Page 36 - Fluid mechanics, heat transfer, and mass transfer

FLUID STATICS 13

indicating needle. Movement of the tip of the tube
can then be used to position a pointer on a circular
calibrated dial or to develop an equivalent electrical
signal to indicate the value of the applied internal
pressure.
& Typical shapes of tubes are spiral or helical.
& This instrument indicates gauge pressure.
& Figures 1.12 and 1.13 illustrate spiral and helical
types of pressure gauges.
& The helical configuration is more sensitive than the
circular Bourdon tube.
& Due to their robust construction, Bourdon tubes are
often used in harsh environments and high pressures,
but can also be used for very low pressures.
. Name other types of pressure sensing devices.
& Bellows.
& Diaphragms.
Two-liquid manometer. & Capsules.
FIGURE 1.11
. Describe a bellows-type pressure sensing device? What
are its characteristic features?
& Pressure applied to the inside of the tube causes & Metallic bellows pressure sensing detectors are de-
change of the flat sections and tends to restore its veloped to provide a device that is extremely sensi-
original round cross section. tive to low pressures.
& This change in cross section causes the tube to & The metallic bellows is most accurate when measur-
straighten slightly. Since the tube is permanently ing pressures from 3.4 to 39 kPa gauge (0.5–75 psig).
fastened at one end, the tip of the tube traces a curve However, when used in conjunction with a heavy
that is the resultof the change in angular position with range spring, some bellows can be used to measure
respect to the center. pressures of over 6890 kPa gauge (1000 psig).
& As the gauge pressure increases the tube will tend to & Figure 1.14 shows a basic metallic bellows pressure
uncoil, while a reduced gauge pressure will cause the sensing element.
tube to coil more tightly. This motion is transferred & The bellows is a one-piece, collapsible, seamless
through a linkage to a gear train connected to an metallic unit that has deep folds formed from very

FIGURE 1.12 Bourdon tube pressure gauge.