Pressure types: absolute pressure, relative pressure, differential pressure

In contrast to liquids, air or gases have a decisively different property: they are compressible. Gas can be compressed. Compressed gas has very different properties. When the gas is compressed, it heats up. But when it relaxes, it cools down considerably. This is how refrigeration compressors are operated, for example. However, gas in closed systems reacts to the surrounding air pressure. If the pressure outside decreases, the pressure inside the container also decreases. However, if the external pressure increases, the pressure in the closed system also increases.

Of course, this reaction is largely determined by the deformability of the container. Nevertheless, it is very important to differentiate clearly between the individual types of pressure. Otherwise, miscalculations and even accidents can happen.

The pressure types commonly used for gases are

– Absolute pressure
– Relative pressure
– Differential pressure

The surrounding air: the absolute pressure

The absolute pressure is the surrounding air pressure. A “zero bar pressure” therefore describes the absolute vacuum. At sea level, the absolute pressure is about 1 bar. At the top of Mount Everest, however, the air pressure is only about 0.3 bar. On the shore of the Dead Sea, the deepest landscape on earth, the air pressure is 1.07 bar. This shows that it is very important for the design of systems to consider the installation site. A system that functions at sea level can malfunction at high altitudes.

How strong the one bar air pressure at sea level is can be demonstrated with a very simple example: The brake booster in a car uses the air pressure to double the driver’s pedal force. To do this, a vacuum is permanently generated in the brake booster. When the brakes are applied, the surrounding air pressure presses down on the master brake cylinder. If, however, the car is allowed to roll without the engine running, the vacuum is released at the latest after the first braking process and considerably more force is required to release the brake.

The internal pressure in relation to the air pressure: The relative pressure

A flat bicycle tire still has a bar air pressure. If you would seal the hole and climb a high mountain, the tire would inflate again. The reason is that the remaining residual pressure in the tire expands and thus widens the tire. So to ride a bike at sea level, the air pressure in the tire must be added to the absolute pressure. The resulting gas pressure in the closed system – in this example the bicycle tire – must therefore always be considered relative to the surrounding pressure conditions. This does not necessarily have to be the atmospheric pressure.

One application where the relative pressure must be considered very precisely is, for example, diving technology. Diving watches, gas pressure cylinders, submarine bodies or diving capsules must always be designed in such a way that they can absorb the external pressure without damaging the internal systems by excessive backpressure. The permanent determination of the differential pressure is therefore indispensable for these applications.

Should and have: the differential pressure

The differential pressure is a calculated quantity which results from the difference between an internal and external pressure. What a pressure gauge indicates is therefore always a differential pressure, as it always measures the difference between an external and internal pressure. The differential pressure can be measured in many different ways. The simplest measuring instruments for determining differential pressure are Bourdon tube pressure gauges. Our high-precision differential pressure gauges work with ring balances as a measuring mechanism to determine the differential pressure. In between, there are numerous other measuring methods to determine the differential pressure according to requirements (piezo, diaphragm, etc.).

Differential pressure gauge – Differential pressure sensor for low pressures from Rixen

Our differential pressure gauges can be used with either analog or digital display.

Available in various housing versions ( IP42, IP65 ) for normal use or for use in Ex zones.

Differential pressure sensors for low pressures from Rixen Messtechnik have the 3 common signal outputs ( 0-10V, 0-20mA and 4-20mA ).