Absolute Pressure vs Gauge Pressure
Pressure is an important concept in physics and finds scores of industrial and daily life. It is defined as force per unit area when it is applied in a direction perpendicular to the surface of the body on which it is applied. But what we measure with devices designed to measure pressure (such as manometer) is gauge pressure and not absolute pressure. This gauge pressure is always relative to the atmospheric pressure. Being a scalar quantity, pressure has no direction, and hence it is wrong to talk of pressure in a particular direction. The units of pressure are Newton per square meter or Pa, but there are many other units (non SI) of pressure such as bar and PSI also. This article will attempt to find differences between absolute and gauge pressure.
Pressure is often measure in terms of depth of a column of mercury because of the high density of mercury but it often gives erroneous results given variations in density and gravity with changes in temperature and location. This is why other units of pressure such as torr and ATM are used instead of mm of Hg.
One can measure either absolute pressure or gauge pressure. It is important to know which pressure you need as otherwise your measurement may be wrong and may have an error of up to one bar. Most commonly used pressure reference is gauge pressure and you know that it is gauge pressure when you see the letter g suffixed after the result (such as 15 psi g). This implies that the pressure measured is obtained after subtracting atmospheric pressure. Absolute pressure is a reading that is taken in reference to absolute vacuum. To measure absolute pressure, it is necessary to seal a high vacuum behind sensing diaphragm of the device.
Absolute pressure = gauge pressure + atmospheric pressure
Gauge pressure = absolute pressure – atmospheric pressure
This is simply because absolute pressure is zero referenced against perfect vacuum whereas gauge pressure is zero referenced against ambient air pressure.
In general, if you want to measure pressure that is influenced by variations in atmospheric pressure, you will have to measure gauge pressure as it will give you a reading that reflects pressure minus atmospheric pressure. However, if you want readings that are not influenced by atmospheric pressure variations, then you will have to use absolute pressure sensor.