Air Fills - Age of Air

[String]

1 year inspection is true for the USA but not elsewhere. Can be up to 2.5 years.

 

[MikeFerrara]

I'd recommend always analysing tanks before a dive no matter what. About the only time I don't is when I dive the local quarry where they have an air only fill station and I don't bother to bring an analyzer with me.

Nitrox stickers don't mean anything and if you fill at a fill everything kind of fill station, you'd better analyze it. We analyze when we mix and we do it again at the dive site especially when we're diving with other people.

 

[Walter]

Lets face it the longest air should stay in a tank is max 1 year. VISUAL INSPECTION.... In my book it is every year.

The visual inspection is not legally required. It's a great idea, but let's face it, most folks doing the inspections don't actually know what they are doing anyway. The inspection is required by dive shops before they fill tanks. Draining a full tank to have it inspected is silly. Use the tank, then have it inspected by someone who actually knows what they are doing.

 

[DEEPLOU]

The visual inspection is not legally required. It's a great idea, but let's face it, most folks doing the inspections don't actually know what they are doing anyway. The inspection is required by dive shops before they fill tanks. Draining a full tank to have it inspected is silly. Use the tank, then have it inspected by someone who actually knows what they are doing.

I would dispute yor

most folks doing the inspections don't actually know what they are doing

although I don't have any statistics to cite.

Tank inspection is not just

required by dive shops

it is part of the SCUBA industry self regulation.

As to how long for "stale air", my rule of thumb is 1 1/2 years. Good, bad, or indifferent, it is stale. A stale loaf of bread is editable, but would you want to eat it?

 

[loosebits]

Tank inspection is not just it is part of the SCUBA industry self regulation.

What exactly is the difference between "self-regulation" and something being insisted upon by most LDS's? How can I tell the difference between something done for safety reasons and something done to simply help LDS's make a few bucks? Does anyone know of any instances of an in-hydro steel cylinder rupturing while being filled to rated pressure?

You know, I may be convinced that this is needed for AL bottles, even the newer alloy but for steel cylinders?

 

[Walter]

I would dispute yor, "most folks doing the inspections don't actually know what they are doing," although I don't have any statistics to cite.

OK, I don't have stats either. Why do you believe most do know what they are doing? I've come to my belief by talking to people who are inspecting tanks. They are usually not only ignorant of proper proceedures, but they believe myths about tanks. How often do you have a tank inspected where they actually remove all the sickers?

Tank inspection is not just required by dive shops, it is part of the SCUBA industry self regulation.

Why do you think there's a difference? Unless you are taking a bottle to a shop to get it filled, how would it come into play?

 

[ClayJar]

As to how long for "stale air", my rule of thumb is 1 1/2 years. Good, bad, or indifferent, it is stale. A stale loaf of bread is editable, but would you want to eat it?

The proteins, carbohydrates, fats, et cetera that make up a loaf of bread degrade easily. Heat, light, air, microbes, and even just the passage of time are enough to break them down into a rather unpalatable jumble of products.

Air in a scuba cylinder, on the other hand, is a much less complex system. Assuming you have a nice, clean fill to begin with, you have 78.084% N2, 20.946% O2, 0.9340% Ar, something like 0.04% CO2, and the rest trace gases [1, 2], plus a certain amount of water vapor (greatly reduced from the local atmosphere). This mix of gases would then be contained in a steel or aluminum cylinder (with a neck O-ring and a valve).

Obviously, we can ignore N2 and Ar, as they're non-reactive. We'll also ignore the trace gases, as their concentrations are to small to make consideration worthwhile. We'll ignore the O-ring, as it has such a miniscule area of contact with the gas in the cylinder (although if you're truly masochistic, feel free to find the precise dimensions and composition of the neck O-ring for your tank, and I'll run some numbers). Lastly, we'll ignore the valve and consider it non-reactive (it has a very low exposed surface area with respect to the cylinder, so this shouldn't be an issue).

This leaves steel, aluminum, O2, and CO2 as the relevant materials for determining when air in a scuba cylinder would be considered "too stale". None of these would impart unpleasant flavor or smell to the air; you would never end up with "stale crypt air", so only O2 and CO2 concentrations are relevant. For an aluminum cylinder, you wouldn't have any carbon in the alloy (aluminum, magnesium, and silicon, if my memory serves), and given the starting CO2 concentrations, you will never end with a problematic CO2 concentration, so we can ignore CO2.

For a steel cylinder, I'd have to know what the percent (by mass is fine) carbon is in order to give worthwhile numbers, but if it were a very high carbon steel (2.1%), and if the iron and carbon oxidized proportionally, you could theoretically acheive problem CO2 concentrations before the mix became hypoxic, but since scuba cylinders aren't made from such steels, with the much lower carbon content of their steels, you would hit hypoxic well before you hit toxic. I'd love to do actual numbers on it, but without the alloy information, I can't.

If you ignore CO2, you're left with O2 concentration as the only relevant metric for air staleness. Assuming 79% N2 and 21% O2 at fill time, the mix will become hypoxic (defined here as a concentration of 10% or less O2) when the pressure drops to .79/.9, or 87.7...% of the original pressure -- a 3000-psi cylinder originally filled with air would be hypoxic when the total pressure drops to 2630 psi, and so on.

My rule of thumb is that if I haven't used my fills in a month, I'm dangerously dry and need to recompress as soon as possible.

 

[rjack321]

Why do you believe most do know what they are doing? I've come to my belief by talking to people who are inspecting tanks. They are usually not only ignorant of proper proceedures, but they believe myths about tanks. How often do you have a tank inspected where they actually remove all the sickers?

I suspect part of the problem being varying definitions of "proper".

I do remove stickers when doing visuals, but then again I don't have those giant "proper" nitrox banners either so its no big deal. Lately, I have been removing all the paint from my remaining painted AL cylinders too. Its pretty easy and the exterior surfaces are easy to see and keep salt free.

 

[lamont]

The >6 month stale air that I mentioned diving above was also in tanks that were several months out of VIP.

Somehow I didn't die because they were out of VIP.

I then stripped off all the bumper stickers any old VIP stickers myself and took them in.

 

[spectrum]

Lets face it the longest air should stay in a tank is max 1 year. VISUAL INSPECTION.... In my book it is every year.

VIP is for filling. That does not address the dormant cylinder that was stored full.

Old air, all things considered it's probably better that what we breathe today.

Pete