Twin Set

[Lost Yooper]

It can actually come down to the thickness of the suit being used. It's only a rule of thumb that most have no idea about, and is why I/we try to bring it up once in while.

Good post Roak.

Mike

 

[devjr]

Yeah, it was a good post and can be used to figure average buoyancy. I have to tell you though, the buoyancy changes of a set of double 70's is almost unnoticeable in salt water. In fresh water, a diver clad in only swim trunks and wearing a set of doubles cannot float on the surface without swimming a bit. Otherwise, the diver slowly sinks. In salt water, the doubles are unobtrusive except for the drag. I've personally never noticed any particular buoyancy change. Swimming up or down is not even an issue regardless of the swing weight.

I've had the same experience with a single HP100 steel(PST). I often swim these in the tropics with no exposure suit. Moving about the reefs the tank feels light, very light. I usually wear a couple of pounds of lead just to keep my body level in a current.

I've even used my old 3000 psi PST tanks without a suit. These tanks look like 70's but are about 10 pounds negative when empty. Obviously, they require some inflation of the BC. Since there are frequent depth changes on most reefs, the bother of venting and injecting air becomes an issue. However, even here, I don't see a problem swimming up in a pinch, at least not if the diver has both fins on.

There is nothing unusual about my natural buoyancy so that should not enter into this. As mentioned in roak's post, the only real consideration is the trimming of buoyancy, eg the weight belt. Weighting for post dive neutral buoyancy at 10 feet is one technique and useful for deco divers. However, it's also possible to go in underweighted, swim down and achieve slight negative trim on the bottom. This is comfortable and doesn't require nearly as much fiddling with the BC when going deeper on the reef or over the sides of a wreck, or returning to the surface. Going back up is doubly easy. The downside occurs if you miss the anchor line. There is no way to do a "safety" stop when buoyant unless a Jersey up line is used. However, most modern computers are so conservative, that may not be needed on a non deco dive. In any case, the diver using doubles should plan ahead for a possible stop.

 

[newton]

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?

 

[100days-a-year]

newton,the point usually brought up is the lower swing weight inherent in a drysuit(shell or compressed neoprene)but your analysis is pretty good.

 

[Lost Yooper]

The only thing that is critical in this discussion is the amount of negative weight at the beginning of the dive. Roakey outlined this very well. A full steel 80 is about -7lbs (-14lbs for doubles) compared to double 80's that are -4lbs full.

You need just enough ditchable weight so you swim up in the event of an emergency at the beginning of a dive. This isn't supposed to be your first option in getting up, but has to an option.

Mike

 

[budgy]

Hi Honkeye,
What kind of sport diving needs twins?
My average dive is around 20 meters and in our cold water is about 30 min long, I don't normally stay longer than 40 min. A 15 litre 232bar on a 20 metre shore dive should last you about the maximum limit for non deco diving on buhlmann air tables, or two 30 min dives
I think a 15 is heavy enough to pick up, and wading in or out in a swell, it can be very difficult to retain your balance. I think twins will be worse. From a boat a pair of twins may be easier, you wont carrry them far and the boat man will pull them in. I see divers using twins on the same dives as I use a 12 litre and I wonder why.
If you just want the redundancy of another reg and cylinder wouldn't you be better off with a pony?

 

[Lost Yooper]

IMO, doubles, such as twin 50's or 63's, are a blast to dive with. They are much more comfortable than singles, and the weight is very managable. Double 72's can be nice for two tank recreational dives too. I don't think doubles necessarily have to be a tech diver thing.

Mike

 

[trymixdiver]

I switched to doubles just to get more dive time. I started out just beach diving with them for longer dives cause i loved stayng down as long as i could. It all led to tech diving for me, not my original plan either, just happened. Now i iuse twin OMS 104 and im happy with them.

I wish you safe diving weather its tech or sport