Page 37 - Fluid mechanics, heat transfer, and mass transfer

FLUID MECHANICS BASICS
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Helical Bourdon tube.
FIGURE 1.13
thin-walled tubing. System pressure is applied to the FIGURE 1.15 Capsule device for measurement of differential
internal volume of the bellows. pressure.
& As the inlet pressure to the instrument varies, the
& The sensitivity range for most capsules is low, only a
bellows will expand or contract. The moving end of
few kPa of differential pressure. Higher differential
the bellows is connected to a mechanical linkage
pressures can damage the capsule.
assembly.
. What is a transducer?
& As the bellows and linkage assembly moves,either an
& A transducer is a device, electrical, electronic, or
electrical signal is generated or a direct pressure
indication is provided. electromechanical, that converts one type of energy
to another for various purposes including measure-
& The flexibility of a metallic bellows is similar in
ments or information transfer.
character to that of a helical, coiled compression
& In a broader sense, a transducer is sometimes defined
spring.
as any device that converts a signal from one form to
. How does a capsule device for pressure measurement
another.
work? What are its characteristics? Illustrate.
& A pressure transducer converts pressure into an
& The capsule consists of two circular shaped, convo-
analog electrical signal. Although there are various
luted membranes (usually stainless steel) sealed tight
types of pressure transducers, one of the most com-
around the circumference.
mon types is the strain gauge-based transducer.
& Pressure is applied to the inside of the capsule.
& The conversion of pressure into electrical signal is
& Figure 1.15 illustrates the principle.
achieved by the physical deformation of strain
& Unlike a diaphragm, which is fixed on one side, a
gauges that are bonded into the plastic diaphragm
capsule expands like a balloon in both directions. of the pressure transducer and wired into a Wheat-
stone bridge configuration.
& The pressure applied to the pressure transducer pro-
duces a deflection of the diaphragm, which intro-
duces strain to the gauges. The strain produces an
electrical resistance change proportional to the pres-
sure. The electrical signal can get altered even if the
transducer is touched by hand.
& Most pressure signals transmitted from the fieldto the
control room are generated by pressure transducers.
& Differential pressure indicators give differential
readings from two transducers and generate a milli-
ampere output signal.
. What is a McLeod gauge?
& A McLeod gauge is an instrument to measure very
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low pressures, as low as 10 Torr. It isolates a
sample of gas and compresses it in a modified
mercury manometer until the pressure is few mmHg.
. What is a Pirani gauge? Where is it used?
& A Pirani gauge consists of a metal wire open to the
FIGURE 1.14 Basic metallic bellows. pressure to be measured. The wire is heated by the