Pressure measurement

What is relative pressure?

In relative pressure measurement, the reference variable is the ambient air pressure p_amb. The ambient pressure is subtracted from the pressure to be measured. This is done automatically via the so-called pressure compensation.

Why is relative pressure measurement important?

The measurement of relative pressure is the most widely used in engineering. To ensure that most processes run properly, it is necessary to measure its value continuously.

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Fig. 1: Schematic diagramm of a relative pressure sensor

p_rel = p_abs - p_amb

Bezeichnung:

p_rel – Relativdruck
p_abs – Absolutdruck
p_amb – atmosphärischer Druck oder Umgebungsdruck

What are examples of relative pressure measurement?

About 90 % of all pressure measurements involve relative pressures, since in most applications the relation of the process pressure to the ambient pressure is relevant. So, for example, in a hydrostatic level indicator without a relative pressure compensation, a fluctuating atmospheric pressure would lead to an incorrect display.

With the usual fluctuations of approx. ±20 mbar, the measured level would change by approx. ±20 cm in the case of water. A typical example is tire pressure: a car tire is filled with a relative pressure of 2.5 bar_rel at sea level in accordance with the manufacturer’s specifications. The tire is now at an absolute pressure of approx. 3.5 bar_abs. On a mountain pass at an altitude of 3000 m, only about approx. 0.7 bar_abs acts from the outside due to the lower atmospheric pressure. The tire expands and its surface area becomes smaller. The relative pressure in the tire is now 2.8 bar_rel and needs to be corrected. The absolute pressure remains unchanged at 3.5 bar_abs.

The relative pressure scale does not have a fixed zero point; it always relates to the existing ambient pressure. From a technical perspective, the relative pressure is a differential pressure since the measurement reading always represents the difference between 2 pressures.