diving double steels

[beche de mer]

I read somewhere that twin steels should never be used without a dry suit. I presume the rationale has something to do with buoyancy.

But why? I have double steels and with my 7 mm wetsuit I still need 7 kg lead. So what's the problem?

 

[Scubaroo]

It's a generalisation of a comment taken out of context, that has somehow become "internet DIR law". The materials of the tanks is irrelevant - it's the buoyancy of the tanks involved that determine if the tanks are an acceptable combination to dive wet.

It's very subjective. Some divers like having ditchable weight, some divers like using channel weights and V weights to eliminate ditchable weight.

 

[Don Burke]

beche de mer once bubbled... I read somewhere that twin steels should never be used without a dry suit. I presume the rationale has something to do with buoyancy.

But why? I have double steels and with my 7 mm wetsuit I still need 7 kg lead. So what's the problem?

You need to be able to solve two problems:

1. You jump off the boat, snag on something, and rip your BC hose to shreds. How do you stay on the surface to get back on the boat?

2. You get the above hose fixed, jump off the boat and immediately upon getting to the bottom, snag on something and rip your BC hose to shreds. How do you get to the surface?

In a wetsuit, the simple answer to both is ditchable weight.

At the start of your dive, you are heavy by the amount of gas in your tanks and at depth, you are heavy by the amount of displacement the compression of your wetsuit has removed.

Most steel tanks don't leave you with enough ditchable weight to take care of the problems, so the general guidance is to only wear steel tanks with a drysuit. It isn't a hard and fast rule.

There are many steel singles that are fine, especially with a jacket type BC. There aren't too many doubles setups that work out. Sometimes you'll need to go with an aluminum backplate to move another four pounds or so to the belt.

 

[DeepScuba]

Deploy your lift bag and reel yourself in, like a fish (Helps to swim up a bit too).

"Climb" the anchor chain until buoyancy returns to a workable level.

Has anyone ever seen this to be a problem in reality?

The thought of ditching too much weight (Weight belt, all or none) is a tad scary if I have an alternative. (The above two methods).

Not too many folks have their weight "split" so they have the option of ditching, say half.

 

[ElectricZombie]

The rule is to always dive a balanced rig, meaning you could swim it up from depth if you had a complete wing failure. Most people can't do this with double steel tanks; I know I can't.

 

[Don Burke]

DeepScuba once bubbled... Deploy your lift bag and reel yourself in, like a fish (Helps to swim up a bit too).

"Climb" the anchor chain until buoyancy returns to a workable level.

Has anyone ever seen this to be a problem in reality?

The thought of ditching too much weight (Weight belt, all or none) is a tad scary if I have an alternative. (The above two methods).

Not too many folks have their weight "split" so they have the option of ditching, say half.

I go with AL80s, a steel backplate and a weightbelt, so yes, I've split the weight, but not in half.

That leaves me enough weight on the belt to drop so I can stay on the surface. If I have a problem early in the dive, coming directly to the surface isn't much of a problem, so dropping the 18 pounds or so on my belt if I really had to wouldn't bother me.

Later in the dive, when I'm concerned about making safety stops, there is less gas to swim up and I would not need to ditch.

I don't do deco stop diving with a wetsuit as I'm not convinced I can do it safely in the ocean.

I do carry a liftbag and reel and it is an option. Being able to get to the anchor line isn't something I count on.

Whenever I see someone with thirty-something pounds on a belt, I wonder if that could be ditched at depth without the Polaris effect.

No, I haven't actually had a wing fail at depth. I have broken a power inflator in half on the boat and I've seen BCs with pretty bad leaks.

 

[Genesis]

As was noted, the real issue is "can you swim that rig up from maximum depth with full tanks?"

If the answer is yes, then its not a problem. If the answer is "no", then the rig is dangerous and if you lose your BC for some reason (e.g. you hole the bladder) it might kill you.

