30 replies to this topic

[nextariel]

I had my tank filled last July, yes it is past the visual date. Plan to use the tank next weekend. The tank is 7 years old and has had about 7 fills. Should I be concerned about the quality of the air?

[kevininpo]

probably its ok...take a few sniffs and a few test tastes and see...you really need to get wet a bit more often, though

Edited by kevininpo, 08 April 2005 - 08:14 PM.

[RichardB]

You should be fine but make sure there's still enough air in the tank for your dive. Although if your LDS can give you a quick vis why not just do that and get the free fill at the same time? Many shops will generally try to work you in or give you a free loaner if your tank is in for an annual.

[nextariel]

you really need to get wet a bit more often, though

Oh, I've been wet, just not using my own tank. August SD trip in Key Largo, didn't bother since I was using Nitrox, NC wrecks in Oct, they tend to frown on tanks on a plane, and then there was Cozy in Jan. But there was the week in Key West planned for last Sept and Ivan had other ideas.

[Walter]

Oh, I've been wet, just not using my own tank. August SD trip in Key Largo, didn't bother since I was using Nitrox, NC wrecks in Oct, they tend to frown on tanks on a plane, and then there was Cozy in Jan. But there was the week in Key West planned for last Sept and Ivan had other ideas.

You really need to get wet a bit more often.

[scubafanatic]

....most dive shops include a free fill with an 'annual', moreover, if the annual is expired, you won't be able to refill your tank at the dive site, assuming you want to do more than one dive there. Also, as already pointed out, double check the air pressure to make sure it's still full, if youi choose to skip the annual for now.

...if you have an O2 analyzer, you can at least check the O2 % as well.

Karl

[Geek]

Oxygen is one of the key components in rust. I suggest getting the visual done since you need it anyhow. You will not only know the air is okay, but that the tank is okay too. When storing a cylinder for a long period consider first draining it, then close the valve.

[Diverbrian]

I have a dive party coming up this weekend and took a quick look at my doubles. They have to be VIP'ed or the quarry won't able to fill them.

So, I have to tear the things down and drain them and then put them back together when the shop is done with vis so that they can fill them prior to going to the quarry. What a pain!

So, remember that if you choose to have it done now. It should make you laugh a bit!

[nextariel]

Since Divers Direct was given VIP for half price with the email I received yesterday. It is there now so it will be ready for Friday.

[Marvel]

Since Divers Direct was given VIP for half price with the email I received yesterday. It is there now so it will be ready for Friday.

No fair! I just checked my email & don't have that one....

[RichardB]

Oxygen is one of the key components in rust.  I suggest getting the visual done since you need it anyhow.  You will not only know the air is okay, but that the tank is okay too.  When storing a cylinder for a long period consider first draining it, then close the valve.

Thanks Geek!

You're right about the rust, but I'm not sure on the draining issue. Of course this isn't a problem with aluminum tanks.

Actually, you need three things for rust (Fe2O3) to form.

First, Oxygen, then iron (from steel tanks) and an electrolyte to carry the electrons. Typically the electrolyte is water. With salt water there is a better electrolyte and items will rust quicker. If there is no electrolyte then there can be no rust....even with high oxygen mixtures. As long as you have a good fill with dry air you're better off keeping the tank pressurized.......at least to a few hundred pounds. This will prevent moisture or moist air from entering the tank and eliminate one of the three ingredients required for rust to form.

[Geek]

Actually aluminum will oxidize, though it is less of a problem than with steel. As far as the draining is concerned, I can understand the desire to keep a couple hundred pounds as some places will not fill a totally drained tank, but more air provides more oxygen to the oxidation process. Some aluminum tanks can suffer from cracking as well. Also, do you really want a cylinder with 3000 psi in your garage or basement for months, or worse years, subjected to temperature variations, etc.? While cylinder failures are not common, they do happen. The result is not worth the cost of a fill or a VIP.

Since the tank that started this thread is relatively new, I doubt this is a serious concern in this case, but I still recommend draining the tank if it is going to be stored for a long time.

Edited by Geek, 11 April 2005 - 05:58 PM.

[RichardB]

Actually aluminum will oxidize, though it is less of a problem than with steel. As far as the draining is concerned, I can understand the desire to keep a couple hundred pounds as some places will not fill a totally drained tank, but more air provides more oxygen to the oxidation process. Some aluminum tanks can suffer from cracking as well. Also, do you really want a cylinder with 3000 psi in your garage or basement for months, or worse years, subjected to temperature variations, etc.? While cylinder failures are not common, they do happen. The result is not worth the cost of a fill or a VIP.

Since the tank that started this thread is relatively new, I doubt this is a serious concern in this case, but I still recommend draining the tank if it is going to be stored for a long time.

Aluminum is a very reactive metal. (In fact, it's so reactive that it is never found in nature as the unreacted element.) However, unlike other reactive metals, aluminum forms a coating of oxide that protects it from further reaction with the oxygen of air. So aluminum oxidation actually forms a protective layer that prevents further oxidation.

Iron is NOT nearly as reactive as aluminum. With iron (the main component in steel tanks) it takes an electrolyte (think electrical connection, water acts like a wire to complete the circuit) to cause the chemical reaction leading to oxidation. Rather than the content of oxygen in the air, it's generally the oxygen in the water, or electrolyte, right at the "edge" of emulsion that initiates the ferrious oxide. The water acts as a corridor...to complete the "circuit." Oxygen in the air can then replenish oxygen consumed in the chemical reaction and continue the process. (Ever see those little rust circles from a rain drop?)

Simply put, No electrolyte, OR no Oxygen is NO rust. Every wonder why nothing rust in dry climates? Scuba air is VERY dry and unless there is an intrusion of damp air or water or other electrolyte into the tank there will be no rust. The pressure inside a tank will keep the water out.

As for storing 3000 psig air in a tank for long periods, I don't think it's a good idea from a safety issue. As for the material science, the design of steel tanks doesn't care and doesn't experience any degrdation over time under stress. (We don't loosen bolts on our car tires to give them a rest, do we?) Steel has an infinate cycle life under non-yeild loading as experienced in scuba tanks. With aluminum it's a bit different as there is no infinate cycle life. Given enough cycles aluminum will eventually fail, but we're talking millions with tanks. Still with aluminum it's the cycles, not the duration that causes failure. (With the possible exception of Luxfers 6351 aluminum alloy which was occasionally prone to very slow crack probagation over time whil under stress.)

[Geek]

I think we are in agreement here. The risk is really small, but why take it? If you are storing a tank for a long period, drain the pressure down to a few hundred psi at most.

There is another risk I did not think of earlier, i.e. you have a full tank in your house and have the misfortune of a house fire. You could wreck some fireman's whole day.

[RichardB]

I think we are in agreement here. The risk is really small, but why take it? If you are storing a tank for a long period, drain the pressure down to a few hundred psi at most.

There is another risk I did not think of earlier, i.e. you have a full tank in your house and have the misfortune of a house fire. You could wreck some fireman's whole day.

Agreed! AND, if you have tanks with full pressure at the end of the season....go diving!!!!