Page 127 - MEMS Mechanical Sensors

116 Pressure Sensors

2
2
1 2
F= PA 1 F = PA = FA/A 1
1
d2
d1

Figure 6.4 Hydraulic force multiplication [1].

shown in (6.5), states that for an inviscid (zero viscosity), incompressible, steady
fluid flow of velocity v with negligible change in height, the static pressure (p) plus
dynamic pressure equals the total pressure (p ), which is a constant.
t
ρ v 2
p+ = p t (6.5)
2

The dynamic pressure is given by the second term. This principle is used in meas-
urement of airspeed using a Pitot tube as shown in Figure 6.5. The tube incorporates
a center orifice that faces the fluid flow and a series of orifices around the circumfer-
ence of the tube that are perpendicular to fluid flow. The perpendicular orifices
measure static pressure, p , while the center orifice measures the total pressure at the
s
stagnation point. Equation (6.5) can be rearranged to calculate velocity v, as shown
by

2 (p − p )
v = s t (6.6)
ρ

p
Fluid flow (velocity ) Static taps ( ) s
v

Stagnation point ( )
p
t
p t p s

Pressure-sensing diaphragm
Figure 6.5 Pitot tube arrangement.