Resistance Thermometer - Advantages and Limitations

Advantages and Limitations

The advantages of platinum resistance thermometers include:

• High accuracy
• Low drift
• Wide operating range
• Suitability for precision applications.

Limitations: RTDs in industrial applications are rarely used above 660 °C. At temperatures above 660 °C it becomes increasingly difficult to prevent the platinum from becoming contaminated by impurities from the metal sheath of the thermometer. This is why laboratory standard thermometers replace the metal sheath with a glass construction. At very low temperatures, say below -270 °C (or 3 K), because there are very few phonons, the resistance of an RTD is mainly determined by impurities and boundary scattering and thus basically independent of temperature. As a result, the sensitivity of the RTD is essentially zero and therefore not useful.

Compared to thermistors, platinum RTDs are less sensitive to small temperature changes and have a slower response time. However, thermistors have a smaller temperature range and stability.

Sources of error:

The common error sources of a PRT are:

• Interchangeability: the “closeness of agreement” between the specific PRT's Resistance vs. Temperature relationship and a predefined Resistance vs. Temperature relationship, commonly defined by IEC 60751.
• Insulation Resistance: Error caused by the inability to measure the actual resistance of element. Current leaks into or out of the circuit through the sheath, between the element leads, or the elements.
• Stability: Ability to maintain R vs T over time as a result of thermal exposure.
• Repeatability: Ability to maintain R vs T under the same conditions after experiencing thermal cycling throughout a specified temperature range.
• Hysteresis: Change in the characteristics of the materials from which the RTD is built due to exposures to varying temperatures.
• Stem Conduction: Error that results from the PRT sheath conducting heat into or out of the process.
• Calibration/Interpolation: Errors that occur due to calibration uncertainty at the cal points, or between cal point due to propagation of uncertainty or curve fit errors.
• Lead Wire: Errors that occur because a 4 wire or 3 wire measurement is not used, this is greatly increased by higher gauge wire.
  • 2 wire connection adds lead resistance in series with PRT element.
  • 3 wire connection relies on all 3 leads having equal resistance.
• Self Heating: Error produced by the heating of the PRT element due to the power applied.
• Time Response: Errors are produced during temperature transients because the PRT cannot respond to changes fast enough.
• Thermal EMF: Thermal EMF errors are produced by the EMF adding to or subtracting from the applied sensing voltage, primarily in DC systems.