Ability to maintain R vs T under the same conditions after experiencing Repeatability: thermal cycling throughout a specified temperature range. Change in the characteristics of the materials from which the RTD is built due to Hysteresis: exposures to varying temperatures. Error that results from the PRT sheath conducting heat into or out of the Stem Conduction: process. Errors that occur due to calibration uncertainty at the cal points, Calibration/Interpolation: or between cal point due to propagation of uncertainty or curve fit errors. Errors that occur because a 4 wire or 3 wire measurement is not used, this is Lead Wire: 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. Error produced by the heating of the PRT element due to the power applied. Self Heating: Errors are produced during temperature transients because the PRT cannot Time Response: respond to changes fast enough. Thermal EMF errors are produced by the EMF adding to or subtracting from Thermal EMF: the applied sensing voltage, primarily in DC systems. 2.3.6. Construction Figure 2.6: Construction of an RTD (Wikipedia.org) These elements nearly always require insulated leads attached. At temperatures below about 250 °C PVC, silicon rubber or PTFE insulators are used. Above this, glass fibre or ceramic are used. The measuring point, and usually most of the leads, require a housing or protective sleeve, often made of a metal alloy which is chemically inert to the process being monitored. Selecting and designing protection sheaths can require more care than the actual sensor, as the sheath must withstand chemical or physical attack and provide convenient attachment points. 2.3.7. Wiring Configuration Two-wire configuration Figure 2.7: Two-wire configuration of an RTD
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. 2.3.6. Construction Figure 2.6: Construction of an RTD (Wikipedia.org) These elements nearly always require insulated leads attached. At temperatures below about 250 °C PVC, silicon rubber or PTFE insulators are used. Above this, glass fibre or ceramic are used. The measuring point, and usually most of the leads, require a housing or protective sleeve, often made of a metal alloy which is chemically inert to the process being monitored. Selecting and designing protection sheaths can require more care than the actual sensor, as the sheath must withstand chemical or physical attack and provide convenient attachment points. 2.3.7. Wiring Configuration Two-wire configuration Figure 2.7: Two-wire configuration of an RTD
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