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Non-Electric Thermometers 2.1 Liquid-in-Glass Thermometers Liquid-in-glass thermometers are based upon the temperature dependent variation of the volume of the liquid which is used . The thermometer consists of a liquid filled bulb connected to a thin capillary with a temperature scale as shown in Figure 2.1 . Assuming that the bulk volume, Vb, is much greater than that of the liquid contained in the capillary, the volume variation, AV, of the liquid corresponding to the measured temperature variation, d6, is: AV = V/3.;AO (2.1) where /3a is the average apparent coefficient of cubic thermal expansion of the thermometric liquid in the given glass | Temperature Measurement Second Edition L. Michalski K. Eckersdorf J. Kucharski J. McGhee Copyright 2001 John Wiley Sons Ltd ISBNs 0-471-86779-9 Hardback 0-470-84613-5 Electronic 2 Non-Electric Thermometers 2.1 Liquid-in-Glass Thermometers Liquid-in-glass thermometers are based upon the temperature dependent variation of the volume of the liquid which is used. The thermometer consists of a liquid filled bulb connected to a thin capillary with a temperature scale as shown in Figure 2.1. Assuming that the bulk volume Kb is much greater than that of the liquid contained in the capillary the volume variation AV of the liquid corresponding to the measured temperature variation AS is AF KJ8aAr 2.1 where Ja is the average apparent coefficient of cubic thermal expansion of the thermometric liquid in the given glass. This coefficient which also covers small changes of the bulb volume as a function of the measured temperature has an average value for a given application range of the thermometer. It equals the difference between the respective coefficients of cubic expansion jGj of the liquid and j3g of the glass so that 2.2 Assume that the inner capillary has a diameter d and that the temperature difference Ar corresponds to a change of length AZ of the liquid column. Using equation 2.1 the thermometer sensitivity is AZ _ AV _ jSi-jgg At toZ2Aj 4 nd2 2-3 Equation 2.3 indicates that the sensitivity increases in direct proportion with increase in bulb volume Vb and coefficient Pa but as the inverse square of capillary diameter d. There are some practical limits to increasing this sensitivity. Firstly an increase in bulb volume increases the thermal inertia of the thermometer. Secondly if the bore of the capillary is too small the liquid column may break easily under the influence of surface tension. In mercury-in-glass thermometers for the Celsius scale the bulb volume is about 20 NON-ELECTRIC THERMOMETERS SCALE CAPILLARY LIQUID COLUMN BULB Figure 2.1 Liquid-in-glass .
