Next to temperature, pressure is one of the most important physical state variables. The pressure is defined as a force (FN) which acts uniformly over a defined area (A). The different types of pressure are differentiated only by the reference pressure.
Whether absolute pressure, gauge pressure or differential pressure. With WIKA, you will find the right measuring instrument for each type of pressure:
The clearest reference pressure is the pressure zero, which exists in the air-free space of the universe. A pressure which is related to this reference pressure is known as absolute pressure. 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.
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 a height 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 %.
The difference between two pressures, p1 and p2, is known as the pressure differential 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. 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, level measurement and monitoring applications.
The most frequently measured pressure in the technological field is the atmospheric pressure differential, 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 the overpressure or gauge pressure .
A positive overpressure is referred to when the absolute pressure is greater than the atmospheric pressure. In the opposite case, one speaks of negative overpressure. In order to avoid misunderstandings, the term vacuum, previously used for this overpressure with a negative sign, is no longer used as a description of a variable. However, the term is still used to describe a state, e.g. “vacuum chamber” or “a vacuum exists in the intake manifold”. In the same way, where there are word compositions with overpressure, the “over” part is omitted if the associated variable is clearly defined as an overpressure, e.g. blood pressure or tyre 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.