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Old 12th December 2014, 02:25 PM
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rich rich is offline
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Join Date: Nov 2008
Location: Shebbear, N Devon
Posts: 1,642

The calculator you have linked to is actually too optimistic, as most of the on-line ones are.

You won't get as many fills at 300 bar as it says. Air does not behave in a linear fashion as the pressure increases. It is reasonably linear up to 230 bar - which by some coincidence (not!) was the standard chosen for dive bottles years ago. Once you go above 230 bar then the behaviour becomes increasingly non-linear.

In school science terms, what happens is this. When you start to fill a cylinder with air, the pressure rises because you are putting more molecules of air inside it, and there is space between the molecules for more to get in. At 230 bar, the spare space is pretty well used up, and the extra pressure arises partly because the molecules are rubbing shoulders with each other, rather like commuters on a packed train, not just because more air is going in.

This means that 300 bar air does not behave as you might expect it to. The extra pressure isn't derived totally from extra molecules in there, and it's the molecules that do the work in propelling the pellet. If there aren't as many molecules as you expected, then there won't be as many fills as you hoped for.

If you want a comparison, then consider that 300 bar air "behaves" usefully as though it was only at about 275 bar. You can see that the headroom above your fill pressure of 230 bar is not 70 bar but more like 45 bar.

This non-linearity gets progressively "worse" as the pressure rises. Take that into account when thinking what benefits 340 bar might offer.
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