Even though most divers will seldom find the need to buy their own Scuba air tank compressor, it often helps to understand the process by which our dive tanks get filled.
A compressor, like a pump, essentially increases the pressure of a gas while reducing its volume so that a larger quantity can be carried by a diver in a smaller air tank. Unlike an ordinary compressor, a scuba compressor needs to perform two tasks: compress air to very high pressures in excess of 3000PSI and filter and purify the air to make it fit for breathing.
The most common type of scuba air compressors involves staged compression, where air is compressed to increasing pressures and cooled after each stage. In fundamental terms, a compressor works in three stages. Air is sucked in through an air filter into the first stage, where a large piston compresses the air down to around 100-140 psi. The air leaving the first stage is hot as heat is generated as a byproduct of the compression. This needs to be cooled before it enters the second stage, or it will ruin the pump. Hence the air is passed through an intercooler coil that dissipates the heat before being sucked into the second stage, where a piston compresses the air again to around 800 to 1,000 psi. Once again, the air is passed through intercoolers and, before entering the third stage, passes through a moisture removal separator, which allows the moisture from the air to trickle out the bottom and sends only the dehumidified air into the third stage, where the air is compressed up to 5000psi. From the third stage, the air is again cooled using the intercooler and passed through a moisture separator or two before being fed into a Scuba tank at 2700-3000Psi using a bleed valve.
As a diver’s life depends on the quality of breathable air in the scuba tank, air filtration is an integral component of scuba air compressors. It ensures clean, odorless, and moisture-free air. A Scuba Air compressor must remove water/moisture and oil, which the moisture separator and desiccant filter handle. Next, Carbon Monoxide must be rendered harmless using a catalyst that oxidizes the Carbon Monoxide (CO) in the air into harmless Carbon Dioxide (CO2). Carbon Dioxide in high concentrations can kill a diver, and hence, that too must be reduced using Activated Carbon filter cartridges and a molecular sieve. Other Hydrocarbons are also removed when passed through the filter cartridges and molecular sieves that render the air fit for diving.
Now you know why scuba compressors are so expensive. Usually larger compressors used at dive shops pump air into a large tank from which it can be re-distributed to smaller scuba tanks, making the process much quicker. Portable air compressors are perfect for a recreational diver but can often take time to fill cylinders.
