Page 30 - Mechanical Engineers Reference Book

Mechanics of fluids 1/19

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(a) U-tube manometer

(c) Enlarged end manometer

PJ

(b) Inverted U-tube manometer (d) Inclined limb manometer

reservoir and the inclined leg by a small-bore tube (Figure
1.34(d)).
Another method is to increase the cross-sectional area of
If ,om < pA and pm < p~ then an inverted U-tube manometer is the ends of the legs (or one of the legs), as shown in Figure
used as shown in Figure 1.34(b). In this case the pressure 1.34(c), so that a small movement of the free surfaces in the
difference is enlarged ends results in a large aovement of the surface of
separation.
PA - PB = PdZA + (& - prn)gh - &gZB (1.9)
and if pA = pB = p then the difference in pressure head is (b) Dial gauges Most pressure dial guages make use of a
Bourdon tube. This is a curved tube with an oval cross section.
(1. loa) Increase in pressure causes the tube to straighten, decrease
makes it bend. The movement of the free end turns a pointer
over a scale, usually via a rack and pinion mechanism. The
of if prn e 0 (the manometer fluid a gas and A and B scale may be calibrated in the required pressure units.
containing liquid),
(c) Diaphragm gauges In these gauges the pressure changes
PA - PB
- z, - 2, + h (1. lob) produce a movement in a diaphragm which may be detected
Pg by a displacement transducer, or by the output from strain
The accuracy of a U-tube manometer may be increased by gauges attached to the diaphragm surface.
sloping one of the legs to increase the movement of the fluid
interface along the leg for a given difference in vertical height. (d) Piezoelectric transducers A piezoelectric crystal produces
This may be further enhanced by replacing the vertical leg by a a voltage when deformed by an external force. This induced