Page 270 - Instrumentation Reference Book 3E

254 Temperature measurement

temperature of the capillary tube and Bourdon The range of an instrument using a particular
tube is above or below that of the bulb. liquid is limited by the fact that the maximum
If the ambient temperature of the capillary and temperature for which it can be used must be well
Bourdon tube is above that of the bulb, then they below the critical temperature for that liquid. The
will be full of vapor. which will transmit the range is further limited by the non-linear nature
vapor pressure, as shown in Figure 14.11(a). of the scale.
When the ambient temperature increases. it will In Table 14.11 the three types of fluid-
cause the vapor in the capillary and Bourdon filled thermometers are compared.
tube to increase in pressure temporarily, but this
will cause vapor in the bulb to condense until the 14.3.5 Solid expansion
pressure is restored to the saturated vapor pres-
sure of liquid at the temperature of the bulb. Thermal expansion of solids, usually metals, forms
Vapor pressure instruments are not usually the basis of a wide range of inexpensive indicating
satisfactory when the temperature being mea- and control devices. These devices are not particu-
sured at the bulb is near the ambient temperature larly accurate: typically errors of as much as rt5"
of the capillary and the Bourdon tube. In particu- or more may be expected, but due to their low cost
lar, significant measurement delays occur as the they find wide application, especially in consumer
measured temperature crosses the ambient tem- equipment. As indicated earlier in this section this
perature. These delays are caused by the liquid technique is also used to provide temperature
distilling into or out of the gauge and capillary, compensation in many instruments.
Figure 14.11(b). The temperature-sensitive elements using solid
If there is a significant level differencre between expansion fall into two groups: rod sensing
the bulb and the gauge, an error will be produced probes and bimetal strips.
when liquid distills into the capillary due to the There are so many applications that only one
pressure head from the column of liquid. or two examples will be given to illustrate the
When rapid temperature changes of the bulb techniques.
occur passing through ambient temperature the
movement of the instrument pointer may be quite
erratic due to the formation of bubbles in the 14.3.5.1 Rod sensing probes
capillary. The widest application of this technique is for
In order to overcome the defects brought immersion thermostats for use in hot water tem-
about by distillation of the liquid into, and perature control. Figure 14.12 shows diagramma-
out of, the capillary and Bourdon tubes, these tically the operation of an immersion thermostat.
tubes may be completely filled with a non- The microswitch is operated by the thermal expan-
vaporizing liquid which serves to transmit the sion of the brass tube. The reference length is
pressure of the saturated vapor from the bulb provided by a rod of low thermal expansion such
to the measuring system. To prevent the non- as Invar. These thermostats, thought not particu-
vaporizing liquid from draining out of the larly accurate and having a switching differential
capillary tube. it is extended well down into of several degrees Celsius, provide a very rugged
the bulb, as shown in Figure 14.11(c), and the and reliable control system for a non-critical
bulb contains a small quantity of the non- application such as domestic hot water control.
vaporizing fluid. The non-vaporizing fluid will Figure 14.13 shows another rod application. In
still tend to leave the capillary tube unless the this case to achieve greater sensitivity the expand-
bulb is kept upright. ing component is coiled.
Vapor pressure thermometers are very widely
used because they are less expensive then liquid-
and gas-filled instruments. They also have an 14.3.5.2 Bimetal strip tlzermonzeter
advantage in that the bulb can be smaller than Bimetal strips are fabricated from two strips of
for the other types. different metals with different coefficients of ther-

m

Snap
action
switch

Figure 14.12 Rod thermostat.