Page 132 - MEMS Mechanical Sensors

6.3 Traditional Pressure Sensors 121

conditioning can include amplification, filtering, and compensation. The frequency
response of the electronics is likely to be a limiting factor only when used with very
high frequency sensors as described above. Sensors requiring ac excitation (e.g.,
capacitive) will be limited in particular by the frequency of this driving signal.

6.2.3 Pressure Sensor Types
Pressure can be measured relative to vacuum, atmosphere, or another pressure
measurand.

• Absolute pressure sensors are devices that measure relative to a vacuum and
therefore must have a reference vacuum encapsulated within the sensor.
Atmospheric pressure is measured using absolute sensors.
• Gauge pressure sensors measure relative to atmospheric pressure, and there-
fore, part of the sensor must be vented to the ambient atmosphere. Blood pres-
sure measurements are taken using a gauge pressure sensor. Vacuum sensors
are a form of gauge pressure sensor designed to operate in the negative pres-
sure region.
• Differential pressure sensors measure the difference between two pressure
measurands. The design of differential sensors often represents the greatest
challenge since two pressures must be applied to the mechanical structure. The
specifications for such devices can also be exacting since it is often desirable to
detect small differential pressures superimposed on large static pressures.

6.3 Traditional Pressure Sensors

Traditional macroscale pressure sensors have been developed that are based on a
wide range of mechanical sensing elements and transduction principles. These are
discussed briefly in this section to illustrate the development of pressure sensors.

6.3.1 Manometer
This is a simple yet accurate method for measuring pressure based upon the influ-
ence of pressure on the height of a column of liquid. The best-known form is the
U-tube manometer shown in Figure 6.9. If pressure is exerted to one side of the
U-tube as shown, the liquid is displaced, causing the height in one leg to drop and
the other to rise. The difference in height h between the fluid-filled legs indicates the
pressure. The measurement is usually taken visually by reading the height from the
scale incorporated into the instrument. Resolution can be improved by inclining one
leg, allowing more precise reading of the scale. Often a liquid reservoir is incorpo-
rated onto one side, making the drop in fluid height on that leg negligible. The unit
of pressure will depend upon the liquid (e.g., inch of water, inch of mercury).
Manometers can be used both as a gauge sensor with one side vented to atmosphere
and as differential sensors with pressure applied to both legs. The disadvantages
associated with manometers include their slow response (they are not suitable for
dynamic applications) and the limited range of pressures for which they are suitable.