SCUBA tanks are metal (usually steel or aluminum) that are filled with compressed air. This is loosely related to a pneumatic system.
Most SCUBA divers dive with compressed air, which is normal atmospheric air compressed into a scuba tank.
Fluid logic.
Scuba divers require increased air pressures in their air tanks while diving because the pressure on their bodies increases.
You could buy your own portable diving compressor, my dive buddy has one, there not silly expensive . You must get fills from a compressor designed for diving (breathing) as the air is filtered and moisture removed.
Because the air is compressed so that the same amount of air fills much less space meaning more can be put into a SCUBA cylinder. Normally a tank will be pressurised up to around 3000psi/200bar of pressure.
The purpose of industrial air compressor is to compress air in large amounts and volumes, later storing the compressed air in special tanks or pressure vessels.
Scuba tanks are typically filled with compressed air, which is a mixture of oxygen and nitrogen. However, some scuba tanks may also contain a noble gas such as helium as part of a special gas mix called trimix, which is used for deep dives to reduce the risk of nitrogen narcosis.
Tanks are used to hold air like you breath above water. They do go thru a filtering process that removes the water and other things in air that can harm a diver. Scuba air from a reputable source is very clean and dry.
Most scuba tanks are filled with simple purified air. However, some are filled with oxygen enriched air called 'nitrox' or (less commonly) a blend of helium, oxygen and nitrogen called 'trimix' or (even less commonly) helitrox or heliox.
Scuba divers use weights to sink during a dive because the air tanks on their backs make them buoyant. By adjusting the amount of weight, divers can achieve neutral buoyancy underwater. This allows them to control their depth and movement throughout the dive.
The air mixture in scuba diving tanks is typically compressed air, which consists of about 21% oxygen and 79% nitrogen. This mixture allows divers to breathe at depth without experiencing adverse effects of high pressure.