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TEMPERATURE-MEASURING
INSTRUMENTS
Temperature is the degree of hotness or coldness
of a substance measured on a definite scale.
Temperature is measured when a measuring
instrument,
such as a thermometer, is brought into
contact with the medium being measured. All
temperature-measuring instruments use some
change in a material to indicate temperature. Some
of the effects that are used to indicate temperature
are changes in physical properties and altered
physical dimensions. One of the more important
physical properties used in temperature-measuring instruments
is the change in the length of a
material in the form of expansion and contraction.
Consider the uniform homogeneous bar illustrated
in figure 8-11. If the bar has a given
Figure 8-11.—Expansion of a bar.
length ( LO)
at some temperature and is heated, it will
expand (Lf).
The amount of expansion  is a function of the original length and
the temperature increase. The
amount a material changes in length
with temperature is called the linear
coefficient of expansion.
The linear coefficient of expansion for a material
is a physical property of that material and
describes its behavior with respect to temperature.
BIMETALLIC EXPANSION
THERMOMETER
If two materials with different linear coefficients
are
bonded together, as the temperature changes
their rate of expansion will be different. This
will cause the entire assembly to bend in an arc
as shown in figure 8-12.
When the temperature is raised, an arc is formed
around the material with the smaller expansion
coefficient. Since this assembly is formed
by joining two dissimilar materials, it is known
as a bimetallic element.
A modification of this bimetallic strip serves as
the basis for one of the simplest and most commonly
encountered temperature-measuring instruments,
the bimetallic thermometer. Figure
8-13 shows a bimetallic thermometer. In
it, a bimetallic strip is wound in the form of a
long helix. One end of the helix is held rigid. As
the temperature varies, the helix tries to wind or unwind. This causes the free
end to rotate. The
Figure 8-12.—Effect of unequal expansion of a bimetallic strip.
free end is connected to a pointer. The pointer actually
indicates angular rotation of the helix; however,
since the rotation is linear and a function of
temperature, the scale is marked in units of temperature.
DISTANT-READING THERMOMETERS
Distant-reading dial thermometers are used when
the indicating portion of the instrument must
be placed at a distance from where the temperature
is being measured. The distant-reading thermometer
has a long capillary, some
Figure 8-13.—Bimetallic thermometer.
as long as 125 feet, which separates the sensing bulb
from the Bourdon tube and dial (fig. 8-14). There
are three basic types of distant-reading thermometers:
the liquid filled, the gas filled, and
the combination liquid-vapor filled. The thermometers
are filled with fluid (liquid or gas) at
some temperature and sealed. Almost the entire volume
of the fluid is in the sensing bulb. As
the temperature of the bulb changes, the volume
of the fluid tries to change. Since the
volume of the thermometer (sensing bulb, capillary,
and Bourdon tube) is constant, a pressure
change occurs within the thermometer. This
pressure change causes the Bourdon tube to straighten
out (with an increase in pressure), working
a system of levers and gears, which causes the
thermometer pointer to move over the dial and register
temperature.
TEMPERATURE SWITCHES
Temperature switches operate from temperature changes
occurring in an enclosure, or in the air
surrounding the temperature-sensing element. The
operation of the temperature switch is similar to
the operation of the pressure switch shown in figure
8-9; both switches are operated by changes in
pressure. The temperature element is arranged so
a change in temperature causes a change in the internal
pressure of a sealed-gas or air-filled bulb
Figure 8-14.—Distant-reading, Bourdon-tube thermometers.
or helix, which is connected to the actuating device by
a small tube or pipe. Figure 8-15 shows a temperature
switch and two types of sensing elements.
A temperature change causes a change in the volume
of the sealed-in gas, which causes movement
of a bellows. The movement is transmitted
by a plunger to the switch arm. The moving
contact is on the arm. A fixed contact may be
arranged so the switch will open or close on a
temperature rise. This allows the switch contacts to
be arranged to close when the temperature drops
to a predetermined value and to open when the
temperature rises to the desired value. The reverse
action can be obtained by a change in the contact
positions.
GAUGE SNUBBERS
The irregularity of impulses applied to the fluid
power system by some pumps or air compressors
causes the gauge pointer to oscillate violently.
This makes reading of the gauge not only
difficult but often impossible. Pressure oscillations
and other sudden pressure changes existing
in fluid power systems will also affect the delicate
internal mechanism of gauges and cause either
damage to or complete destruction of the
Figure 8-15.—Temperature switch with two types of sensing
elements.
A. Bulb unit. B. Helix unit.
gauge. A pressure gauge snubber is therefore installed
in the line that leads to the pressure
gauge.
The purpose of the snubber is to dampen the oscillations
and thus provide a steady reading and protection
for the gauge. The basic components of
a snubber are the housing, fitting assembly with a
fixed orifice diameter, and a pin and plunger assembly
(fig. 8-16). The snubbing action is obtained
by metering fluid through the snubber. The
fitting assembly orifice restricts the amount of
fluid that flows to the gauge, thereby snubbing the
force of a pressure surge. The pin is pushed and
pulled through the orifice of the fitting assembly
by the plunger, keeping it clean and at a
uniform size.
Figure 8-16.—Pressure gauge
snubber.
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