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Pressure measurement is critical in many processes and essential in many industries. There are many techniques used to measurement pressure and vacuum (negative pressure). Instruments used to measure pressure are called pressure gauges, switches and transmitters.

Fluid pressure is defined as the measure of force on a surface, per some unit of area, perpendicular to the surface. The standard unit of measurement for pressure measurement in the English system is PSI, or pounds per square inch. In countries that use the Metric system, the Pascal (Pa) or the Newton/meter (N/m2) is used.

Pressure measurement has three sub-categories defined as:

 Industrial Pressure Switch (Winters)
• Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure. It is equal to gauge pressure plus atmospheric pressure.
• Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. To distinguish a negative pressure, the value may be appended with the word “vacuum” or the gauge may be labeled a “vacuum gauge.”
• Differential pressure is the difference in pressure between two points.

The term “static pressure” refers to a uniform force measured in all directions. Static pressure is independent and unrelated to flow and only refers to the forces of the static fluid.

A directional component of pressure, when the flow of a moving fluid is present, is called dynamic pressure.
Total pressure takes into account the static pressure and dynamic pressure and adds them together. Thus, total pressure is the sum of static and dynamic pressures.
 Pressure Transmitter (Winters)
There are many types of pressure sensing and controlling instruments. These include switches, transmitters, and indicators (gauges). A pressure switch generally refers to an instrument that includes “on/off” switching mechanisms to open or close a circuit. Transmitters are a type of instrument that provide an analog electrical or pneumatic signal proportional to the range of the transmitter. Gauges are visual indicators.
To properly select the right pressure instrument you must consider:
• Process compatibility with the “wetted” (in contact with the process media) parts.
• The pressure range.