Gauge Versus Absolute Pressure Measurement
Everyday pressure measurement, such as measuring your car tyre pressure, is usually made relative to the ambient air pressure. In the world of Instrumentation and Control however, often pressure is measured relative to some other specific reference, e.g. a vacuum. To avoid confusion it is important to distinguish between these zero references, so the following terms are widely used when measuring pressure:
An absolute pressure measurement is zero-referenced against a perfect vacuum, using an absolute scale. You are most likely to measure absolute pressure when measuring a partial vacuum.
A gauge pressure measurement is zero-referenced against ambient air pressure, known as the Barometric Pressure. Negative signs are usually omitted. To distinguish a negative pressure, the pressure unit may be appended with the word "vacuum" or the gauge may be labelled a "vacuum gauge",
Generally, when measuring pressure the zero reference in use is implied by context, and the words absolute or gauge are added only when clarification is needed. However it is common to see the letters "g", or "a" added after the units of presure to distinguish gauge or absolute respectively e.g. 15 barg.
The difference between gauge and absolute pressure measurement can be shown graphically as below:
For most process fluids where the fluid exists in a closed system e.g. within the pipework of a process plant, gauge pressure measurement prevails. Pressure instruments connected to the system will indicate pressures relative to the current atmospheric pressure.
It is worth noting that Atmospheric pressure is variable with altitude and weather. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes. For most situations within the oil and gas, and process industries changes in Atmospheric pressure can be ignored.
Converting between Gauge and Absolute Pressure
Many calculations performed by instrument or process engineers use absolute pressure rather than gauge pressure.
To convert between gauge and absolute pressure is straightforward; just add atmospheric pressure.
Generally, for the purposes of calculations this means adding the Standard Atmospheric Pressure. The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa; 1,013.25 mbar), which is equivalent to 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth, that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.
See our page on Units for Pressure Measurement for details on how to convert between the various common units of pressure.
Another commonly encountered term is Differential Pressure. A differential pressure measurement is the difference in pressure between two points. Since differential pressure can be the difference between two gauge pressures, or two absolute pressures it will never be expressed as a guage or absolute reading.
Differential pressures are commonly used in industrial process systems, and indeed differential pressure measurements form the basis of many instrument systems e.g. level measurement, and flow measurement. Differential pressure gauges (often referred to as DP gauges) have two inlet ports, each connected to one of the volumes whose pressure is to be measured. The DP gauge performs the mathematical operation of subtraction through mechanical means, obviating the need for an operator or control system to monitor two separate gauges and calculate the difference in pressures.