If you can't swim up 22 lbs when empty, what do you think the chances are that you're going to be able to swim up ~35 lbs?
No, these are full numbers.
Let me do 'em again....
From my "single tank" HP dives, I KNOW what I need. I need 2lbs on a weight belt with a SS BP + STA, single HP 100 or 120, and a 3 mil wetsuit and booties. Without the 2lbs, I cannot stay down at the surface with a nearly empty tank - even on a full exhale. With it, I am neutral at the surface with 300 psi in the tank, and can control my position in the water with nothing other than breathing mechanics.
Now I KNOW that the tank is -1.5 empty, as I dove with 6lbs with an AL80 on the same rig, and the AL80 is +4 empty. That's what's published, and its about right. Both of those numbers are with valves (which are about a pound each.)
Nitrox weighs about 1lb for every 13cf.
The plate weighs 6lbs, and the STA weighs 2 (measured). The harness is effectively nil, as it is very close to completely neutral.
I am (personally) about +1 or so. I am very close to neutral in fresh water; I can sink with a full exhale in the pool.
The exposure protection (3 mil full suit with booties) thus must be roughly +10 at the surface. (-1.5 - 6 - 2 - 2 + 1) = -10.5, which means the suit must be +10, give or take 1.
Ok.
The twinset in question is twin HP100s, and so is -3 for the tanks. The two valves .vs. the manifold is probably a push. The bands and the STA are pretty much a push. This bears out, in that when I did the weight check with the twinset and my exposure protection, with the tanks intentionally filled to only 500 psi, with no gas in the wing I was just barely negative (maybe -0.5) at the surface. So, within my measurement tolerance, removing the 2lb weight belt was an almost perfect adjustment.
Now, with 200cf of Nitrox in the tanks, I have 15lbs (roughly) of gas in there. If we assume my exposure protection loses 75% of its buoyancy at 4 atm, and very little thereafter (probably accurate or close enough for this purpose) then my
maximum negative buoyancy is -22.
My boat's anchor is -22 (FX-37). I CAN swim it up, but its not much fun. I've done it to move it off a wreck when I didn't have a bag with me (yeah, I know, silly me), and was successful without using the wing as a lift bag (which I don't do as I KNOW that's grossly unsafe.) But it was WORK. And actually, it is somwhat worse than -22, since it has chain attached to it, and I have no idea how much THAT weighs; all of it wasn't in free water (some on the wreck), but some was, and it definitely counts!
Chain is HEAVY!
So, assuming the tanks are CHOCK FULL, I can swim up the twinset from the bottom with a TOTAL buoyancy failure. I do not know exactly how much margin I have on that; it depends on how much the chain contributes to my experience with the anchor. I will, however, find out this weekend by simply trying to swim the kit up with a dumped wing from the bottom and see if I can make headway, and if I believe I could make it from 100' down, assuming no shortage of gas (if the tanks are more empty, its much easier of course.)
If the tanks are EMPTY, I should be negative ONLY the loss of buoyancy from my exposure protection. I can EASILY swim that up; that's no different than a single steel or even a single aluminum - you're gonna be negative your exposure protection's loss in any event.
Barely isn't good enough, so I need alternatives. But ditchable weight isn't necessary a good alternative. The problem isn't the tanks, its the weight of the gas and the buoyancy shift of the exposure protection, neither of which I can avoid except by carrying less of either, or getting rid of the shift in exposure protection buoyancy (diving dry.)
Ditching weight at the bottom is only reasonable IF you know you will the weight of gas to offset it when you get up in the water column - otherwise its very dangerous.
That means you must be able to make a prospective calculation on your gas consumption rate before you dump the belt, but your consumption is going to be radically higher at that point due to the brown-water syndrome. Get it wrong and you do the missile imitation. I think the odds of being right on this one suck.
As for why dive a twinset at all for these exposures, the answer is convenience as much as anything else. I've been doing some "two dives on a single HP120" stuff recently, and really like it. No screwing with the tank between dives, etc. There is also another advantage - I get much, much better overall runtimes.
