= Thermocouple voltage calibration
<Thermocouple voltage calibration> relates an actual assembled sensor's <electromotive force> to independently established junction <temperatures>. It must be distinguished from selecting a nominal letter-type conversion curve. In <Thermocouples Calibrations Services by National Institute of Standards and Technology> (opening method description and bath-calibration table), NIST describes comparison with a maintained reference <thermocouple>, comparison with a standard platinum <resistance thermometer> in controlled <heat baths>, and fixed-point calibration at the freezing points of zinc, aluminum, silver and gold on the <International Temperature Scale of 1990>. These are independent thermal references, unlike a test that checks only a voltage-input circuit.<Thermocouples Calibrations Services by National Institute of Standards and Technology>[method description; table for base-metal thermocouples compared with an SPRT in a temperature-controlled bath]{ref}
For a concrete electrical check, the SCC-TC guide describes shorting the input after 30 minutes of powered operation and adjusting its offset to a zero-voltage readout. Its separate calibrator procedure requires equal connection <temperatures> at the calibrator and module, measures the supplied voltage, converts it to <temperature>, and compares with the calibrator setting. Such an injected-signal test checks the readout chain; it does not expose a real measuring junction to an independently known bath <temperature>.<SCC-TC Series Thermocouple Input Modules User Guide by National Instruments>[p. 9, Calibrating System Offsets; Calibrating the SCC-TC0X Using a Thermocouple Calibrator]{ref}
Useful additional checks follow from the model: log the raw voltage, the reference <temperature>, and the final inferred <temperature> separately; hold $T_h$ stable while changing $T_c$ and test whether <cold-junction compensation> preserves the inferred $T_h$; compare against an independent <thermometer> at several stable <heat bath> settings. Check immersion and repeatability instead of assuming a steady display establishes <thermal equilibrium>. These are proposed diagnostic checks, not new experimental results. The equal-<temperature> and polarity controls are already described under <Seebeck effect>.
Limits arise separately from the <thermocouple reference function>, sensor-wire changes, thermal contact, terminal <temperature> differences, and the voltage-input circuit. In particular, the SCC-TC guide warns of connector-to-module <temperature gradients> and voltage drops caused by the fault-detection <electric current> flowing through lead <electrical resistance>.<SCC-TC Series Thermocouple Input Modules User Guide by National Instruments>[p. 6, Uncompensated Connectors and Accuracy; p. 7, Errors Due to Open-Thermocouple Detection Circuitry]{ref} A fitted curve or compensation algorithm cannot make all these errors vanish. No single accuracy is assigned to every <thermocouple>.
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