Maintenance, validation, and recalibration of liquid-in-glass thermometers

Abstract

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.

LiG Metrology, Correlated Error, and the Integrity of the Global Surface Air-Temperature Record

“…Resolution Until relatively recently, the U.S. National Institute of Standards and Technology (NIST) carried out detailed calibrations and evaluations of LiG thermometers [99,100]. NIST publications list the visually indistinct physical imperfections that can degrade the accuracy of a LiG thermometer, including "changes in bulb volume, microscopic alterations in glass geometry at elevated temperature, microscopic cracking, degradation of the thermometer liquid", and endogenous solids, such as glass particles in the capillary [101,102]. Microscopic inspection to detect such flaws preceded calibration of LiG thermometers at NIST [102].…”

“…Prior to year 2000, LiG thermometer calibrations at NIST employed visualization by eye, which is the standard method used to obtain LiG thermometer readings at meteorological stations. The uncertainty attending a LiG thermometer reading taken by eye (visual repeatability) is 1/4 of a scale division [101,105]. For a calibrated 1 • C/division mercury or alcohol (spirit) meteorological LiG thermometer, the true air temperature is taken to be somewhere within ±0.125 • C of the measurement as read by eye.…”

“…If a LiG thermometer suffers ice-point calibration drift and is recalibrated, the uncertainty from visual repeatability again enters. For a 1 • C/division LiG thermometer, the new lower limit of uncertainty following recalibration is then 2σ = 1.96 × (0.055) 2 + (0.072) 2 + (0.072) 2 = ±0.23 • C [101].…”

Termometry szklane cieczowe – charakterystyka przyrządów

“…Jeśli w termometrze zaobserwuje się zmianę barwy cieczy lub jakiekolwiek produkty uboczne utleniania lub rozkładu cieczy w kapilarze lub zbiorniku (fot. 3), również należy niezwłocznie wycofać go z eksploatacji (Cross et al, 2009).…”