= Platinum resistance thermometer comparison calibration
A 1998 NIST account describes stirred comparison baths, a reference <SPRT> with a 30 Hz <resistance bridge>, and a temperature-controlled $100\,\Omega$ reference <resistor>. The test <IPRTs> used a $1\,\mathrm{mA}$ <current source>, a scanner, and an 8½-digit <voltmeter>. Separate current and voltage leads made each test probe a four-terminal device; readings in both current directions supplied voltage ratios to the reference <resistor>.<A New NIST Automated Calibration System for Industrial-Grade Platinum Resistance Thermometers by Gregory F. Strouse and others>[A New NIST Automated Calibration System for Industrial-Grade Platinum Resistance Thermometers, §§3.1, 3.3 and 4.2, printed pp. 2–6: baths, readout, four-terminal devices and reversed current]{ref}
Its program bracketed test readings with reference <SPRT> readings and repeated a block if bath drift was excessive. A check <SPRT> monitored the system; reference water-<triple point> measurements checked stability. Sensor depths were matched, and unfamiliar probes underwent immersion tests. Ice and water-<triple point> readings before and after calibration checked changes. Additional points could test the fitted curve. Reported calibration uncertainties excluded transport, subsequent drift, <hysteresis>, and the user's measurement system. These are controls and limits of that documented apparatus, not an accuracy promise for all <platinum resistance thermometers>.<A New NIST Automated Calibration System for Industrial-Grade Platinum Resistance Thermometers by Gregory F. Strouse and others>[§1, printed p. 2; §4.2, printed pp. 5–6; §5.4, printed p. 9: excluded effects, drift rejection, check thermometer, immersion and extra fit checks]{ref}
The inference is a pair: reference <temperature> from the <SPRT>, test <electrical resistance> from the electrical readout. Record measuring <electric current>, wiring, immersion, and the stated reference scale with each pair. A bath controller's setpoint is not itself the calibrated reference <temperature>. Returning to a reference and checking intermediate points distinguishes a fitted relation from a reproducible calibration over the intended interval.
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