LIG – Liquid in Glass Thermometers
Liquid in glass thermometers are the sensor one visualizes most often for temperature measurement.
A glass cylinder with a bulb at one end, a capillary hole down the axis, connected to the reservoir in the bulb filled with silvery mercury or perhaps a red-colored fluid, an engraved temperature scale.
That’s what I picture in my mind’s eye as a proper thermometer.
There are many designs and a significant range of capabilities and uses of those designs.
There are specifications and standards, too. The image at the left depicts a sample of the General Purpose Laboratory LIG thermometers made by the Miller & Weber company of New York, USA.
These types of thermometers are used primarily in QA, R&D and Testing Laboratories and are always certified traceable to a National Standard for temperature. In that way, the lab can provide an unbroken chain of calibration traceability to the fundamental standards that underpin the International Temperature Scale.
Up until quite recently, almost all these kinds of precision thermometers, including the more run-of-the-mill clinical or fever thermometers used a filling of liquid mercury. the silvery liquid metal.
However, the well-documented hazards from mercury contamination of lakes, rivers and ponds has prompted an environmental effort to completely eliminate mercury from all products, such as these.
In even more recent years (2007 and on), the standards organizations of the world and the USA in particular have been working to rewrite the standards that specify the use of mercury filled glass thermometers.
ASTM International in the USA has a major task force identifying standards and developing alternative ones.
The LIG Thermometer makers have been equally challenged to develop and bring to market suitable alternative materials that are environmentally safe.
New designs must undergo rigorous testing to be certain that they can provide satisfactory precision and repeatability in use as the time-proven mercury-filled devices.
The story is far from complete as of the end of 2009.