Pressure is the measure of force applied perpendicularly over a surface area. The pressure exerted by a liquid or gas is a result of the movement of individual molecules and their collision with the walls of the container or with other molecules in the system. Common units used to express pressure are Pascal (Pa) and psi (pounds per square inch). The most common systems of pressure measurement are absolute pressure and gauge pressure. This article will help you learn the difference between the two.

Definitions

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Absolute pressure is the measure of pressure relative to a perfect vacuum. A perfect vacuum is defined as absolute zero in the absolute pressure scale. At this scale, there is no molecular movement present at any point in the system and no pressure is exerted on the surface of the container. Therefore, pressure cannot be lower than absolute zero; there is no negative absolute pressure. Absolute pressure implies that the pressure measured will be the same no matter what the surrounding atmospheric conditions are.

Units for absolute pressure are sometimes suffixed with the letter “a”; for example, “kPaa” for absolute pressure in kiloPascal or “psia” for absolute pressure in pounds per square inch.

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Gauge pressure is the measure of pressure relative to the ambient atmospheric pressure. It is the difference between absolute pressure and the atmospheric pressure. Therefore, a zero value on the gauge pressure scale means that the absolute pressure of the system is equal to the absolute pressure exerted by the surrounding atmosphere. A gauge is the instrument used in measuring pressure. A gauge always needs a reference point since the reading is made with a deflection in the gauge caused by a difference in pressure. Usually, a gauge is vented, meaning it uses the pressure of the air as reference. This is why it is called gauge pressure.

Since gauge pressure is measured relative to the ambient pressure, changes in the weather result in different readings on the gauge pressure. A lower atmospheric pressure would make your tires have higher gauge pressure, whereas they would actually have the same absolute pressure. A positive gauge pressure refers to a pressure measurement that is greater than the ambient pressure. A negative gauge pressure refers to pressure lower than the ambient pressure, and is sometimes called “vacuum pressure.”

Gauge pressure units sometimes use the letter “g” as a suffix, such as “kPag” or “psig.” Some gauges are sealed so that the reference pressure may be a value other than the ambient air pressure. The reference pressure may even be the standard atmospheric pressure (1 atmosphere) which represents the pressure at sea level at standard temperature. Sealed gauges would allow one to measure the pressure independent of the current actual condition of the environment and may be used to show absolute pressure.

Absolute Pressure vs Gauge Pressure

What is the difference between gauge pressure and absolute pressure? Absolute pressure cannot be lower than absolute zero as that is its zero point. On the other hand, gauge pressure uses atmospheric pressure as its zero point. Even with varying atmospheric pressure, absolute pressure is always definite. Meanwhile, due to varying atmospheric pressure, the measurement of gauge pressure is not precise. Absolute pressure units are sometimes suffixed with the letter “a” whereas units for gauge pressure use “g” as a suffix.

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