Pressure:Units, Conversions and Types

 Pressure in a process generally refers to fluid pressure. Fluid pressure is defined as the force exerted by the fluid on unit area, acting perpendicularly to any surface it contacts.
The standard SI Unit of Pressure is Pascal which is 1 newton per meter square. There are other units which are being used globally in different systems and different locations.
The conversion methods of different units are shown in the following table:

Pressure Units and Conversions

Head Pressure:
Head Pressure, or Pressure Head, is the static pressure of a liquid in a tank or a pipe. head Pressure,P, is solely a function of height of the fluid, i.e. h, and its density ,i.e. w.

An important factor to consider while we measure pressure is with what reference are we measuring it. Some measurements are done relative to air pressure at that location, while some may be done relative to vacuum.
There are different types of Pressure depending upon the zero reference being considered.
  • Absolute Pressure
    This measurement is with zero-reference to vacuum. Therefore,
    Absolute Pressure = Gauge Pressure + Atmospheric Pressure.
  • Gauge Pressure
    This pressure is measured relative to ambient atmospheric pressure. The ambient atmospheric pressure depends upon altitude, temperature and local weather conditions. In this type, it is critical to specify what pressure the measurement is with reference to. In the United States, for example, the standard atmospheric pressure at sea level and at 59 deg.F is 14.696 pounds per square inch absolute(psia) or 101.325 kPa absolute.
    Gauge Pressure = Absolute Pressure - Atmospheric Pressure.

    We need to specify if the measurement is absolute or gauge by adding an 'a' or 'g' respectively, after the standard unit. Thereby, 'psia' stands for pound per square inch absolute and 'psig' stands for pound per square inch gauge.
  • Differential Pressure
    In this type, pressure is measured relative to a specific reference pressure or an unknown pressure. The output of this measurement is the difference between the two. In this type, pressure reference need not be mentioned. This type of measurement is generally used for measuring pressure drop in a fluid system.
    Differential Pressure = P1 - P2
Now that we know some basic fundamentals of Pressure, its units and types; let us look forward to different sensors that are available to measure pressure and understand them. The different pressure sensors that we are going to see are: