LostYooper writes:
Well, what's going to happen to you if you're wearing twin steel tanks with a wetsuit and your BC fails at depth? How do you plan on getting up? Don't bother making something up. Ultimately, what you're after is a balanced rig, which basically means that you should be able to swim up your full tanks with no aid from your BC. It ain't gonna happen with double steel tanks and a wetsuit.
<Emphasis added by me>
Double steels and wetsuits are a bad combo.
</Emphasis>
The preferred set up is double AL80's if using a wetsuit. With this combo (cheaper, BTW), you can ditch a weightbelt and swim up with full tanks. There won't be any reason for double bladders or whatever (also keeping things less expensive).
So, all things being equal, the amount of compression on the wet suit during the dive (decreased buoyancy at depth) is the same, and the amount of swing weight is the same (since it's just the amount of air you breathe, which doesn't change with steel vs. al).
So, the only different between double Steel and AL tanks is that steels are by definition heavier at the end of the dive than AL tanks. All this means is that the Steel guy has to carry less weight (ditchable or otherwise) at the beginning of the dive. However, the amount of weight carried should be the same.
I'm confused by the above statement. It would seem to me that regardless of the type of tanks used (steel vs. AL), the amount of swing weight is the same with Steel vs. AL.
Empty Steel 80 = -2 Empty AL 80 = +4
So, assuming the manifolds and bands are the same weight (2 lbs), we have the following setup for each:
(Note, I picked the tanks with the greatest difference in weight above. There are nuetral ALs, and there are nuetral Steels, which would seem to negate the argument somewhat.)
2 Steel 80's = (-2 * 2) + -2 = -6 lbs
2 Al 80's = (+4 * 2) + -2 = +6 lbs
Let's assume that our 3mm wetsuit buoyancy is +5 at the surface, and +1 at depth. (I have *NO* idea if this is even close to correct, but let's go with it.)
Twin AL diver, at 15' SafetyStop w/empty tanks:
* twin Als will be +6 lbs
* 5mm +5
---------
+11 (Not good, can't stay under)
So, this diver needs *AT LEAST* 11 lbs. of weight (on his rig) to have a good safety stop.
Twin Steel diver, at 15' SS using the same setup:
* Twin Steels will be -6
* 5mm +5
------
-1 (OK, just a tad bit overweighted)
Pre-dive now:
* twin Als will be +6 lbs (as before)
* 160 cft AIR = -13 (full tank weight difference)
* weight (for SafetyStop) -11
* 5mm +5
-------
-13 lbs (No worries getting down)
-VS-
* Twin Steels will be -6
* 160 cft = -13
* 5mm +5
--------
-14 lbs (Remember, this guy is -1 at SS, so is 1 lb over)
This is as expected. Note, the amount of weight carried by both divers at the beginning and end of the dive is roughly the same for both kind of setups, so the amount of weight they'd have to 'swim up' from the bottom is also the same.
However, the twin Al dude has the ability to ditch 11 pounds *IF* the weight is setup to be ditchable. This would seem to give an advantage to the AL diver *assuming* his weight was ditchable. However, I would consider ditching 11 pounds at depth to be as *serious* a problem as OAA, since you'd be rocketing towards the surface, making the weight dropping's effect just an easier way to find my body.
That would lead me to the conclusion that *IF* you need additional weight, it's would be better to have something that can be ditched in increments (vs. everything all at once). (IMO, this means that if you carry a weight belt, it shouldn't contain alot of weight for safety reasons.)
In summary it seems than 'wetsuits and twin steels == BAD' vs 'wetsuits and twin AL == BETTER' isn't really the statement to make. It's really an argument of having wetsuits *AND* incrementally ditchable weight, if I'm not mistaken.
So, if you dive wetsuits, you need to have ditchable weight, regardless of whether or not you are using twins, and the reason for the ditchable weight is because of the buoyancy loss of wetsuits at depth. If you don't have ditchable *any* weight, then you'd better be able to swim up your entire kit (including full cylinders) minus any exposure suit buoyancy from depth.
(This implies that it's not just a problem with wetsuits, but this also is something that must be considered in the unlikely chance your drysuit floods. I'll leave that topic for another time.)
Did I miss something in my analysis?