Here's why that works:
Let's say you took a single HP100 with a 19cf pony. You go down to 130. You are limited not necessarily by your gas consumption, but by your safety factor. IF your primary tank/reg fails, you must be able to make the ascent AND ANY REQUIRED DECO on the pony. I can make the ascent on the 19, but deco? Only if its a small amount! Anything else is grossly unsafe, as if I end up having a problem with the primary reg down (e.g. freeflow, first stage problem, etc) I'm hosed. I have to be on my way to the surface immediately on that pony. Second, if I fill that pony with 50/50 as a deco gas to use starting at 70', and the problem happens at 130, I'm skerewed with no way out of the hole I dug for myself. If I pull the deco bottle down there I am at severe risk of a tox hit, and besides, even if I don't tox I've just blown my dive plan in terms of gas consumption, and may not have enough mix to complete the deco. So, for planned deco this doesn't work, as my deco gas isn't breathable at the bottom and I have no redundant bottom gas.
Second, safety demands that I not push the reserves on that tank. If I am buddy diving, I must leave gas for my buddy as I am his redundancy. Thus, I cannot invade the last 500psi or so of the tank, because its HIS. If he breathes faster than I do (and most of my diving partners breathe at anywhere from half again to twice as much gas as I consume) then the ratios get even worse; a couple of guys I dive with would be unsafe to buddy with if I didn't allow for 1000 psi on an airshare ascent. If I'm solo then my pony is my redundancy, but its ALL I have, so I have to be conservative there too, because I COULD have a failure on the backgas.
If I come back on the boat with that 500 psi, its unusable. If I have three tanks, and do three dives, I have the equivalent of almost half a full fill that I cannot utilize. If I'm buddy diving, I may come back on the boat (depending on my buddy's consumption) with 750 psi or more, simply because I have to allow for a potential ascent sharing air with him, and in that case its even worse.
Now consider the twinset.
Same three dives.
I require only TWO tanks instead of three. On the first two I have lots of extra backgas. If something breaks, I have redundancy; I can turn off the offending post or in the extreme case isolate. I lose my second (and possibly third) dive, but I don't lose my safety.
I can also go all the way to the wall on all EXCEPT the last dive on that twinset without risk, as in THIS case the redundancy is my "next dive"'s gas supply!
Let's say my "rock bottom" reserve is 30% - that is, I want to leave 30cf in the tank for brown water (non-overhead environments here.) This is 90cf between three 100cf tanks! But on a twinset, I leave only 30cf - on the last dive - on the first two, I have godawful amounts of redundant gas. So I have 170cf to breathe.
In most cases, I can do three dives on one twinset, yet I am carrying less gas in total than between three AL80s, and far less than three HP100s. My safety factor is much higher on the first two dives, and identical on the last.
If I decide to do planned deco, and sling a deco bottle, I have redundant backgas and no longer need to fear a failure at depth with a gas supply I can't breathe down there. In fact, I can now do a first dive of the day as a decompression dive, and do another, on the same backgas, as a no-deco dive, provided that I have enough to breathe. While I won't get THREE dives out of a set of doubles doing this, I will likely get two, with the second being a "NDL" dive.
Double HP120s get too heavy for this to work, as the negative buoyancy gets out of control and they are long besides. But the funny thing is that the HP100s, because they are so much shorter than the AL80s, are actually MORE comfortable in the water (at least they were in the bay yesterday!) than either AL80s or a single HP120. The double HP100s are above my butt with the valves right at head level, I can get to everything easily, the isolator is right at my head, I don't bang my head on the tank or valve, and in general its a very comfortable setup.
Yeah, its heavier out of the water. So for the 2 minutes I spend climbing in and out, its heavier. I humped the entire kit from my dock back to my pool yesterday afternoon, about 300' or so, and from where I stood up to the dock (about 200' out) and it wasn't bad. The 20' I have to carry it on the boat deck is a non-issue.
I don't see the downside to diving the twins, but I do see plenty of upside. Besides, if I take 'Mix training, twins are a necessity - may as well get used to them now.
