Double LP85's or HP100 - Diving Wet-Steel Backplate

[tbone1004]

why does fresh vs salt matter? If you are diving a balanced rig, it is balanced regardless of the density of the medium you are immersed in. Everything has a buoyancy adjustment when you get into fresh or salt water, the difference for us that are typically cave diving, and especially in warm water, is very few of us can remember the last time we actually dove a balanced rig.

Now to your point, if you don't require additional ballast in salt water, then you are no longer diving a balanced rig which changes things, but in cold water it is quite possible that you could be in a balanced rig and if in OW, you should be able to kick up the rig. I'd still take a 50lb lift bag with me, but I dive a 5mm wetsuit with kydex plate and HP120's *independents without a manifold and with plastic cam bands instead of steel bands so it is about 6lbs less negative than a typicaly set of doubles* on a fairly regular basis and I can kick them up with no problem in the event of a wing failure.

 

[kensuf]

The buoyancy will vary between fresh and salt water. A set of double 85s with an aluminum back-plate might be close enough to a balanced rig in salt water, but nowhere near close enough in fresh.

A 180 pound diver, plus full gear is probably going to weigh close to 320 pounds dry weight. For the sake of argument, let's say that diver is 5 to 7 pounds negative in salt water with LP85s and an aluminum back-plate. Put the same diver with the same rig in fresh water and he's now 15 pounds negative without any changes. Personally, I wouldn't sweat being 5 pounds negative. 15 and we've got a problem.

 

[Scared Silly]

I agree with what others have written in that you will be overweighed with steel cylinders and a steel backplate and require some form of redundant buoyancy (second wing or a lift bag/SMB). A dry suit becomes one's redundant buoyancy.

Remember your wing has two functions - to float the rig at the surface without you in it and to compensate for the loss in buoyancy at depth.

That said T-bone why did you add in ditchable weight to compensate for the wetsuit? If one were to dive a rig as the OP proposed there would be no need for ditchable weight. Which there in lies the problem - at depth the wetsuit would have zero effective buoyancy and a wing failure would result being -40 lbs buoyancy. Which would not be a reasonable amount to swim up.

Here are the same numbers with aluminum cylinders.

Bands -3 lbs
Manifold/valves -6 lbs
Aluminum Backplate -3 lbs
Regulators -4 lbs
Weight of gas -12 lbs
Buoyancy of AL80 tanks +8 lbs

You are at a minimum of -20 lbs buoyant. Now figure in your wetsuit say which say it takes 20 lbs to sink. At the beginning of the dive everything is balanced. But at the end of the dive you need to compensate for the gas so you need 12 lbs of ditchable weight.

Now at depth the wet suit has no buoyancy so you are -32 lbs buoyant. With a wing failure it would not be reasonable to swim that up but you have 12 lbs of ditchable weight so the effective buoyancy is -20 lbs which is getting to be a reasonable amount to swim up.

 

[macado]

@macado where are you getting those numbers from? That is only true with the Worthington LP85's against normal HP100's or the newest FX100's *which are closer to 120's than 100's* from Faber against their LP85's. "normal" HP100's are 2" shorter and bigger around than LP85's, but anyway, that's neither here nor there.

Updated my post above. Sorry I quickly spoke without looking at some updated specifications. I was basing my information off Worthington LP85s. I still maintain they're at least similar diameter (7.25 inch vs. 7 inches), 2 inch height difference not huge. I still admit that I've always found LP85s trimmed out very similar to HP100s

In any case, I think a lot of posters agree that LP85s may still be a better choice than HP100s if you're only going to be diving wet. Neither are optimal.

I will fully admit to diving double HP100s in a wetsuit on occasion but I always hated how negative I was. I'm still waiting for the scuba police to issue me my citation.. These were shallow dives in comparison and I always carried a lift bag. The reality is people do it every day. It doesn't make it right or a best practice.

 

[NAM001]

With a wetsuit, I'd go with double AL80's. Diving steel tanks with a wetsuit can open yourself up for a bunch of issues if you lose your wing.

I second that . I have lived it and it aint pretty if it catches you off guard. Really not good if you don't have a hard bottom.

 

[stuartv]

I don't understand the need for all the arithmetic on weight of bands, manifolds, regs, etc..

IF the diver is weighted properly, then when he gets to the bottom at the start of his dive, he will be negative by the weight of his gas plus the buoyancy lost from his wetsuit.

Tbone threw out 16# as an estimate for the weight of the gas and 20# for the positive buoyancy of a 7mm suit.

