Methods of Reading Liquid-in-Glass Thermometers

The objective of this paper is to analyse the different methods of reading Liquid-in-Glass Thermometers (LiGTs). The aim is to minimise uncertainty contributions in calibrations of LiGTs and make them easier to use. The inherent problem of the LiGTs is the objectivity of reading and hence the resulting uncertainty contribution. Most commonly, LiGTs are read with the naked eye or by using a magnifying glass. These are classified as subjective methods. Major improvements could be achieved by using specially designed stereoscopes. A stereoscope and standard platinum resistance thermometer together represent the reference standard in calibration of precise LiGTs. The entire procedure is considered as the reference method. As a new method, a CCD camera with specially developed image processing software is introduced. It combines the objectivity of the stereoscope readings and automation of LiGT measurements and calibrations.