Absolute vs Gauge Pressure

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 (by Wikipedia) as:

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.

What is Gauge Pressure?

The most common pressure reference is gauge pressure which is signified by a “g” after the pressure unit e.g. 30 psig. Gauge pressure is measured in relation to ambient atmospheric pressure. Changes of the atmospheric pressure due to weather conditions or altitude directly influence the output of a gauge pressure sensor. A gauge pressure higher than ambient pressure is referred to as positive pressure. If the measured pressure is below atmospheric pressure it is called negative or vacuum gauge pressure.

Gauge pressure sensors only usually have one pressure port. The ambient air pressure is directed through a vent hole or a vent tube to the back of the sensing element. A vented gauge pressure transmitter allows the outside air pressure to be exposed to the negative side of the pressure sensing diaphragm so that it always measures with reference to the ambient barometric pressure. Therefore a vented gauge pressure sensor reads zero pressure when the process pressure connection is held open to atmospheric air.

A sealed gauge reference is very similar except that atmospheric pressure is sealed on the negative side of the diaphragm. This is usually adopted on high pressure applications such as measuring hydraulic pressures where atmospheric pressure changes will have only a slight effect on the accuracy of the sensor. The definition of sealed-gauge pressure is the pressure measured through a sealed device in which the zero point is set. This set point is whatever the pressure inside of the device was before sealing, which the manufacturer of the sealed pressure gauge decides.

What is Absolute Pressure?

Absolute pressure is gauge pressure plus atmospheric pressure. An absolute pressure reading of zero can only be achieved in a perfect vacuum and only naturally occurs in outer space. If you want to measure pressure in an environment where the atmospheric pressure is constantly changing, you’ll want to measure in pounds per square inch absolute (PSIA). One of the most common uses of absolute pressure measurements is to understand and predict weather patterns. Absolute pressure measurements are also used in food packaging, aeronautical equipment, and gas analysis systems.

What is Differential Pressure?

Differential pressure (DP or ∆P) is the difference in pressure between two points of pressure. To generate a reading one of the two pressures is either higher or lower than the other. If they are the same then no pressure reading will be generated.

Pressure, as discussed above, is called absolute pressure. Often it will be important to distinguish between absolute pressure and gauge pressure. In this article, the term pressure refers to absolute pressure unless explicitly stated otherwise. But in engineering, we often deal with pressures that are measured by some devices. Although absolute pressures must be used in thermodynamic relations, pressure-measuring devices often indicate the difference between the absolute pressure in a system and the absolute pressure of the atmosphere existing outside the measuring device. They measure the gauge pressure.

Absolute Pressure. When pressure is measured relative to a perfect vacuum, it is called absolute pressure (psia). Pounds per square inch absolute (psia) is used to clarify that the pressure is relative to a vacuum rather than the ambient atmospheric pressure. Since atmospheric pressure at sea level is around 101.3 kPa (14.7 psi), this will be added to any pressure reading made in air at sea level.

Gauge Pressure. When pressure is measured relative to atmospheric pressure (14.7 psi), gauge pressure (psig), the term gauge pressure is applied when the pressure in the system is greater than the local atmospheric pressure, patm. The latter pressure scale was developed because almost all pressure gauges register zero when open to the atmosphere. Gauge pressures are positive if they are above atmospheric pressure and negative if they are below atmospheric pressure.

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.

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.
Sensitivity (deadband).
Output type (if required).
Visual indication.
Shock and vibration.
Area classification.

In order to properly specify any pressure instrumentation, an applications engineer should always be consulted.

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