Liquid-in-Glass Thermometers

About liquid-in-glass thermometers

Thermal expansion methods make use of the fact that the dimensions of all substances, whether solids, liquids or gases, change with temperature. Instruments operating on this physical principle include the liquid-in-glass thermometer, the bimetallic thermometer and the pressure thermometer.


Liquid-in-glass thermometers
The liquid-in-glass thermometer is a well-known temperature measuring instrument which is used in a wide range of applications. The fluid used is usually either mercury or colored alcohol, and this is contained within a bulb and capillary tube. As the temperature rises, the fluid expands along the capillary tube and the meniscus level is read against a calibrated scale etched on the tube. The process of estimating the position of the curved meniscus of the fluid against the scale introduces some error into the measurement process and a measurement accuracy better than 1% of full-scale reading is hard to achieve.


However, an accuracy of 0.15% can be obtained in the best industrial instruments. Industrial versions of the liquid-in-glass thermometer are normally used to measure temperature in the range between -200 C and +1000 C, although instruments are available to special order which can measure temperatures up to 1500 C.


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