# 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 ).

• ### Differential Pressure Gauge for all ex-zones – RW65_EX_II

Ringbalance with built-in intrinsically safe 2-wire 4-20mA loop current transmitter. Approved according to ATEX II 1 GD Ex ia IIC/IIIC T4 Ga Da for all ex-zones. Housing steel/brass in RAL5015 or stainless steel. Supply via intrinsically safe power supply unit
• ### Differential Pressure Gauge with build-in threepoint-step-controller – Rixotact_4

Rixotact_4: Ringbalance with digital display for pressure, vacuum and differential pressure with integrated Threepoint-Step-Controller and additional transmitter output 0-10V.

• ### Differential Pressure Sensor – DDS10

The differential pressure sensor DDS10 is a multifunctional electronic pressure sensor. It is designed for monitoring and measuring over-pressure, under-pressure and differential pressure. With transmitter outputs in the range of 0-10V, 4-20 mA and a variably adjustable measuring range, you remain extremely flexible in practical use. The compact design makes the DDS10 easy to mount anywhere. With an auxiliary power of 24V DC, our differential pressure sensor can be easily integrated into existing control technology.   The versatile fields of application of the differential pressure sensor DDS10 - Air conditioning and ventilation technology - Clean room and laboratory monitoring - Filter monitoring - and everywhere where precise pressure measurement is required.
• ### Differential Pressure Transmitter – MU-Analog-65

Differential pressure gauge  to converts pressure into standard Transmitter output signals of 0-20mA, 4-20mA and 0-10V

• ### Differential Pressure Transmitter – MU-Digital

Ringbalance Differential Pressure Transmitter negative-, over- and differential pressure into standard signals of 0-20mA, 4-20mA and 0-10V.

• ### Differential Pressure Transmitter in 2-wire – MU-Analog-65-2L

MU-Analog-65-2L: Ringbalance Differential Pressure Transmitter “Loop-Powered” (2-wire) version.