Scuba Tech Tips: Is Tank O2 Cleaning Necessary?

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    Alec Peirce - Tank Tips

    Hi guys, Alec Peirce Scuba Tech Tips, talking to you from Simcoe Diving in the village of Barrie up on Lake Simcoe in Ontario, Canada.

    We’re talking today about whether or not O2 cleaning is necessary. Here is an O2 tank. I know it’s an O2 tank because it says O2 right on it: nitrox, which is short for enriched air.

    Now, what that means is that this tag has been especially clean. It’s not just had a normal cleaning, not just a normal visual with a cleaning. This has a nitrox clean sticker on it. Says right on there that this “cylinder has been vision inspected to so many standards and suitable for oxygen service.” OK, so this tank can be used for nitrox. You’re going to be diving with enriched air.

    Here’s the question: Is it necessary for you to have your tanks cleaned for a nitrox service? Here is the controversy. I love controversial topics. I get lots of comments. You hear from dive stores because it’s a fairly substantial charge to clean a tank for oxygen use; and for those who make a fair bit of money from that, they don’t want to give it up. The short answer is no, not necessarily. You don’t necessarily need to have your tank clean if you’re using nitrox.

    Why do I say that? The tag needs to be clean if it is going to be subjected to 100 percent oxygen. In fact, it really should be clean for oxygen service if it’s going to be used for higher than 40 percent oxygen. At those concentrations, higher than 40 percent and certainly 100 percent, any impurities, any oil, anything inside the tank that might be flammable or might increase temperature for a variety of ways, an oxygen rich environment could be very, very dangerous. Note what I said is only essential if the oxygen level is 100 percent or more specifically, let’s be more specific, if it was above 40 percent. So what’s the problem here? In most cases, your tank will never get 100 percent oxygen in it, and it’s very seldom, if ever, it’ll get higher than 40 percent. That’s right. When you go into a dive store and say, “Hey, I want to make a nitrox dive this afternoon, I’m going to be diving 35 feet, and I want it 32 percent.” In most cases that dive store will put 32 percent oxygen in.

    That means air, instead of being at 21 percent, will have 32 percent. You’ll have about 10 percent more oxygen by volume than normal. So the guy goes and he hooks up your tank to the stick or to his banked air, whatever system he’s using. He’ll hook it up and he’ll fill your tank to 3000 PSI at 32 percent; and then he’ll mark it and sticker it because you can’t tell what percentage is in there and the percentage is important. So usually he’ll put a piece of tape on there and you this particular tank was filled to 30.3: not even 32, it’s probably going to be a 30 percent, which has an M.O.D. (maximum operating depth) of 124 feet. So this is this tank has air at 30 percent. Well, that’s a long, long way from 100 percent.

    That’s a heck of a long way from even 40 percent. So this tank is perfectly safe with 30.3, 30 percent, enriched air. It’s even safe.

    So why did dive stores insist that you have oxygen clean? They insist you have this sticker and the visual on it. First of all, I already mentioned money. Secondly, there are a number of dive stores, and still a good number of dive stores, because at one time it was the most common method, inexpensive and easy to use what’s called the partial pressure system to fill your tank. The partial pressure system of filling a tank, a nitrox tank getting enriched to the partial pressure system, does use pure oxygen. There’s this very simple mathematical formula. You come into the store and say, “Hey, my tank has got 500 PSI in it, I want to fill it to 3000 PSI, and I don’t want 32 percent.” They punch those figures in.

    They check, first of all, to make sure that your tank only has 500 so they punch in 500 starting pressure; they punch in 300 finished pressure; they punch in current oxygen level, which they test quickly. They test. And so let’s say 21 percent regular air level and then they punch in what you want. So they got the four numbers, they put them under a formula, and out spits the answer. It says, “step one, fill the tank to, pick a number, 900 PSI with pure oxygen.” So you do that, then step two: fill the tank to 3000 PSI using oxygen compatible air. If you do that, you now have a tank filled to 3000 PSI at 32 percent. It’s tested for pressure and level to make sure that it worked properly and it does.

    Using that particular system, think back for a moment with what I just said a minute ago. You put in 100 percent oxygen in the tank. That’s right. A hundred percent oxygen. Even though it had some air in it, you’re still putting in 100 percent. One hundred percent oxygen goes through the valve down near the tank and mixes in there. Now, if the tank was almost empty, you put in 100 percent oxygen… I have oxygen all over all the surfaces inside the tanks so anything in there in the presence of pure oxygen could be a problem – heat and igniton.

    That can be a problem, but that only applies to the partial pressure method. Partial pressure method. All of the systems don’t do that. They put in the level of oxygen, nitrox, enriched air that you ask for. Sure, you have a dive store that uses the stick or you have a dive show that uses banked enriched air. They may have a big tank full of 32 already pre-mixed and you want to 32 there’s no oxygen going in. For many of those systems and other systems, as well as a membrane system, they put 32 in – 32 is perfectly safe to use without cleaning it.

    It’s not a bad idea to clean it. It’s not necessary. Some dive stores will argue and even some tech divers will argue. The simple fact of the matter is, from a pure physical and chemical point of view, if you’re only using low level enriched air and you’re always getting pre-mixed enriched air, oxygen cleaning of your tank is not that necessary.

    See how many comments I get about that? This should be fun anyway. There you go. Just some thoughts from Alec Peirce Scuba Tech Tips from Simcoe Diving in Lake Simcoe.

    How about that? OK. See you.