Nitrox and Air Tanks

[iztok]

After the shop puts pure O2 into your tank for partial pressure blending, it puts it on the compressor and tops it off with air.

What is the difference between these two choices? Why will the second contaminate the tank if the first one will not?

1. putting a few hundred PSI of O2 into the tank and then topping it off with air
2. putting air into the tank without putting a few hundred PSI of O2 in first

I think you've misread what I've said.

Tanks only need to be specially cleaned if they are exposed to high concentration of O2. If they are not they don't need to be O2 cleaned.

If shop uses partial pressure and you have O2 clean tanks there is no reason why they can't put just clean air and keep your clean tanks clean.

Issue arises when you fill your O2 clean tanks somewhere that they don't have O2 clean system/air to put in them. So if you have your tanks filled somewhere where they just have air fills or where they just do banked nitrox you might have your tanks contaminated and no longer suitable for high O2 concentration (partial fill).

 

[iztok]

After the shop puts pure O2 into your tank for partial pressure blending, it puts it on the compressor and tops it off with air.

What is the difference between these two choices? Why will the second contaminate the tank if the first one will not?

1. putting a few hundred PSI of O2 into the tank and then topping it off with air
2. putting air into the tank without putting a few hundred PSI of O2 in first

If you are filling from the same system then there is no difference. However if second one is fill of air that is not clean then second one is going to contaminate your tanks.

 

[kanonfodr]

To the OP: If you are having a fatigue problem after diving, Nitrox isn't the only answer. Check your ascent profile, if you are doing a Haldanean-style ascent to 15 feet where you wait for 3 minutes before continuing to the surface, that could be what is causing your fatigue. I use an ascent strategy where I stop at half of my average depth, wait for a minute, then make 1 minute stops at every teen feet up until 10 feet where I hang for a minute before making a slow ascent to the surface.

The key is not only the stops but maintaining a controlled ascent rate of 30 fpm. The Haldanean decompression model (Navy, PADI, etc. tables) does not account for Free Phase Nitrogen (really super small bubbles) so while a 30 fpm ascent rate is still recommended, the tables only have the 3 minute stop as a recommended safety measure because they found divers were just rushing to the surface and getting bent, the 3 minute stop is designed to prevent that.

AFAIK, the reason most divers doing a typical ascent profile feel tired because their bodies are still trying to offgas nitrogen whereas using an ascent that includes stops produces a better decompression (all diving is decompression diving) so you are doing your deco in the water where it is more efficient.

Peace,
Greg