Page 128 - MEMS Mechanical Sensors

6.2 Physics of Pressure Sensing 117

Many of the principles discussed so far rely on fluids being incompressible.
Gases, however, such as the Earth’s atmosphere mentioned above, are compressi-
ble. Boyle’s law relates pressure to volume, V, as shown by
1
p∝ or pV =constant (6.7)
V

The value of the constant depends upon the mass of the gas and the tempera-
ture. This is shown by

pV = nRT (6.8)

where n equals the mass of the gas divided by the molar mass, and R is the universal
–1
–1
molar gas constant (8.31 J mol K ). The relationship between pressure, volume,
and temperature can be shown graphically in Figure 6.6.

6.2.1 Pressure Sensor Specifications
A wide variety of pressure sensors have been developed to measure pressure in a
huge range of applications over many years. In order to select the correct type of
sensor for a particular application, the specifications must be understood (i.e., what
makes a good pressure sensor?). The fundamental specification is the operating
pressure range of the sensor. Other specifications are also obvious: cost, physical
size, and media compatibility. Specifications relating to performance, however, are
not so obvious. and this is exacerbated by subtle differences in definitions used by
manufacturers. The performance will depend upon the behavior of the sensor
element, the influence of the material from which it is made, and the nature of
the transduction mechanism. Common performance specifications are therefore
explained next.

6.2.1.1 Zero/Offset and Pressure Hysteresis of Zero
Zero or offset is defined as the sensor output at a constant specified temperature with
zero pressure applied. Pressure hysteresis of zero is a measure of the repeatability of
the zero pressure reading after the sensor is subjected to a specified number of full
pressure cycles. This is typically expressed as a percentage of full-scale output (% fs).

p

Increasing
temperature

V
Figure 6.6 Pressure versus volume for a compressible gas.