How long will a fill last ?

[Keith Whittaker]

probable Numpty alert!!!!!

Apropos of nothing at all, the following ha just occured to me.

IIRC Oxygen is incredibly reactive. it only exists in our atmosphere because it is constantly replenished as a byproduct of life processes, Photo-synthesis etc. Therefore if you put air into a closed environment, lets say, For the sake of arguement, a ...............Cylinder! Then shouldn't the Oxygen eventually all react with the Nitrogen to create Nitrous Oxide??
I'm no scientist and am probably missing something really obvious and fundamental here but if the above is true,( and thats an incredibly large if!), i wondered just how long you can leave a cylinder filled and still be able to dive with it?

Any stupendously large intellects out there in YD land able to shed light on this?

[dry suit diver]

would take a long time or a tremendous amount of energy to do, there was a discussion on here and UKRS a while back about length of time for air in a cylinder to go off and someone posted that they had a 15 year old cylinder with a full fill of air and it was still 21% and breathable.

NigelH would probably have the energy required figures as he is a top physics/chemistry bod

[Duncan Price]

Originally Posted by Keith Whittaker

Any stupendously large intellects out there in YD land able to shed light on this?

I'm a scientist and have a stupendously underused intellect.

Originally Posted by Keith Whittaker

IIRC Oxygen is incredibly reactive. it only exists in our atmosphere because it is constantly replenished as a byproduct of life processes, Photo-synthesis etc. Therefore if you put air into a closed environment, lets say, For the sake of arguement, a ...............Cylinder! Then shouldn't the Oxygen eventually all react with the Nitrogen to create Nitrous Oxide??

Nitrogen is incredibly unreactive and you need very forcing conditions to get it to react directly with oxygen. A spark such as lightning or in a car engine is the best bet. The conditions inside your cylinder are rather mild by comparison. Nitrous oxide and other oxides of nitrogen are in fact oxidising agents in themselves. N2O4 is used in rocket engines for this purpose.

Originally Posted by Keith Whittaker

I wondered just how long you can leave a cylinder filled and still be able to dive with it?

The main problems are moisture in a steel cylinder causing oxygen depletion due to the formation of rust. Helium molecules might preferentially leak out through valve seats.

It in doubt anaylse your mix. Formation of nitrogen oxides is not an issue though.

[Paddy Diver]

We were told and tell our students that your air is good for a least 2 & 1/2 years and then you'll need a visual test so the air would be replaced then but as Dry Suit Diver said

Originally Posted by dry suit diver

that they had a 15 year old cylinder with a full fill of air and it was still 21% and breathable.

Thats a long bloody time so I guess would dont need to worry.

[ray]

Hi.

Get yourself off diving at every opportunity, then you only have to worry about filling the cylinder

[Paddy Diver]

Originally Posted by ray

Hi.

Get yourself off diving at every opportunity, then you only have to worry about filling the cylinder

Ya I'd have to agree with Ray on this, now since the weather is shite does the bath count?

[Scuba1]

And if you have some nitrox left, it is a great thing for helping the bbq along

[Reikimaster]

Originally Posted by Scuba1

And if you have some nitrox left, it is a great thing for helping the bbq along

sod the bbq, a 50% does wonders the following morning!

[nigelH]

Originally Posted by Keith Whittaker

Therefore if you put air into a closed environment, lets say, For the sake of arguement, a ...............Cylinder! Then shouldn't the Oxygen eventually all react with the Nitrogen to create Nitrous Oxide??

That's not a net energy gain reaction that's a net loss. If Oxygen and Nitrogen reacted there would be a detectable level of Nitrous Oxide in the atmosphere while, in fact, there is not.

i wondered just how long you can leave a cylinder filled and still be able to dive with it?

Decades.

I bought a set of vintage scuba gear, I really wanted a twin hose reg, and it came with a suit and tank. The tanks last test date, on a brass collar as was the system at the time, was 1975 and when I turned the valve is hissed at me so we put the old nitrox analyser on it. 21%. It was boring old air. Now I admit I have no idea when it was filed but I would guess it was the late 70s.

Anyhow, as somebody else pointed out, if you are asking these sought of questions you clearly aren't diving enough. The only fills that should last more than a fortnight are the ones in my bailout tanks and they go two and a half years between cylinder tests.

[Duncan Price]

I should add that we've got bailout tanks stashed down caves that have been there ten years. They still get used on a semi-regular basis i.e. a reg is put on them, pressurised and a couple of breaths taken to check them before we set off on a dive.

Needless to say they were out of test before they went underground and they are not coming out until they need filling.