Changing Gas in a Tank

[garyw90]

I would like to see what some of the opinions are on this subject.

I have one steel tank. It is my understanding that it can be filled with Nitrox and does not have to be O2 clean if filled with a membrane system.

So if I want to switch the tank from a normal air fill (21%) to Nitrox (32%), is it safe to have the tank completely drained of air and then immediately filled with 32%? Going under 500 PSI requires that the tank get visually inspected because of risk of contamination, but is this the case when switching gases? How about the other way? Going from Nitrox to air? Is it safe to empty the tank and then immediately fill with air?

Let's hear what you think!

 

[Thalassamania]

Having to visual if you go under 500 psi is horse pucky. If you've at a shop with such a rule, take the tank in with 500 psi, let them drain the rest and refill it with EAN.

 

[Doc Intrepid]

"I have one steel tank. It is my understanding that it can be filled with Nitrox and does not have to be O2 clean if filled with a membrane system."

Gary,

Your steel tank does not have to be O2 cleaned if you do not intend to fill it with pure O2 - as you would do initially if you were partial-pressure blending nitrox. (To partial-pressure blend nitrox you initially fill the tank with X amount of O2, and then top it off with air to achieve "enriched air".)

The alternative is to fill your tank with nitrox that is already mixed. Many shops bank nitrox "pre-mixed" to EAN32 or 30 or whatever is in vogue in that area. If you're filling your tank with premix, then it does not need to be O2 cleaned because you're never introducing pure O2 into your tank.

I'm not sure where you got the "membrane" part. (A membrane system is one means of extracting O2 from air, and can be used to premix nitrox. Is this what you were referring to?)

"So if I want to switch the tank from a normal air fill (21%) to Nitrox (32%), is it safe to have the tank completely drained of air and then immediately filled with 32%? Going under 500 PSI requires that the tank get visually inspected because of risk of contamination, but is this the case when switching gases? How about the other way? Going from Nitrox to air? Is it safe to empty the tank and then immediately fill with air?"

If your tank is filled with air, it is safe to drain the air out of the tank and then immediately fill it with nitrox. The reverse is also true. (But I'm uncertain why you'd want to drain a nitrox fill and then fill the tank with air.)

Both statements assume that you've had your tank inspected routinely and are aware that the tank is in good condition.

Hope this helps,

Doc

 

[deepstops]

Is it safe to empty the tank and then immediately fill with air (or premixed nitrox)?

Let's hear what you think!

Yes it is.

 

[knowone]

If you empty them quick you freeze the orings. That is bad. If you empty them slow and miss the boat, that is bad.

 

[halocline]

The membrane is a continuos nitrox blending system that many shops use; you do not need O2 clean tanks to use it.

Did the shop really say that any tank with less than 500PSI needs to be inspected? That's an instant classic on the dumb LDS hit list.

 

[wedivebc]

Having to visual if you go under 500 psi is horse pucky. If you've at a shop with such a rule, take the tank in with 500 psi, let them drain the rest and refill it with EAN.

Or better yet find a shop that knows what they're doing

 

[dannobee]

...Did the shop really say that any tank with less than 500PSI needs to be inspected? That's an instant classic on the dumb LDS hit list.

Oh, we have a local shop that, when returning renting tanks, will immediately check the tank pressure and charge you for a VIS if it has less than 500 psi. It has become a new profit center for said shop, from what I've seen/heard.

 

[GrumpyOldGuy]

I have seen some shops charge for a vis if a rental tank is returned empty. This makes sense as there is a very slight possibility the tank might have water in it and if you use a tank down to empty you are probably an idiot anyway and they should be cautious.

To extend this to 500PSI or draining the tank on dry land is ludicrous. If your LDS is so stupid or greedy to insist on this, then treat it as fair warning to find a new LDS. To continue and trust them for other things that you cannot see is stupid.

 

[Scared Silly]

When switching mixes I rarely worry draining out whatever was in there before because in the long run the combining the two typically does not change the mix significantly enough to alter the dive profile. Showing this statement to be true is left as an exercise to the reader.

As for the silly dive shop rules, they can make up their rules but folks should really start talking to the owners and ask what gives?