Posted by roakey on August 16, 2001 at 11:41:41:
In Reply to: Another question about Nitrox blending. posted by Maciek on August 16, 2001 at 09:55:17:
From a pure physics perspective the order of gas addition doesn't matter.
On paper it would appear that adding O2 to air would be safer.
Unfortunately there's a practical reason for not putting the O2 in last. The cheapest O2 is "off the shelf" like ABO (Aviator's Breathing Oxygen). Bulk cylinders of O2 come in + rated 2400 psi cylinders which means *if you're lucky* you'll get one filled to 2640 psi. PP blending of 36% in a 3k bottle means you put in 570 psi of O2 then fill with air up to 3000 psi. To do it the other way would require you to put air in it up to 2430 psi, then O2 up to 3000 psi. Well, as I mentioned the best you can hope for in an off the shelf bulk bottle of ABO is 2640, more typically 2400 so youíre SOL right out of the gate since youíre starting at 2430 psi!
Now, gas companies will sell you just about anything you want (except nitrous oxide :-)) and there are suppliers out there that will sell you O2 at 6000psi, but this comes with a whole host of problems. First, it costs three to five times as much for a cubic foot of O2. Second, the fittings cost three times as much as the normal 3000psi brass fittings that most PP shops and individuals use. Third, you can only use half the O2, because once the supply drops to 3000 psi itíll be unusable for filling bottles anymore (ignoring folks like me with LP steels).
And lastly, no one will pay $50 a cylinder for Nitrox to support all this additional cost! :-)
You could also boost O2 your with a Haskel, but theyíre very expensive too (on the order of $8-10k for a new O2 clean one). Boosting O2 makes me nervous because of the heat thatís generated. Iím more than willing to boost He since it only goes kaboom via pressure, not combustion. Bottom line the price/performance tradeoff of doing the O2 second makes it just not worth it, because in reality there isnít that much of a problem with PP mixing to begin with!
MHKís and my O2 comfort levels may differ, but they donít differ all that much and we both agree that O2 is one heck of a lot safer than the dive shops seem to think it is. Iím hoping (Iím sure MHKís hoping the same thing) that as the public becomes better informed on Nitrox and O2 handling that the voodoo-like mystique that surrounds it will be dispelled. Then weíll have a more level headed understanding of it at the diver, shop and gawd let us hope at the government level.
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