at the same time, the metal is not easy to change, with a relatively stable chemical inertia.
Rtd-resistance temperature sensor working principle
RTD works according to one basic principle; When the temperature of the metal increases, the resistance to the current also increases. Current passes through the sensor, and the resistance element is used to measure the resistance of the current passing through it. As the temperature of the resistance element increases, the resistance also increases.
Resistance is measured in ohms. The resistance value can then be converted to temperature based on the characteristics of the component. Typically, the response time of an RTD is between 0.5 and 5 seconds. This makes them ideal for many applications.
RTD standard tolerance
RTDS are built according to several standardized curves and tolerances. The most common standardized curve is the "DIN" curve. This curve describes the resistance and temperature characteristics of platinum at 100 ohm sensors, standardized tolerances, and measurable temperature ranges.
The DIN standard specifies a base resistance of 100 ohms at 0°C and a temperature coefficient of 0.00385 ohms/ohms /°c. The nominal output of the DIN RTD sensor is as follows:
DIN RTD has three standard tolerance classes. These tolerances are defined as follows:
DIN Class A: ±(0.15 + 0.002 |T|°C)
DIN Class B: ±(0.3 + 0.005 |T|°C)
DIN Class C: ±(1.2 + 0.005 |T|°C)
RTD component type
When determining the type of RTD element, first consider the instrument used to read the sensor. Select a component type that is compatible with the sensor input of the instrument. By far the most commonly used RTD is 100 ohplatin with a temperature coefficient of 0.00385.
RTD accuracy
Second, determine the required measurement accuracy. Accuracy is a combination of the base resistance tolerance (resistance tolerance at calibrated temperature) and the resistance tolerance temperature coefficient (characteristic slope tolerance). Any temperature above or below this temperature will have a wider tolerance band or lower accuracy (see figure below). The most commonly used calibration temperature is 0°C.
Sensor connection
RTD sensors are available in a variety of different lead configurations. The most common configuration is the unit three-lead configuration. The schematic diagram of the available lead configuration is shown below:
Two-wire sensors are often used in applications where accuracy is not important. The two-wire configuration enables the simplest measurement technique, but there are inherent inaccuracies due to the resistance of the sensor leads. In a two-wire configuration, it is not possible to directly compensate the lead resistance that increases the offset of the resistance measurement.
The three-wire sensor has a compensation loop to remove lead resistance during measurement. With this configuration, the controller/measuring device can take two measurements. For the first measurement, the total resistance of the sensor and the connection leads is measured. On the second measurement, the resistance of the compensating loop resistance is measured. The actual net resistance can be determined by subtracting the compensation loop resistance from the total resistance. The three-wire sensor is the most common configuration, which combines precision and convenience.
The four-wire sensor configuration and measurement technology measure the sensor resistance without being affected by the lead. Although this technique is more accurate, many industrial controllers/measuring devices are unable to achieve true four-wire measurement.
The transition from the sensor lead to the field wiring is usually done at the connector connected to the sensor. Terminals are used for easy connection.
What are the application areas of RTD sensors?
• Car
• Power electronics
• Consumer Electronics
• Food handling and processing
• Industrial electronics
• Medical Electronics
• Military
• Aerospace
Due to the linear change of the positive temperature coefficient of the RTD sensor, as well as the advantages of the stability of the material and the wide temperature range and small size, its application field is also very wide. Our ERTD2 series of RTD sensors can be tested in the temperature range of -200 to +300 ° C, of course, versions from -70 to 850 ° C and more are also available. You can find more information about RTD sensors at https://www.lkrelec.com.
Conclusion
RTDS are more linear than thermocouples and are the most accurate and stable temperature sensors available. But because the change in resistance takes time, its response is slower. At the same time, its price is relatively expensive, and it is suitable for occasions where there are certain requirements for precision and cost control is not strict.
