Fluid pressure can be defined as the measure of force per unit area exerted by a fluid, acting perpendicularly to any surface it is in contact with.

Pressure can be expressed in many different units including in terms of a height of a column of liquid.

• The standard SI unit for pressure measurement is one Pascal (Pa)
  which is equivalent to one Newton per square meter (N/m2).

• In the English measure system, pressure is usually expressed in pounds per square inch (psi).

• The table below lists commonly used units of pressure measurement and the conversion between the units:

Categories of pressure measurements:

• Absolute pressure refers to the absolute value of the force per-unit-area exerted on a surface by a fluid.
  

• Gage pressure measurement is the measurement of the difference between the absolute pressure
  and the local atmospheric pressure.
  

• Differential pressure measurement is simply the measurement of one pressure
  with reference to another pressure.
  

Standard atmospheric pressure at the sea level and the temperature of 20oC is 101.325 kPa

Types of fluid systems and fluid pressures:

• Static systems - fluid is at rest - the pressure measured in a static system is called static pressure.
  
• Dynamic systems - fluid is moving - the pressure is more difficult to measure
  
  Typically is defined the term total pressure that is a sum of the static pressure and the dynamic pressure.
  
• Transient systems - most difficult to obtain exact results of a measurement.

Classes of silicon micromachined pressure sensors:

• Piezoresistive
  
• Capacitive