• Welcome to ScubaBoard


  1. Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

    Benefits of registering include

    • Ability to post and comment on topics and discussions.
    • A Free photo gallery to share your dive photos with the world.
    • You can make this box go away

    Joining is quick and easy. Login or Register now by clicking on the button

Trim weights

Discussion in 'Advanced Scuba Discussions' started by flyboy08, Oct 29, 2016.

  1. stuartv

    stuartv Seeking the Light

    # of Dives: 200 - 499
    Location: Manassas, VA
    8,132
    3,769
    113
    I calculate for an empty tank, so that I can use it ALL if I need to, without fighting to maintain my depth. Why would you carry gas with you that you can't use (because the lighter tank is pulling you to the surface)? If you don't have weight to match it, then it's not "reserve gas", it's ballast.
     
  2. DevonDiver

    DevonDiver N/A

    # of Dives: 5,000 - ∞
    Location: Subic Bay, Philippines
    15,396
    8,159
    113
    Because it's sidemount.

    There's a level of flexibility and problem solving available because we aren't bolted onto our cylinders.

    You forsee a low on air problem. Maintain some gas in 1 tank. Breath the other tank down to zero. Pass off, or float, that empty tank. Eliminate instantly a few lbs of positive buoyancy. Breath down the second tank at your leisure.

    I do this sometimes just for the heck of it. Pop a DSMB, remove and tidy hoses... and float cylinder/s up the line to the surface. Finish the dive single tank.

    Besides which, we're talking of a couple lbs, at most, 500psi versus empty. This is why we learn to use our lungs for buoyancy control. Our lungs can compensate a few lbs +/- easy.

    That's, of course, for 500psi reserves.... not 1/3rd tech reserves. But even for tech, I'll go with 500psi for weighting.

    How does a recreational sidemount diver even drain both tanks to empty on a no-stop dive? Really?

    Its just not an issue that demands more lead in the harness.
     
    Last edited: Oct 31, 2016
  3. Basking Ridge Diver

    Basking Ridge Diver Solo Diver

    # of Dives: 200 - 499
    Location: New Jersey
    1,898
    922
    113
    Although you mentioned you are in a wet suit. In a dry suit - I had been taught to split the BCD/Wing inflator and dry suit inflator on separate tanks. I am sure you could put them all on one tank but you are aborting that dive if you lose all your gas on that tank. That said it should not be an issue on the ascent as much since you should be dumping air from the dry suit and the BCD/Wing. Once on the surface you best be able to inflate that BCD/Wing manually...
    In my conditions I am not sure floating a tank is an option.
     
  4. DevonDiver

    DevonDiver N/A

    # of Dives: 5,000 - ∞
    Location: Subic Bay, Philippines
    15,396
    8,159
    113
    Maintain gas in the drysuit tank. Oral inflate the wing if necessary.

    In reality, you shouldn't have to float a tank. That's just an option.. a last resort.

    And... needless to say... it's only for aluminium cylinders really. You'd be diving steel in a drysuit, right?

    Breath control will deal with 500psi worth of gas.
     
  5. stuartv

    stuartv Seeking the Light

    # of Dives: 200 - 499
    Location: Manassas, VA
    8,132
    3,769
    113
    I didn't realize we're talking about SM. I looked back at the OP and I thought we were talking about backmount. Recreational BM, actually.

    And you are certainly right. 500 psi in an AL80 is only about 1# difference vs empty. But, I have seen your way of thinking repeated by numerous experienced tech divers. "My plan will leave me with 1200psi in my tanks, so I'm weighting based on having that much gas left, not empty." And, "I know my SAC, I've done this dive before. I know I'm going to finish with 1200psi, so I don't need to carry extra weight to counter balance the 6 # of gas that is still in my double 120s when they are down to 1200psi."

    I've also talked to pure recreational divers who do things the same way. "I always have at least 1000psi left when I get out, so I'm weighting based on that. I don't want to carry the extra 2 pounds I would need to stay neutral with normal breathing if I have to breathe my tank down to nothing while hovering at 10'."

