Pressure types

Absolute pressure

The zero point of the absolute pressure scale is a vacuum. Absolute pressure data is therefore always measured in relation to airless space.

Relative pressure

When measuring relative pressure (also referred to as gauge pressure), the reference value is the ambient air pressure pamb. The ambient pressure is subtracted from the pressure to be measured.

Differential pressure

The difference between 2 absolute pressures p1 and p2 is called differential pressure Δp. It is often used in technical applications for control purposes.

Static and dynamic pressure in fluids

Fluid is the overarching term for liquids and gases. The physical laws of fluid mechanics apply to both aggregate states. When fluids are in a resting position, i.e. when there is no flow, only the static pressure (pstat ) acts on the fluids, evenly in all directions.

Hydrostatic pressure

Hydrostatic pressure is the pressure that is generated by gravity, i.e. by weight force, in a non-flowing liquid. The hydrostatic pressure ph is a measurand that results from the density ρ of the medium, the gravitational constant g, and the height h of the liquid column.


Basics

How high is the pressure?

Pressure is one of the most frequently measured and controlled physical measurands in automation technology. It has a direct influence on the flow of production and industrial processes. This article explains the basics of pressure and its measurement.

Definition

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Der Druck p ist definiert als Quotient aus dem Betrag der Kraft F und der Größe der Fläche A. Dabei ist die Kraft senkrecht auf die gesamte Fläche verteilt. Die SI-Einheit für den Druck ist das Pascal (Pa).

Generation of pressure: the force F always has one direction (here: vertical) and is therefore a vector. The pressure p, however, has no direction – it is a scalar. Within a closed system, for example a pressure accumulator, the pressure always acts evenly in a vertical direction across all surface areas. In practice, this means that the installation position of the pressure transmitter and the membrane can be chosen at random. For example, the pressure in the accumulator of a compressor can be measured by installing the pressure transmitter on the upper side of the container with the membrane pointing downwards.

Fig. 1: Definition of pressure

International pressure units

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If you load an area of 1 m² with 100 g weight force (F = 0.1 g × 10 m/s² = 1 N), this results in a pressure of 1 Pa.* The commonly used unit for higher pressures in Europe is the Bar (bar). As can be seen in the calculation, the Pascal (Pa) is too small a classification for most technical applications.


1 bar = 105 N/m² = 100 000 Pa

*Comment: the exact value of the gravitational acceleration of g = 9.81 m/s² has not been used for the calculations on this
page. Instead a rounded value of 10 m/s² has been used.