Evaluating Temperature Coefficient Wander in Thin Film Platinum Resistance Elements
Evaluating temperature coefficient wander in thin film platinum RTDs requires tracking substrate strain, glass passivation diffusion, and multi-point calibration.
Self-Heating Uncertainty
Metrological Definition
Temperature measurement error arises when electrical current passing through a resistive element generates unwanted internal heat. Self heating uncertainty quantifies the residual variance remaining after applying correction factors to compensate for that parasitic temperature rise. Calibration laboratories determine this dispersion by recording resistance changes across multiple excitation currents in a controlled liquid bath.
Thermal dissipation depends heavily on fluid velocity and sensor packaging materials. Operating conditions in the field rarely match the laboratory environment where baseline coefficients were established.
Thermal Resistance
Conduction losses through lead wires dissipate a fraction of the thermal energy generated at the sensing junction. Mathematical models treat this heat transfer mechanism as an electrical network of thermal resistors in series. Immersion depth dictates the rate at which heat escapes from the sheath into the surrounding medium.
Gas streams present higher thermal contact resistance than agitated liquids because fluid density restricts local convection currents. Manufacturers publish self heating coefficients derived from standardized immersion tests in circulating water.
Excitation Current
Power dissipation scales with the square of the electrical current supplied by the measuring bridge circuit. Instrument designers select lower excitation levels to minimize internal heating effects inside miniature resistance thermometers. Signal conditioning electronics amplify the resulting voltage drop to preserve overall measurement resolution without driving excessive current through the sensing element.
Standardized testing protocols require laboratories to verify that applied electrical power does not alter the measured temperature beyond stated repeatability limits.
Error Propagation
Uncompensated thermal shifts introduce systematic biases into process control loops and custody transfer metering systems. Calibration certificates specify the exact thermal dissipation coefficient for a given probe under reference conditions. Field technicians apply corrections using real time current values and known sensor characteristics to subtract the estimated self heating offset from the final output signal.
Residual uncertainty bounds combine with other instrumentation tolerances to establish the expanded measurement uncertainty reported on official calibration documentation.