Gases differ from liquids and solids in that they are compressible. Liquids and solids, on the other hand, can hardly be compressed even under high pressure.

This can be easily demonstrated with the following experiments ¹⁾:

While the syringe filled with air (the two left drawings in the picture) can be compressed, this is not possible with the right syringe, which is filled with water.

Take an empty thin plastic bottle and close it. Do the same with a second bottle. Fill this completely with water and then close it.

Squeeze the bottle with your hand.

Now increase the external pressure. The bottle filled with air is squeezed, while the bottle filled with water retains its shape.

The pressure can be increased in various ways. You could close the bottles before descending in a balloon flight, or on a mountain.

Or you can let the bottles sink to the bottom of a swimming pool.

A difference in height of 5 metres in water causes the same difference in pressure as the difference in height in the atmosphere from sea level 0 m/msl to 5500 m/msl.

• A container filled with a gas or gas mixture that can no longer withstand the internal overpressure would burst, resulting in explosive decompression. For this reason, pressure vessels are filled with water for pressure testing, as water is incompressible and the overpressure is released abruptly when the vessel bursts, preventing a pressure wave from being released into the environment.
• Liquefied gas cylinders must always have a gas cushion above the liquid propane so that the expansion of the liquid propane due to an increase in temperature can be compensated without the pressure in the cylinder rising. If this gas cushion did not exist, the compression pressure would rise extremely quickly and the cylinder's pressure relief valve would open. Although a temperature increase also leads to an increase in pressure in the cylinder even if a gas cushion is present, this is not the compression pressure but the vapour pressure, as in a pressure cooker. Incidentally, although this has nothing to do with ballooning, a pressure cooker should not be filled completely with water.
• Because gases are compressible, increasing the pressure allows more gas or gas mixture to be stored in a pressure vessel.

Pressure times volume equals constant (P x V = constant)

This means that if I double the pressure, the volume is halved.

Wherever gases are stored in compressed form, the contents can be calculated using pressure times volume:

If a hydrogen tank has a volume of 73 cubic metres and the maximum pressure is 42 bar, it contains 42×73 cubic metres = 3066 cubic metres at 1 bar (corresponding to 1000 hPa).

If a 5-litre oxygen cylinder shows 200 bar, it contains 200 x 5 litres = 1000 litres of oxygen at a pressure of 1 bar. Whether this is also the usable amount depends on various factors. On the one hand, there is the minimum pressure that must remain in the container so that the container cannot draw in air from the environment, which could be particularly dangerous with flammable gases. Then, of course, it depends on the current external pressure and also on the adiabatic cooling when the gas is removed.

Note: In the past, the term ATÜ was used to refer to pressure measured from the ambient air pressure. Even the pressure gauges on pressure vessels usually do not indicate the absolute pressure, but rather the excess pressure. If a pressure gauge set in this way shows ZERO, there is no vacuum in the container; instead, the residual pressure corresponds to the ambient pressure.

Here too, if I fill the tyre with twice the amount, i.e. twice the mass of air, the pressure also doubles.

The lifting gas in gas balloons and the air in the envelope of hot-air balloons is also subject to different pressures depending on the altitude and thus the ambient pressure in which the balloon is located. The image below shows a gas balloon on the left filled with approximately 1000 cubic metres of hydrogen and, on the right, after descending from approximately 5800 metres, filled with approximately 500 cubic metres of hydrogen.

The effect of decreasing air pressure is also illustrated by the experiment with the syringe as a mini pressure chamber.

The plunger of the syringe is partially pulled out. A balloon is located in the space that is now free in the syringe. The outlet of the syringe is closed, in the video by a finger, but a tap could also be used. If the plunger is now pulled out further, the air pressure in the syringe decreases and the balloon inflates. Conversely, the volume decreases when the plunger is pushed into the syringe, as the air pressure then increases.

When the volume increases and the pressure decreases, not only does the pressure decrease, but also the density of the gas. This is a factor that plays a role in calculating buoyancy and lifting force.

The pressure gradient in the atmosphere also plays a role in altitude measurement. An altimeter measures the pressure and then assigns an altitude to the pressure. The pressure corresponding to a specific altitude has been defined by the International Civil Aviation Organisation (ICAO) in the standard atmosphere.

What is the difference between gases and liquids that is responsible for the pressure gradient in the atmosphere?

Density is mass per volume. How does the density of a gas behave when it is compressed?

Is a gas cylinder containing hydrogen at a pressure of 200 bar lighter or heavier than the same cylinder containing hydrogen at a pressure of 50 bar?

Answer form for gas theory - pressure