Topping off Tri-Mix with air

[Michael_Lambert]

I think like anything the shop has the choice to do what it wishes. With the popularity of Scuba these days and the popularity of Trimix and CCR. shops are crazy to piss off the already small community of diving like this. Instead of bitching whining they should consider getting better educated.

I had an issue not too long ago, where i wanted a standard air fill in my doubles and the shop insisted on a card saying i was trained to dive the double tanks. lol

 

[Bretagus]

Its a matter of partial pressure. The net flow of gas will be into the tank, but there will still be a diffusive force on the He to move into the space where there is less helium. The degree to which this occurs could be negligible, but concentration gradients can oppose bulk flow.

 

[Blackwood]

Its a matter of partial pressure. The net flow of gas will be into the tank, but there will still be a diffusive force on the He to move into the space where there is less helium. The degree to which this occurs could be negligible, but concentration gradients can oppose bulk flow.

Assuming an appreciable amount of helium diffuses from a set of doubles (relatively small) into an air bank (relatively large), what's the problem?

The diver getting the fill will (should) still analyze his tanks, and the next tanks will get negligible increases in helium (over that which exists in atmospheric air).

Much death will ensue?

 

[Gombessa]

Its a matter of partial pressure. The net flow of gas will be into the tank, but there will still be a diffusive force on the He to move into the space where there is less helium.

I think in this instance you may be lacking the context to understand how much or how little "risk" is involved here, considering the movement of gas, length of the whip, and effects of what would happen even if measurable amounts of helium did make it into the bank. If you truly consider this "too risky, especially for a commercial operation," you should also be aware of the numerous, significantly more deleterious and higher probability risks involved (moisture entering the tank, contaminants entering the compressor, metal fatigue of pressure vessels and lines, etc.

Practically speaking, if helium backflow against a pressure gradient is on the level of being too risky to offer scuba air fills, there would be no compressed gas industry at all, for any purpose.

 

[darkmed5]

Well that's something new

 

[UCFKnightDiver]

That's exactly what I was thinking. I've never seen it made except by PP blending.

Terry

I have, you can continuously blend it

http://www.amigosdivecenter.com/stick.html

 

[Jim Lapenta]

I also still want to know why a MSDT would not know the basics about mixing gas. I can see the new shop monkey with no dives asking about this but an instructor? Even if the shop does not have nitrox, what happens when a student wants a nitrox course? This is all covered. It was in the class I took anyway.

 

[Blackwood]

I see you've never been to Catalina, Jim.

 

[Wookie]

I also still want to know why a MSDT would not know the basics about mixing gas. I can see the new shop monkey with no dives asking about this but an instructor? Even if the shop does not have nitrox, what happens when a student wants a nitrox course? This is all covered. It was in the class I took anyway.

You'd be surprised the number of shops that find anything besides air evil. I know of a shop in Houston that is a Dive Rite dealer that WILL NOT order Dive Rite gear because it is for "tech". Period. They don't do or teach nitrox because it's for "deep diving". In the years I dove BP/W before advanced classes, I bought from any shop besides my local one because they wouldn't sell "tech". And if you wanted a DIN reg, good luck.

I'll bet you could walk into that shop and ask any of their 100 instructors about air topping trimix and get a very similar response. They would then call the owner, who would tell them that they don't do "tech", and the doubles owner would have to find another fill station.

Further, I'll bet 1/2 the LDS's in the United States would have a similar attitude. Any question why the industry is stagnant? I've come to expect the question. I have a couple dive shops in KW that refer ALL their "tech" questions to me, they just don't want to deal with it.

Frank

 

[mark99]

Topping with air is doing an air fill. I am not getting a trimix fill, nor am I getting a nitrox fill... I come in to the shop asking for an air fill, and that's what I want. I can analyze the gas myself, and calculate my subsequent dive based on that mix.

To me it seems like topping up a trimix fill with air is blending trimix. I agree from a practical standpoint a diver could add his O2 and He and go to the local shop to top with air (or add He and go to the shop to top with Nitrox), and he's already done the calculations so the mix will come out correctly. However, I just have to wonder what the shop liability is if the shop-person adding the air isn't certified as a trimix blender. (assuming they were told that they were adding air to trimix). Is it common that shops have employees making mixes (nitrox or trimix) without some formal certification?