Evaluation of thermocouple junction temperature

The ice point (0(0)C) is the most important temperature in thermocouple measurements. For interlaboratory reproducibility most thermocouple calibration curves are referenced to the ice point. Therefore, before one can accurately interpret a thermocouple's voltage output, the accuracy of the reference junction (0(0)C) must be known. The purpose is to evaluate and report on the variety of ways the Rocky Flats Physical Metrology Laboratory (PML) creates or simulates a 0(0)C environment. The following methods are evaluated in this report: ice bath of distilled water + distilled ice; ice bath of tap water + tap ice; and thermistors. In addition, the effects of using various Type SX extension leads between the thermocouple and the reference junction are evaluated. This was undertaken because all PML automated Thermocouple Calibration Systems show an offset at 0(0)C of between 0.2 and 0.6(0)C for Type S thermocouples. Justification for subtracting out this bias is the specific goal of this investigation.