The most accurate glass thermometers we have at the secondary transfer level are in the
total immersion set. This set has a combined range of -10,C to 360,C with an accuracy
of 0.2,C. The total immersion set is classified as a secondary reference standard.
How can the immersion set indicate up to 360,C when mercury boils at 357,C?
Standard thermometers which indicate over 150,C contain mercury and an inert gas
rather than a vacuum. Expanding mercury compresses the gas; this is very important, for
near the boiling point of mercury, the gas pressure keeps the mercury from vaporizing.
The thermometers we use contain nitrogen gas.
5. BIMETALLIC THERMOMETERS.
Bimetallic and mercury thermometers operate on the same basic principle; that is, they
operate on the principle of different expansion rates of materials.
In bimetallic thermometers, usually, two types of nickel alloys are used: one for its
thermal characteristics and the other one for a reference.
One of the most common reference alloys is called Invar because, at normal atmospheric
temperature, its length is invariable (it neither expands nor contracts). Of course, what we
really mean is that the expansion rate is negligible.
When a straight bimetallic strip fastened at one end is heated, it bends to form an arc or a
circle. This bending action or curvature, by a mechanically suitable system and scale, can
be used to move a pointer tip so that it describes an arc. (Figure 3)
Obviously, as temperature is increased, the bimetallic strip must bend downward because
the top layer would expand at a greater rate than the base metal. (Figure 4)
If metal B had the higher coefficient of expansion, it would expand at a greater rate and
cause the pointer to move upward. (Figure 5).