Page 264 - Instrumentation Reference Book 3E

248 Temperature measurement

change in its volume. The Bourdon tube, the bulb,
and thc capillary tube are completely filled with
mercury, usually at a high pressure. When suitably
designed, the capillary tube may be of consider-
able length so that the indicator operated by the
Bourdon tube may be some distance away
from the bulb. In this case the instrument is
described as being a “distant reading” or “trans-
mitting” type.
When the temperature rises, the mercury in the
bulb expands more than the bulb so that some
mercury is driven through the capillary tube into
the Bourdon tube. As the temperature continues
to rise, increasing amounts of mercury will be
driven into the Bourdon tube, causing it to
uncurl. One end of the Bourdon tube is fixed, J
while the motion of the other end is communi-
cated to the pointer or pen arm. As there is a large
force available the Bourdon tube may be made Figure 14.6 Multi-turn Bourdon tube.
robust and will give good pointer control and
reliable readings. into two coils of several turns. The coils are
The Bourdon tube may have a variety of forms,
and the method of transmitting the motion to the arranged one behind the other so that the free
end of each is at the center while the other turn of
pointer also varies. Figure 14.5 shows one form of the coils is common to both, as can be seen in the
Bourdon tube in which the motion of the free end illustration. One end of the continuous tube-the
is transmitted to the pointer by means of a seg- inner end of the back coil-is fixed and leads to
ment and pinion. The free end of the tube forms a the capillary tube, while the other end-the inner
trough in which a stainless steel ball at the end of end of the front coil-is closed, and is attached to
the segment is free to move. The ball is held the pointer through a small bimetallic coil which
against the side of the trough by the tension in forms a continuation of the Bourdon tube. This
the hair-spring. By using this form of construc- bimetallic coil compensates for changes brought
tion lost motion and angularity error are avoided, about in the elastic properties of the Bourdon
and friction reduced to a minimum. Ambient- tube and in the volume of the mercury within
temperature compensation may be obtained by the Bourdon tube due to ambient temperature
using a bimetallic strip, or by using twin Bourdon changes.
tubes in the manner described under the heading This particular formation of the tube causes
of capillary compensation. the pointer to rotate truly about its axis without
Figure 14.6 shows a Bourdon tube having a
different form, and a different method of trans- the help of bearings, but bearings are provided to
keep the pointer steady in the presence of vibra-
mitting the motion to the pointer. This Bourdon tion. The friction at the bearings will, therefore,
tube is made of steel tube having an almost flat be very small as there is little load on them. As the
section. A continuous strip of the tubing is wound
end of the Bourdon tube rotates the pointer
directly, there will be no backlash.
hi Therrnonieter bulbs The thermomctcr bulb may

have a large variety of forms depending upon the
Pmter alummum
Bourdoo
Iprinq
Gear wd prnron cold row Itd chopmanacd use to which it is put. If the average temperature
of a large enclosure is required, the bulb may take
the form of a considerable length of tube of
smaller diameter either arranged as a U or wound
into a helix. This form of bulb is very useful when
the temperature of a gas is being measured, for it
presents a large surface area to the gas and is
therefore more responsive than the forms having
a smaller surface area for the same cubic capacity.
In the more usual form, the bulb is cylindrical
Figure 14.5 Construction of mercury-in-steel in shape and has a robust wall: the size of the
thermometer. Courtesy the Foxboro Company. cylinder depends upon many factors, such as the