bimetallic expansion thermometer is a bimetallic
There are two basic types of filled-system
strip consisting of two layers of different metals
thermometers. One has a Bourdon tube that
fused together. When such a strip is subjected to
responds primarily to changes in the volume of
temperature changes, one layer expands or
the filling fluid; the other is one in which the Bour-
contracts more than the other, thus tending to
don tube responds primarily to changes in the
change the curvature of the strip.
pressure of the filling fluid. Clearly, some pressure
Figure 5-2 shows the basic principle of a
effect will exist in volumetric thermometers, and
bimetallic expansion thermometer. When one end
some volumetric effect will exist in pressure
of a straight bimetallic strip is fixed in place, the
thermometers.
other end tends to curve away from the side that
has the greater coefficient of linear expansion
A distant-reading thermometer (fig. 5-4)
when the strip is heated.
consists of a hollow metal sensing bulb at one end
For use in thermometers, the bimetallic strip
of a small-bore capillary tube, which is connected
is normally wound into a flat spiral (fig. 5-3), a
at the other end to a Bourdon tube or other device
single helix, or a multiple helix. The end of the
that responds to volume changes or to pressure
strip that is not fixed in position is fastened to
changes. The system is partially or completely
filled with a fluid that expands when heated and
the end of a pointer that moves over a circular
scale. Bimetallic thermometers are easily adapted
contracts when cooled. The fluid may be a gas,
for use as recording thermometers; a pen is
an organic liquid, or a combination of liquid and
attached to the pointer and positioned in such a
vapor.
way that it marks on a revolving chart.
T h e device usually used to indicate
temperature changes by its response to volume
Filled-System Thermometers
changes or to pressure changes is called a
Bourdon tube. A Bourdon tube is a curved or
In general, filled-system thermometers are
twisted tube which is open at one end and sealed
designed for use in locations where the indicating
part of the instrument must be placed some
distance away from the point where the
temperature is to be measured. For this reason
they are often called distant-reading ther-
mometers. However, this is not true of all filled-
system thermometers. In a few designs, the
capillary tubing is extremely short, and in a few,
it is nonexistent. In general, however, filled-system
thermometers are designed to be distant-reading
thermometers. Some distant-reading ther-
mometers may have capillaries as long as 125 feet.
Figure 5-3.--Bimetallic thermometer (flat, spiral strip).
Figure 5-4.--Distant-reading, Bourdon-tube thermometer.
5-3