Page 125 - MEMS Mechanical Sensors

114 Pressure Sensors

Physical Electrical
Pressure Sensing movement Transduction signal
element mechanism

Figure 6.1 Block diagram of key pressure sensor components.

with the conversion factor to Pascals. The chosen mechanism for measuring pressure
depends upon the application. Typically, pressure is measured by monitoring its
effect on a specifically designed mechanical structure, referred to as the sensing ele-
ment. The application of pressure to the sensing element causes a change in shape,
and the resulting deflection (or strain) in the material can be used to determine the
magnitude of the pressure. A block diagram of this process is shown in Figure 6.1. A
range of sensing elements designed to deform under applied pressures can be
fabricated using micromachining techniques, the most common by far being the dia-
phragm. The transduction mechanisms suitable for measuring strain or displace-
ment described in Chapter 5 can be used to measure the resulting deflection of the
sensor element. Other techniques such as using micromachined airflow sensors to
measure pressure will also be discussed later in this chapter.

6.2 Physics of Pressure Sensing

The pressure at a given point within a static fluid occurs due to the weight of the
fluid above it. The pressure at a given point depends upon the height of the fluid
above that point to the surface, h, the density of the fluid, ρ, and the gravitational
field g (see Figure 6.2). The pressure, p, is given by [1]

p=ρ (6.1)
h g
This pressure acts in all directions, which leads us to Archimedes’ principle,
which states that when a body is immersed in a fluid it is buoyed up (i.e., appears to
lose weight) by a force equal to the weight of the displaced fluid. Figure 6.3 shows a
block of material area A and thickness t submerged in a fluid. The buoyancy
pressure acting upwards is given by (6.2). The net pressure, shown in (6.3), is given

h

Figure 6.2 Pressure in a static fluid.