Fine.

So, if he loses 80% of suit buoyancy at depth, he'll be 16 + 20*0.8 = 32# negative. If he pulls the elbow off his wing inflator, he's probably not going to swim that up. But, for that matter, in the same situation in a 3mm suit, even if he's only 16# negative, he still might not be able to swim it up.

That seems to be the reason not to dive any doubles in a 7mm suit (or any wet suit) unless you've got redundant buoyancy. It doesn't matter whether the BP is steel or aluminum, and the tanks are 85s or 100s or AL80s. Those choices would potentially just make you even more negative. But, does it matter if you're 32# negative or 52? You're not swimming it up, either way. You could have a Kydex BP and AL80 doubles. With proper weighting, you'll still be on the bottom at the start of the dive, at least 32# negative (using the previous example).

And if you do have redundant buoyancy (presuming enough lift), then, again, does it matter? Obviously, being overweighted has its problems. But, I'm talking about the difference between AL BP+AL80 doubles vs SS BP + HP100 doubles. If you have adequate redundant buoyancy, is the difference between those two options (using a 7mm wetsuit) the difference between safe (enough) and (too) dangerous?

If you don't have redundant buoyancy, neither one is safe. If you do have adequate redundant buoyancy, then it seems to me you could use either one. No? What am I missing?

 

[Scared Silly]

I don't understand the need for all the arithmetic on weight of bands, manifolds, regs, etc..

If you don't have redundant buoyancy, neither one is safe. If you do have adequate redundant buoyancy, then it seems to me you could use either one. No? What am I missing?

What you are missing is that with proper weighting using AL80s one will probably have ditchable weight. Which when Davy Jones comes a calling may allow one to drop the weight and be able swim their rig up whilst not relying on a redundant system. That may be important if one has a deco obligation.

 

[stuartv]

What you are missing is that with proper weighting using AL80s one will probably have ditchable weight. Which when Davy Jones comes a calling may allow one to drop the weight and be able swim their rig up whilst not relying on a redundant system. That may be important if one has a deco obligation.

I considered ditchable weight. I left it out for several reasons.

One, numerous very experienced people here, whom I quite respect, have said many times that you should never need to ditch weight.

Two, if your suit loses 16# of buoyancy at depth and you're 32# negative, how much weight are you going to ditch when your wing has totally failed and you have to swim yourself up? And how are you going to keep from corking to the surface once you get 2/3 of the way to the surface and your suit has regained a bunch of buoyancy? Remember, your wing failed, so you're not using it to go up, so you don't have any air in it that you can dump to keep yourself down.

Three, weight is expensive. I prefer to plan so that I won't have to ditch weight, no matter what. I mean, it's not unobtainium expensive. But, it's not cheap and it seems easy to plan properly so that you never have to ditch it, even if the shtuff hits the fan.

If one has a deco obligation, that is all the more reason to not ditch weight. However, you're the most negative at the very beginning of the dive - when you're not likely to have any deco obligation. You'll be at least somewhat less negative before you have a deco obligation. But, that's neither here nor there. The issue is what happens in the worst case - total wing failure at the very start, when you first get to depth. Can you get off the bottom? Even with AL80s, is there an amount of weight that you can ditch that is enough to let you swim up, but little enough that you can avoid corking when you get shallower?

 

[Scared Silly]

Even with AL80s, is there an amount of weight that you can ditch that is enough to let you swim up, but little enough that you can avoid corking when you get shallower?

Yes, the exercise is left to the reader but is diver specific .

That said, I have done dives where with in the first ten minutes I have deco obligation yet plenty of gas. As for the need and cost of weight, it is negligible when comparing it to the cost of a chamber ride or permanently swimming with the fish.

 

[Lorenzoid]

Define "corking"? You wouldn't be able to hold a safety stop, but you might not shoot up like the proverbial Polaris missile either. I'm sure there is some increased risk from the faster ascent as one nears the surface. You could try to flare your body to increase drag. It seems to me that people who choose to dive in wetsuits, double Al 80s, and some ditchable weight are accepting a little more risk than if they were diving with a single tank, but not as much as with steel doubles. I don't dive doubles with a wetsuit and haven't done the arithmetic, but from what I have read the conventional wisdom seems to be that double Al 80s in a wetsuit is a safe enough configuration.

I, too, have heard it asserted that if properly weighted "you should never need to ditch weight" (during a dive). Maybe diving doubles in a wetsuit is outside the scope of that assertion?