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LP vs HP tanks on doubles...

Discussion in 'Advanced Scuba' started by DC53, Mar 26, 2010.

  1. DC53

    DC53 Solo Diver

    # of Dives: 200 - 499
    Location: Naples, Florida
    Beware of the amount of detail below and thanks for reading through it…

    I am in the process of setting up and learning doubles. Currently my setup uses two steel HP 120’s as they are the only tanks that I have in an identical pair. However for the future I am wondering about input on tanks. I get lots of input although I am not sure how much of it is totally informed vs tradition and so want to solicit more information. For intended use, I am starting tech training, and while I anticipate that most of my dives will remain recreational / light deco, I anticipate expanding to include more challenging dives and a possible move south might also include cave training, although that is still a ways off. I am physically fit but older and need to be aware of weight issues, although I can walk around on land wearing the double 120’s without much discomfort, and I can pick them up alone & put in cars or on benches ok.

    Anyway, (for possible future tanks) everyone seems to favor LP steel tanks. My priorities for tank choice are less weight on land, weight underwater depending on exposure suit needed, and more air underwater. The touted LP 95’s weigh 38.7 lbs on land-empty- and buoyancy-full- underwater is negative 8.3 lbs. They need to be overfilled 10% to reach the 95 cu ft and they aren’t supposed to be overfilled after 5 years old. (Pardon to those who know all this, I’m just clarifying details.) The HP 100’s could be short filled (but usually aren’t) and weigh only 33 lbs empty on land, negative 10 lbs -full- underwater. The HP 120’s have huge gas, weigh 38 lbs empty on land and 11 lbs negative underwater. I like long and skinny tanks as for some reason I need lots of weight “forward” in order to float level.

    As far as insulation and weight needed, much of the time I dive drysuit and with the necessary insulation I still need 35 lbs of weight with a steel 120, and that’s squeezed with an empty BCD at the end. (Please, it’s not an issue of a newer diver -130 dives- that is overweighted; I need only 7 lbs in salt water with an Alum 80 and a 3 mm wetsuit and hood.) Obviously if I were to get the chance to dive doubles in “warm” water say 5 mm I would need air in the BCD but otherwise most of the time the extra weight in the tank means less lead on my body.

    With this info, is it just tradition that speaks so much against the HP tanks or is there some reason that I am missing? Also given that I can’t buy and unlimited number of tanks, if I were to stay with HP tanks, what are thoughts about choosing 120’s as opposed to 100’s?

    Thanks for any input!
  2. Captain12Pk

    Captain12Pk Divemaster

    # of Dives: I'm a Fish!
    Location: Orange, CA
    First of all, at this level you should really consider putting the wetsuit away. It's unlikely that a Tech. instructor would let you train like that anyway.
    But regarding your tanks; if you're comfortable in 120's why would you want to change them? I say as long as you can handle them, dive 'em.
  3. UCFKnightDiver

    UCFKnightDiver ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Florida
    Question, can you get overfills where you are located?

    Also this comment doesn't make sense to me, if you need weight forward, why would you want a long tank?
  4. TSandM

    TSandM Missed and loved by many. Rest in Peace ScubaBoard Supporter

    Cave folks love LP tanks, because with a compliant shop, you can stuff them full of gas. Unlike most tech dives, which at least around here are limited either by the amount of deco you're willing to incur, or how cold you get, most cave dives are simply limited by gas. The more, the merrier . . .

    Around here, HP100s are popular tech diving tanks, for just the reasons you list. We have no problem getting full fills on HP tanks, and the 100s are a pleasant size to move around, and have enough gas for dives in the 150 range with some left over for a second, recreational dive. They ARE more expensive than LP95s, though. But 95s are beasts.
  5. rongoodman

    rongoodman ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Albany, NY
    The OP commented that you're only supposed to be able to overfill the LP tanks by 10% for the first five years, which isn't accurate. You can get the "+" rating renewed at hydro, although you might have to check that the facility you're using is willing to do so and has the information they need.
  6. UCFKnightDiver

    UCFKnightDiver ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Florida
    Just to clarify to the OP what I mean by an overfill, a fill on a LP tank that's anywhere from 3000psi-4200psi most commonly at 3600psi and up, at least in cave country anyway.
  7. DA Aquamaster

    DA Aquamaster Directional Toast ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: NC
    I have both LP 95's and X7-100s.

    If you can't get 3600 psi cave fills then the LP 95 has few if any advantages over an X7-100.

    The X7-100 weighs 33 pounds compared to 38 pounds for a faber LP 95 and 42 pounds for a Worthington LP 95 and 43 pounds for an X8-130. At 3600 psi, a Faber LP 95 holds 129 cu ft, so it is in essence a lightweight 130 - provided you can get it cave filled.

    If you can't get cave fills and need lots of gas, or can get cave fills but can't get a Faber LP 95, then the X8-130 makes the most sense as it is only 1 pound heavier than the Worthington LP 95. And if don't need the gas, th X7-100 is hard to beat as it offers an honest 100 cu ft at basically the same weight as an AL 80 with valve.

    As noted above, as long as the hydro test facility does the extra testing for the plus rating, a 3AA steel the tank can continue to be overfilled. I have seen 70 year old tanks still qualify for the plus rating. Faber stamps the REE number on the tank, which simplifies the process. Special permit tanks like the X series worthington tanks don't get a plus rating, but they also do not figure the volume at a 10% overfill either.

    3600 psi is more or less the cave fill standard, but it does seem to be creeping higher. Personally, given the test pressure of 4000 psi used for a 2400 psi tank, I am not comfortable with much over 3600-3700 psi and 4200 psi is bordering on the insane "crash test dummy" fill range.
  8. ssturges

    ssturges Solo Diver

    # of Dives: 100 - 199
    Location: Portland, OR
    My tech instructor recomended LP because you could a much more accurate mix at low pressures. Some what important when doing things such as Hyperbolic trimix and not having to start over with the mix. In fact he was kind of anti-HP. This was a guy who trains hundreds of tech divers.
  9. UCFKnightDiver

    UCFKnightDiver ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Florida
    I don't know if accuracy is the issue, more so you can have lower pressure O2 and Helium to mix with, though you would still need to have the same amount (volume) of gas regardless, respectively. Also I think the word you were looking for is Hypoxic trimix?
  10. spectrum

    spectrum Dive Bum Wannabe ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: The Atlantic Northeast (Maine)
    I'm not a doubles diver but here are a few observations

    You can find lot's of divers here on the board that have doubled HP cylinders, especially the 100's they are an awesome package. It is a rare (stout ) diver that doubles HP80s since the package is so short they are hard to trim.

    LP cylinders are rated at 2400 PSI but come new with a + rating that permits 10 extra pressure. It's at this 2640 fill that you get the adverted capacity. The + rating may be sought in any future hydro test. It's important to request it in advance, many shops won't volunteer to do the extra work. I recently had a steel 72 from 1965 achieve the + rating but it had not been applied to the 2 interim test dates.

    Once you fill it such that it cools to over 2640 PSI at 70F it's an overfill. What you can get is subject to local practice.

    You may want to do some rudimentary gas planning knowing your consumption rate. It may be that a pair of 100s is good for the vast majority of your future dives. On the few extreme dives you may consider adding a stage. This could be better than lugging overly large cylinders most of the time. You may not really have the training to get to those numbers but they are something a good dive shop person/instructor can walk you through.


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