    Personally, if something hit the fan and I end up needing to hang for final deco literally as long as possible then I want to be able to breathe normally and hold comfortably at 10' while I'm doing it, right down to the end. And whether I was diving SM or not, my plan would be based on not ditching anything I'm carrying - unless there's just no other way to do things. If carrying 1 or 2 extra pounds ensures that I won't have to ditch any gear under any foreseeable contingencies, I'll take the extra 1 or 2 pounds.

    I definitely don't "get" the attitude that you have a plan to finish with, say, 1200psi so you weight yourself based on ending with 1200psi. In that case, why even bother taking the 1200 psi with you? If you're going to make assumptions about your dive based on "knowing" that you'll finish with 1200psi, what else are you compromising besides your weighting?
     
    packrat12 likes this.
  6. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
    8,013
    5,712
    113
    Where did this come from? First mention of SM, as far as I can see.
     
    DevonDiver likes this.
  7. packrat12

    packrat12 Rebreather Pilot

    # of Dives: 1,000 - 2,499
    Location: Merritt Island
    415
    199
    43
    I am with you here Stuartv. If I can breath the gas, I need to be weighted for the gas. I will plan on my tanks being empty or nearly so. If I have a couple of pounds extra, so be it. I watched a guy doing his deco upside down this month. I would call that underweighted! He and his buddy had run out of all of their gas and was using my BOs to deco on! Not my plan but at one point I had no ballast from my BOs.

    It is fine to pride one's self of being well weighted... It is another to not plan for the worst case, e.g. no gas. I am not about to throw away a cylinder and regulator for the sake of 1 - 2 lbs of lead extra to be carried normally.
     
    stuartv likes this.
  8. DevonDiver

    DevonDiver N/A

    # of Dives: 5,000 - ∞
    Location: Subic Bay, Philippines
    15,396
    8,159
    113
    My bad. That's a hilarious gaff on my behalf. Too many tabs open on my phone.

    Please disregard everything I've said for about 2 pages LMAO
     
    flyboy08 likes this.
  9. DevonDiver

    DevonDiver N/A

    # of Dives: 5,000 - ∞
    Location: Subic Bay, Philippines
    15,396
    8,159
    113
    People are talking about a couple lbs of lead, as if.... without it.... You'd be unable to arrest some sort of ballistic ascent.

    Its well within lung compensation parameters. The buoyancy comes on slowly. Chances are you'd even barely notice it....providing, of course, you had a tiny modicum of innate buoyancy control.

    Technical weight checks are based on minimum gas, not zero gas (... holding final shallow stop with no air in BCD). Finishing the dive neutral. The concept being that you'd never need more weight than that.

    In tech.... You're likely to have multiple gasses in multiple cylinders. Total the number of non-related failures you'd need to experience for ALL your cylinders to be empty.. it's a lot. Actually....the chance of that outcome is beyond reasonably likely.

    In recreational diving.. a single cylinder.... there is potential for gas to deplete. But that'd HAVE to coincide with large emergency deco for the situation to get critical. And you'd HAVE to have no team/buddy support. (and no personal capacity to cope with a couple lbs positive through breathing/buoyancy control).

    That's also a string of worst case scenarios hobbled together....a solitary no-stop diver who somehow needs to desperately hold a stop until the very last gramme of gas is sucked out of their tank.

    If that scenario happened, would their weighting really be their biggest failure? :wink:

    To me, weighting for empty tanks seems like a part-understanding of tech concepts being incorrectly applied in a recreational context... leading to over-complication and sub-optimal results (too much weight carried).

    Recreational divers can benefit a lot from adopting tech diving practices, but sometimes things get wrongly interpreted or applied.
     
  10. spectrum

    spectrum Dive Bum Wannabe ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: The Atlantic Northeast (Maine)
    11,376
    817
    113
    Change with purpose....

    You need what you need so set yourself up with some smaller weights adding up to your exiting load. At the end of a future dive when you are wet, your tank is down low and you are about 10 feet of water with an empty BC see what you can live without to stay down. That's what you need.

    With the total known, do you need trim weights? The most common need is droopy feet. Moving 4-6 pounds up higher on your back can help a lot. Sometimes it's not about weight, your cylinder height may have room to move up for instance. Picture yourself as a seesaw and move things around until you have balance.

    More here.


    Pee
     

Share This Page