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Does Air Go Stale

Now the numerous elemental and molecular parts of air - by themself in a scuba cylinder don't delaminate nor deliquesce. However dewing may be possible at the extremes. The problem is that the container is largely inert, however still slightly reactive to oxidation and hence very minor rusting (for steel). That said Iron Oxide (Rust) doesn't itself pose any major risk nor significant odour to turn Air Stale. Even thouigh you can decant Oxygen and other gases and keep these longer, the problem seems to be with the Hydrocarbons.

You have to look further at the Hydrocarbons and unfortunately some compressors do have oil in them - and no matter how well filtered - a small trace level (and trace level of byproducts) such as Carbon Monoxide, Carbon Dioxide and other Hydrocarbons will in tiny levels be present. It is believed these cause the air to go 'off'. In the main if water vapour is to the HSE standard DVIS 9 (EN12021) then chemical reactions with the Steel or Aluminium will be very negligible. The Nickel Plated Phosphorbronze and Nitrile Rubber, PTFE and Stainless Steel components of the Valve are inert at these pressures and storage temperatures.

Personally Stick to the practice of 6 months and a year for 100% O2 - its a sensible and good rule of thumb, for the price of a fill is your life really worth a few quid? Take them to a reputable dive shop for refilling - drain them completely before you go there. Ask the dive store how does they know when to change the filters - total and utter bull will be them saying they use the indicator strip actually on the disposable cartridge - this is not calibrated anywhere close to DVIS 9 (EN12021) - so ask again. If they go on hours run this on its own is iffy as its all about volume of gas, temp, source relative humidity and dewing factors to name but a few - Water vapour content indicators are so cheap to install in between filters for about £60 to £80 so the store can visually prove to you and themselves that the filters are not expired from a water perspective; Oil/Hydrocarbon breakthrough would have to be a succession of tests to know when best to swap the filter based on volume of air dispensed (or the iffy hours run). I have yet to see a scientific and conclusive study that shows this oil/hydrocarbon breakthrough is totally mitigated with active carbon and other zeolites/absorbants and hence isn't possible even after dewing as filters cool and are left dormant from use (but at a given 'back pressure').

As a RIX is Oil Less and Oil Free and is the compressor of choice for air purtiy analysis (as it really doesn't add any contamination at all to the air upon compressing it), we face here at our facility really no major challenge other than the two filters are removing any air bourne contamination and any residual water vapour. We have double visual indication (after each filter stage) so their is no debate as to when to change the filter for our customers and staff alike.

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