I dive double steels wet and have no problem doing it. With a light (3 mil) wetsuit I use an AL BP to control the weight. With a hooded vest under that 3 mil I wear my steel BP. This is with HP100s or double 72s. Neither requires any weight on a belt for me. I'm comfortable with either configuration. I do "light" deco in a wetsuit in the gulf in this configuration during the summer months - but we have water that exceeds 80F much of the time! If you're diving where water at deco depths are much below 80F you probably want a drysuit for warmth if no other reason when doing decompression diving, as during deco you're not exerting yourself and I get COLD! Cold is bad for both comfort and decompression in general.

The HP100s are the more serious problem, since they hold 15lbs of Nitrox (in gas). The 72s only hold 10lbs of gas, which means there is less to swim up.

LP104s would be WAY too heavy.

BTW, I CAN swim up HP100 doubles with a steel BP, but its damn uncomfortable and the amount of the gas I have in the wing makes the dive uncomfortable as well, as buoyancy becomes hypersensitive to depth change due to the expansion/contraction of that volume of gas.

I consider ditching at depth not to be an acceptable option and take the appropriate gear to avoid the possibility that I might be tempted to do that. For most dives, this means carrying a bag, as that can be looped under your arm to provide a redundant source of lift if necessary. (BTW, the idea of shooting a bag and "climbing it" is, IMHO, a bad one - instead, insert your left arm through the straps of the bag and put some gas in it - this lifts you by the shoulder, puts the dump right where you can get to it for control purposes, and, if you put in just enough to make swimming up easy, not to actually lift you, it avoids the risk of a runaway ascent. Practice this before you need to use it!) However, that does not negate the need to be able to swim the kit, since until you can get things stabilized you still need to be able to avoid an uncontrolled decent into a "bottomless" dive site. Ditching at the surface, of course, IS acceptable, but in that situation I'm prepared to ditch the entire kit and kaboodle (plate, tanks and wing) if it has no buoyancy ability (e.g. the wing is critically damaged.)

 

[Rick Murchison]

As has been mentioned earlier, you must be able to swim to the surface without benefit of your BC.
A word on lift bag usage - if your redundant buoyancy is a lift bag (rather than a dry suit) and you actually have to use it, I recommend that you don't hook your arm through it, but rather clip it off to your BC (if you're gonna dive this way you must have a way to clip off your lift bag). The reason is at the surface. The lift bag will get itself to the surface, but it won't get you to the surface. No matter how you slice this pie, you're going to be a few feet below the surface when you run out of lift, and if you're too negative to swim the rig up and stay there you're still in trouble - so, you must be able to jettison those steels in order to float. In other words, once the lift bag has made it to the surface, you need to get out of the BC and leave it below. If you have the lift bag hooked with your arm, you're most likely going to have to unhook it to get out of the BC (especially if diving a BP/harness with no quick releases), with the distinct possibility of losing the bag and being back in the "rock" condition while trying to get out of the BC.
Having the bag clipped to the BC solves two problems - makes it easy to get out of the BC and float up the last few feet, and, when you do jettison your tanks they're still attached to the lift bag (you did fill up the lift bag all the way when you got to the surface, didn't you? If you didn't then you can wave your rig goodbye when you remove your buoyant self from it).
And remember to keep that airway open during this final bit of ascent.
Rick

 

[Genesis]

The problem with doing that is simply one of being able to release it if you need to.

The "can't swim up" issue usually resolves itself on the way up, unless you are GROSSLY overweighted. As your suit expands, the buoyancy comes back.

This isn't such a big thing with a light wetsuit, but with a heavy one its a REALLY big deal. I have friends that dive 2-piece 7 mil suits here in the winter, and they're wearing 25+lbs of weight to do it. If they were diving a heavy double setup instead of a belt, they might have a real problem down there - but at the surface, with a nearly empty tank, they'd be ok.

Yes, you're a few feet below the surface (like 1 or 2) at the end of this, but your suit is no longer compressed and as such you should be ok. If there is no boat you can get to and board, then you're going to have to ditch the kit. If there is then the problem is pretty much solved.

 

[Rick Murchison]

With a set of 95's or 104's ranging as high as 30 pounds negative when full, wearing no wetsuit or a thin wetsuit makes matters worse. There is no jettisonable weight, and without help from a BC or lift bag the average diver *must* separate from the tanks to stay on the surface.
Rick