Wastewater Conveyance - Wastewater Operators - page 217

gauge pressure (psig) and absolute pressure (psia). Gauge pressure
refers to the pressure level indicated by the gauge; however, even when
the gauge reads 0, it is subject to ambient atmospheric pressure (i.e.,
14.7 psi at sea level). When a gauge reads 50 psi, that is 50 pounds gauge
pressure (psig). The true pressure is the 50
pounds shown plus the 14.7 pounds of atmo-
spheric pressure acting on the gauge. The
total pressure is the absolute pressure, which
is the gauge pressure plus the atmospheric
pressure (50 psi + 14.7 psi = 64.7). It is written
as 64.7 psia.

Key Point: Pressure in any fluid pushes
equally in all directions. The total force
on any surface is the psi multiplied by the
area in square inches; for example, a fluid
under a pressure of 10 psi, pushing against
an area of 5 in. 2 , produces a total force
against that surface of 50 lb (10 × 5).

4.15.2.1 Spring-Loaded Pressure Gauges

Pressure, by definition, must operate against a surface. Thus, the
most common method of measuring pressure in a piping system is to
have the fluid press against some type of surface—a flexible surface that
moves slightly. This movable surface, in turn, is linked mechanically to
a gear-lever mechanism that moves the indicator arrow to indicate the
pressure on the dial (i.e., a pressure gauge). The surface that the pres-
sure acts against may be a disk or diaphragm, the inner surface of a
coiled tube, a set of bellows, or the end of a plunger. No matter the ele-
ment type, if the mechanism is fitted with a spring that resists the pres-
sure and returns the element (i.e., the indicator pointer) back to the zero
position when the spring drops to zero, it is a spring-loaded gauge.

4.15.2.2 Bourdon Tube Gauges

Many pressure gauges in use today use a coiled tube as a measur-
ing element called a Bourdon tube . (The gauge is named for its inventor,
Eugene Bourdon, a French engineer.) The Bourdon tube is a device that
senses pressure and converts the pressure to displacement. Under pres-
sure, the fluid fills the tube (see Figure 4.41). Because the Bourdon tube
displacement is a function of the pressure applied, it may be mechani-
cally amplified and indicated by a pointer. Thus, the pointer position
indirectly indicates pressure.

Note: The Bourdon tube gauge is available in various tube shapes: heli-
cal, C-shaped or curved, and spiral. The size, shape, and material of the
tube depend on the pressure range and the type of gauge desired.

4.15.2.3 Bellows Gauge

Figure 4.42 shows how a simplified bellows gauge works. The bel-
lows itself is a convoluted unit that expands and contracts axially with
changes in pressure. The pressure to be measured can be applied to
either the outside or the inside of the bellows; in practice, most bellows
measuring devices have the pressure applied to the outside of the bel-
lows. When pressure is released, the spring returns the bellows and the
pointer to the zero position.