There are several different types of pressure measurement found in process control and industrial applications. Pressure is the measurement of force on a defined area, however, depending on how it’s referenced is often the challenge. The most common types are gauge, compound, vacuum, differential, and absolute. Let’s explore how each pressure type is slightly different

 

Gauge, compound, and vacuum are referenced to the ambient or local atmospheric pressure. Several applications use one of these three types.

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Gauge pressure is higher than ambient and is referred to as positive pressure. Checking the pressure in your vehicles tire is an example of measuring gauge pressure. Gauge sensors have a single port and the value is usually expressed in PSI or BAR.

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Vacuum is the opposite of gauge pressure. It’s the pressure lower than ambient or negative pressure. An example would be vacuum pumps that create suction for several manufacturing applications to control a process. Torr, PSIA, or InHg are common engineering units used in vacuum applications.

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Compound pressure refers to the ability to measure both gauge and vacuum with one device. It’s not true type of measurement. It’s commonly used in gauges and calibrators with sensors that have the ability to measure both vacuum and gauge pressure.

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Differential pressure is measuring the difference between two points without a fixed reference point. Pressure sensors have two ports that are commonly referred to as high and low.  Measurements can be negative or positive. Several flow applications are measuring differential pressure. An orifice plate used in a natural gas pipeline will create differential pressure as the gas flows through the fixed diameter hole.  Differential pressure is usually expressed in InH2O defined at a temperature reference 4°C, 20°C, or 60°F.

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Absolute pressure (Pabs) is probably the most confusing of the bunch. Absolute pressure is always referenced to a fixed value defined at sea level at a known temperature to represent total vacuum. Standard Atmospheric Pressure is 1 atm, 760mmHg, or 14.696 PSI. It does not reference the local barometric pressure that is always changing. Absolute pressure is used in applications where accuracy and safety are critical.  The sensor will have a sealed vacuum behind the diaphragm that is not influenced by ambient conditions.