Making Glass
Liquid in glass thermometers are made to read
temperatures form -200oC to +630oC, and can vary in length from 100
to 2500mm. The stem of the thermometer consists of a fine bore capillary tube with an
outside diameter between 3mm and 9mm with a bore diameter between 0.05mm and 0.40mm.
Depending on the range of temperature to be measured and the length of the divided scale,
a suitable size of bore is selected, carefully measured and a bulb of precalculated volume
fused to the stem.
After careful annealing, to remove strain from the glass,
the thermometer is filled with mercury or a coloured organic liquid and the height of the
liquid column adjusted so that the scale is in the correct position. When mercury is used,
various processes are involved to make sure that the mercury is dry and free from gas so
that it moves smoothly in the bore without breaking up into small globules. The space
above the liquid is then filled with an inert gas, or for some special applications
remains a vacuum. The thermometer is finally sealed to the required length.
The next very important process is to calibrate the
thermometer at three or more temperatures to provide reference point for making the scale.
Very accurate and special laboratory thermometers have an etched scale. Laboratory and
clinical and thermometers, produced in large numbers, generally have a scale printed onto
the glass which becomes permanent after firing just below the annealing temperature. There
are literally thousands of different types of thermometers made to suit various
applications and specifications.
Glass as a material in its own right will always exist. But
many new applications and manufacturing processes will involve glass in combination with
other materials. Optical fibres, for example, are currently manufactured with one or more
different coating, which are often plastics. With the increasing sophistication of
opto-electronic devices, there is an increasing need to combine optical and electronic
devices for many applications such as transmission of audio, video and data information.
Glasses and ceramics, either alone or composite with other materials, will find increasing
application in biological and medical areas. Materials such as photochromic,
electrochromic and thermochrominc glasses, which respond to external stimuli, are being
developed with various, sometimes unusual, applications.
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