Liquid-in-glass thermometers are pervasive throughout industry. These thermometers provide a convenient method for measuring temperature over a broad range of applications with reasonable stability and accuracy. However, like all thermometers, liquid-in-glass thermometers are subject to drift as a function of time and maximum temperature of use. This paper describes methods for the validation and recalibration of previously calibrated liquid-in-glass thermometers to maintain the metrological traceability established with the initial calibration. Procedures and examples are given for visually inspecting a thermometer, using an ice point for a single-point recalibration, and determining uncertainties for thermometers with a single-point recalibration. We discuss when a single-point recalibration is acceptable, and when a full recalibration is needed.

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The use of liquid-in-glass (LIG) thermometers is described in many documentary standards in the fields of environmental testing, material testing, and material transfer. Many national metrology institutes, including the National Institute of Standards and Technology (NIST) and the National Research Council of Canada (NRC), list calibration services for these thermometers among the Calibration Measurement Capabilities of Appendix C of the BIPM Key Comparison Database. NIST and NRC arranged a bilateral comparison of a set of total-immersion ASTM-type LIG thermometers to validate their uncertainty claims. Two each of ASTM thermometer types 62C through 69C were calibrated at NIST and at NRC at four temperatures distributed over the range appropriate to each thermometer, in addition to the ice point. Collectively, the thermometers span a temperature range of −38 • C to 305 • C. In total, 160 measurements (80 pairs) comprise the comparison data set. Pair-wise differences (T NIST -T NRC ) were formed for each thermometer at each temperature. For 8 of the 80 pairs (10 %), the differences exceed the k = 2 combined uncertainties. These results support the claimed capabilities of NIST and NRC for the calibration of LIG thermometers.

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