What is a Platinum RTD?
Platinum RTDs (Resistance Temperature Detectors) are sensing elements that are made of pure platinum wire coil (wire wound) encapsulated in ceramic or glass, or a thin film platinum deposited on a ceramic substrate. Platinum RTDs have a positive temperature coefficient. The electrical resistance increases as temperature rises in a known and repeatable manner. Their linearity and unmatched long term stability firmly establish platinum RTD elements as an ideal sensor for most industrial applications. Thin film elements offer performance equal to standard wire wounds, but with improved cost, size and convenience.
Wide Temperature Range:
Platinum RTDs cover temperature ranges from
- -200C to 650C Ceramic encapsulated wire wounds
- -200C to 350C Glass encapsulated wire wounds
- -50C to 650C Thin film elements
Standardization:
Repeatability and Stability
Platinum Resistance Temperature Detectors are the primary interpolation sensors used by the National Bureau of Standards for temperatures ranging from -260C to 630C. Platinum RTDs provide excellent long term stability.
High Signal Output
RTD elements provide a much larger voltage drop signal than do thermocouples.
Linearity
Platinum RTD elements follow a more linear curve than thermocouples and most thermistors. RTD non-linearity can be corrected by the proper design of resistive bridge networks or mathematical corrections with a microprocessor.
Overall System Costs
Platinum RTDs can utilize standard copper wire extension leads and do not require special connection considerations like the thermocouples. Platinum RTDs are best suited for most industrial applications over a wide temperature range, especially when stability and linearity are essential.
What are Thermocouples?
A thermocouple is made of two dissimilar metals that are welded together. The reference junction is typically part of the instrumentation and the opposite junction is called the sensing junction. When the sensing junction is heated, the opposing metals create a known thermoelectric effect proportional to the temperature difference between the two junctions. By compensating for the known temperature of the reference junction, the temperature at the sensing junction can be determined. Different calibration types provide differing thermoelectric effects for differing temperatures and environments. Because of the cold point compensation and special extension wires, designing thermocouples into systems can be complicated.
Extremely high temperature range Thermocouples can be rated as high as 1700C. The inherent simplicity of thermocouples allow them to withstand extreme shock and vibration. Small size/fast response Thermocouples with exposed or grounded junctions offer nearly immediate response to temperature changes. Thermocouples are best suited to high temperatures and extreme environments.
What are Thermistors?
Thermistors are resistive devices made up of metal oxides that are formed into a bead and encapsulated in epoxy or glass. Thermistors generally have a negative temperature coefficient; the resistance decreases as temperature increases. While thermistors are very sensitive, they are typically non-linear and have a limited temperature range.
Low sensor cost Thermistor elements are typically less expensive than RTD elements in the sensor form. High resistance equates to high sensitivity. Thermistors provide a larger voltage drop over a specific temperature using the same measuring current. Self heating is more problematic with a thermistor due to its small size. Point sensing is a major attribute of thermistors within limited temperature ranges. One typical application is biological monitoring where the small size and mass are critical. Thermistors are best suited for low cost applications over limited temperature ranges.
| RTD | Thermocouple | Thermistor | |
| Temp. Range | -200 to 650C | -270 to 1800C | -80 to 150C |
| Sensor Cost | Mid Range | Low | Low |
| System Cost | Mid Range | High | Mid Range |
| Linearity | High | Mid Range | Low |
| Stability | High | Low | Mid Range |
| Sensitivity | Mid Range | Low | High |
| Purpose | Best Accuracy; Most Applications | Extreme Temperatures | Low Cost, Biology |
Stability and Interchangeability
Platinum is an inherently stable metal. Platinum RTDs possess unparalleled long term stability and consequently a platinum resistance thermometer is the primary interpolation instrument used by NIST, the National Institute of Standards and Technology. Interchangeability describes how closely an RTD element follows its nominal resistance/temperature curve and the maximum tolerance variation allowed within the DIN and IEC standards specifications. Interchangeability consists of both the tolerance at 0°C and the slope of the resistance/temperature curve.