RTD (Resistor Temperature Detector)
All metals produce a positive change in resistance for a positive change in temperature. An RTD sensor is used to measure temperature by correlating the resistance of the RTD element with temperature. In 1871, Siemens choose Platinum as the RTD element in a resistance thermometer and still today it is the most common used metal for RTD's besides the more economical Nickel or Nickel alloys. They consists of fine wire wrapped around a ceramic or glass core. The resistance of the wire at various temperatures is known and therefore predictable. This predictable change in resistance is used to measure temperature. Common values of resistance for a Platinum RTD ranges from 10 ohms for the bird-cage model to several thousand ohms for the film RTD. The most commonly used element is a wire wound, or thin film element, conforming to DIN standard and with a 0.00385 temperature coefficient.
An RTD needs a power source while a thermocouple contains two different electrical conductors which generate a small current when exposed to temperature.
RTD's are used where high repeatability and accuracy are important. (example: labratory applications) RTD's are relatively immune to electrical noise and therefore especially suited around motors, generators and other high voltage equipment (like induction heating). They are also less sensitive to vibration and shock applications. An RTD can be more expensive than a thermocouple with today's pricing of Platinum.
The RTD is a more accurate temperature sensor and is more linear than a thermocouple and has a wider temperature range than a thermistor.
- Process temperatures of less than 400˚F (240˚ C) generally require the use of low temperature range RTD construction because moisture is usually present around this lower temperatures.
- Sensors for a suspected high vibration process should be ordered as high temperature range construction because the highly compacted MgO construction provides superior vibration resistance when compared to low temperature range constructions.
- Minimum insertion of the RTD sensor assembly should be nominal 5-10 times the OD of the sensor sheath to avoid heat dissipation problems.
- The temperature sensitive portion of the RTD assembly is typically a 1 inch length at the hot tip.
- A three wire construction is recommanded to compensate for the resistance lead wire. 2-wire construction can not compensate for the lead wire resistance and therefor can create errors in the temperature measurement.
Shown here is a duplex element RTD designed for two different process temperatures