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Topping off Tri-Mix with air

Discussion in 'Advanced Scuba' started by marcus0453, Dec 2, 2009.

  1. Michael_Lambert

    Michael_Lambert Contributor

    # of Dives: 500 - 999
    Location: Bradford, Ontario, Canada
    381
    7
    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
     
  2. Bretagus

    Bretagus Contributor

    # of Dives: 100 - 199
    Location: Bloomington, IN
    109
    0
    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.
     
  3. Blackwood

    Blackwood DIR Practitioner

    # of Dives: None - Not Certified
    Location: Southern California
    5,535
    318
    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?
     
  4. Gombessa

    Gombessa DIR Practitioner

    # of Dives: 200 - 499
    Location: NorCal
    4,436
    225
    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.
     
  5. darkmed5

    darkmed5 Registered

    11
    0
    Well that's something new
     
  6. UCFKnightDiver

    UCFKnightDiver ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Florida
    1,091
    145
    Last edited: Dec 4, 2009
  7. Jim Lapenta

    Jim Lapenta Dive Shop

    # of Dives: 500 - 999
    Location: Canonsburg, Pa
    17,172
    9,233
    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.
     
  8. Blackwood

    Blackwood DIR Practitioner

    # of Dives: None - Not Certified
    Location: Southern California
    5,535
    318
    I see you've never been to Catalina, Jim.
     
  9. Wookie

    Wookie Curmudgeon Apprentice ScubaBoard Business Sponsor ScubaBoard Supporter

    36,125
    63,104
    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
     
  10. mark99

    mark99 Instructor, Scuba

    # of Dives: 500 - 999
    Location: Redlands, CA
    330
    3
    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?
     

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