Types of pressure: Absolute pressure, gauge pressure, differential pressure

What is atmospheric pressure?

The probably most important pressure for life on earth is the atmospheric pressure, pamb (amb = ambiens = ambient). It is created by the weight of the atmosphere which surrounds the earth up to an altitude of approx. 500 km. Up to this altitude, at which the absolute pressure pabs = zero, its magnitude decreases continuously. Furthermore, the atmospheric pressure is subject to weather-dependent fluctuations, as is only too well known from the daily weather report. At sea level, pamb averages 1013.25 hectopascal (hPa), corresponding to 1013.25 millibar (mbar). With “cyclones” and “anticyclones”, this pressure varies by about 5 %.

What is absolute pressure?

The clearest reference pressure is the pressure zero (vacuum), which exists in the air-free space of the universe. A pressure which is related to this reference pressure is known as absolute pressure. Thus, with absolute pressure, it is always the difference to ideal vacuum that is measured. This means that the ambient pressure, and thus external influences such as the weather or the height above sea level, do not affect the pressure measurement. For the required differentiation from other types of pressure, it is denoted with the index “abs”, which is derived from the Latin “absolutus”, meaning detached, independent.

For the pressure absolute formula, the parameters gauge pressure (pg) and Atmospheric pressure (patm) are required. The pressure absolute formula is thus: pabs = pg + patm.

What is gauge pressure (relative pressure)?

The most frequently measured pressure in the technological field is the atmospheric pressure difference, Pe (e = excedens =exceeding). It is the difference between an absolute pressure, pabs, and the relevant (absolute) atmospheric pressure (pe = pabs - pamb) and is known, in short, as gauge pressure. Since gauge pressure does not measure the difference to the vacuum, but rather to the current ambient pressure (atmospheric pressure), the pressure measurements change due to external influences such as weather or altitude above sea level. If a gauge pressure sensor is mistakenly used in an application where absolute pressure is actually measured, one must reckon with additional errors of +/- 30 mbar due to weather changes and with measurement errors of up to 200 mbar in the case of changes in location, e.g. from sea level to 2000 m.

A positive pressure is referred to when the absolute pressure is greater than the atmospheric pressure. In the opposite case, one speaks of negative pressure.

The indices of the formula symbols “abs”, “amb” and “e” clearly define the reference point of the respective pressure. They are only attached to the formula letter p, and not to the unit symbols.

What is differential pressure?

The difference between two pressures, p1 and p2, is known as the pressure difference, Δp = p1 - p2. In cases where the difference between two pressures itself represents the measured variable, one refers to the differential pressure, p1.2

Accordingly, in order to measure differential pressure, at first two different pressures are captured in a measuring instrument (e.g. differential pressure gauge or differential pressure sensor). Only if the measured values differ from each other, will a differential pressure be indicated. Instances where it is required to measure differential pressure are, for example, filter monitoring and level measurement in closed vessels.