Sensor - The Electronic Nose Artificial Olfaction Technology - page 42

is more appropriate to describe the pressure in a relative sense that is compared to
atmospheric pressure. This is called gauge pressure and is given by:

P g ¼ P abs P at

where,
P g gauge pressure,

P abs absolute pressure,

P at atmospheric pressure.

Methods for measuring pressure:

1. Direct-measuring pressure instruments determine the pressure from the basic
equation:

P ¼ F = A

Types of Direct-measuring pressure instruments

• Liquid column manometers (U-tube, inclined-tube, Multiple liquid, Float-type)
• Pressure balances with liquid separation (immersed segment, immersed cylin-
der, and the cylindrical)
• Piston-type pressure measuring instruments (dead weight tester).

2. Indirect-measuring pressure instruments use the deflection of a flexible material
or an electrical, optical, or chemical effect to determine the measured pressure.
Measuring converters are instruments which convert the pressure acting on
them into an output which is generally an electric or pneumatic signal. This
output is a function of the input pressure and can be either digital or analog [ 8 ].

• Diaphragm
• Strain guage
• Piezo resistive element
• Inductive
• Capacitive
• Hall effect

Many other types of instruments exist for measuring low and high pressures in
addition to those based on electrical pressure measuring systems presented above.
They include:

• McLeod compression gauges
• Pirani vacuum gauges based on the principle of thermal conductance
• Ionization pressure gauges
• Friction pressure gauges which use the internal friction of gases as the basis for
pressure measurements.