Absolute pressure

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The zero point of the absolute pressure scale is a vacuum. Absolute pressure data is therefore always measured in relation to airless space. Typical applications have always included atmospheric pressure measurements, such as barometers or altimeters in airplanes. The weight force of our atmosphere determines the atmospheric pressure pamb. At sea level, atmospheric pressure is about 1013 mbarabs and it decreases as the altitude increases. At the peak of Mount Everest, for example, the atmospheric pressure is only approx. 400 mbarabs.

Fig. 1: Isobars on a weather ©map

In weather forecasts, the course of atmospheric pressure is depicted using isobars (Fig. 1). Places with the same air pressure are connected by lines. These isobars are usually subdivided in increments of 5 hPa. If an area is surrounded by a circle of isobars, the center is identified as a high or low pressure area, depending on the direction of pressure (falling or rising).

Examples of absolute pressure applications

Processes in process engineering often occur in evacuated containers, such as in sterilizers or autoclaves. The process pressure to be measured is then isolated from the atmosphere and is determined using absolute pressure transmitters. Automotive manufacturers use robots with vacuum grippers to transport large elements, such as the engine hood. In this case, the negative pressure is monitored to ensure that the suction capacity of the grippers is maintained. For plastic packaging machines, the level of the negative pressure is measured to control the shrink-wrapping process. The term vacuum is often used colloquially to describe a largely empty space. For example, people say that something has been “vacuum-packed”.

Fig. 2: Schematic diagramm of an absolute pressure sensor