wet suit & steel doubles who dive with that kit

[TSandM]

The underlying concept is balanced rig, which means that, in the event of a wing failure, you can get to and remain on the surface. A thin wetsuit with a set of Al80s, or neutral steels like the little 72s, will work, because the rig has little negative buoyancy to begin with, not much gas weight, and the suit can lose very little buoyancy.

On the other hand, a set of steel 100s and a 7 mil wetsuit can be a mess, because the wetsuit may be able to lose 20 lbs or more of buoyancy, and the rig itself is quite negative from its materials, as well as having significant gas weight.

Risk also goes up as dives get deeper, meaning that having immediate redundant buoyancy becomes more valuable. Thus, very deep dives, even in warm water, might merit consideration of a dry suit, even if they are being done with aluminum tanks and stages.

The original discussions I read, years ago, about balanced rigs, did discuss the possibility of using a lift bag or SMB as redundant buoyancy, but I believe that whole idea has gone by the wayside, as it probably ought.

 

[lucca brassi]

In a DIR context, the above statement is incorrect. Here's why.

the issue is serious for any diving below about 30ft because of the amount of buoyancy the wetsuit loses, has absolutely nothing to do with warmth in a DIR context. You try to dive a balanced rig, you choose to dive wet which is a problem with steel bottles because it is difficult to get to a balanced rig with said bottles because now none of you weight is ditchable in the event of a wing failure and you have the potential to be very negative at the bottom to the point that you can't kick the rig to the surface. This is bad.

In open water, if you're diving big steel doubles you NEED to have redundant buoyancy, whether from a drysuit, or lift bag. I'm not familiar enough with euro style cylinders, but your twinset is already offsetting about 9kg/19lbs of lead with them empty. This is not factoring in the bands, manifold, first stages, which are easily another 3kg, so you're really at 12kg/26lbs. This is with empty tanks. You mention 2kg are on drop weights, so if you feel that you can kick 10kg/22lbs of weight up from the bottom when your wing fails, then fine. I can barely do it, and it requires a LOT of effort including continuous effort at the surface. If you feel that you can safely do that, then fine. This is not including air in the cylinders, which is going to be another 7kg/16lbs if they are twin 100's which are about as small as bottles realistically get. So now you have a worst case scenario of a wing failure at depth, you ditch your drop weights, and have to swim 17kg/38lbs up to the surface. I don't know a single person that can do that and I've worked with Navy EOD divers and SEAL's, and USCG Rescue Swimmers. I don't know a single one of them that can maintain 38lbs of thrust out of their legs, some can give it in a burst, but not maintain it.

Situation above is your situation, this scenario if you choose to dive wet will require a 25kg/50lb lift bag minimum to be safe *this is not "DIR" because it still points to an unbalanced rig*. If you dive dry, you have constant buoyancy, so at depth in a suit failure you'd still have the wing, though for a drysuit to hold 0 air is quite difficult, and in a wing failure, you would at least have the drysuit offsetting a significant amount of weight with the ability to realistically take on the rest of the weight from the wing in an emergency. It will be unwieldy, but you'll get to the surface. Now, if you were diving aluminums in this case, you'd have a total of 7kg/15lbs required of extra ditchable weight from your current scenario, less the 3kg from your bottles, so net extra 4kg. So now you're at 16kg total ballast, but 9kg of it is ditchable, so now instead of having to swim up to the 12kg best case scenario, you now only have to swim up 7kg/15lbs. Many agencies require students in a bathing suit to acquire a 10lb diving brick and keep it at the surface, so 15lbs with fins is easy. This is why to dive steel bottles you need redundant buoyancy, easiest form is a drysuit. Sometimes you have to work around if you're diving in warm water, drysuits aren't accessible for whatever reason but you need the gas volume, in that case you have to carry a lift bag if you're adamant about using a wetsuit with those tanks.

all units in kg ... but agree

 

[leviathann]

What about alu backplate and 2x8.5 liter or 2x7 liter and a 7mil wetsuit?
Is it correct you should be less negative with this?
Would you dive with that kind of configuration with regard to buoyancy.

 

[lucca brassi]

Hi

Double 8,5 l a just more negative buoyant when empty



I have dive also , 8,5 l in our LDS pool , they are superslim almost unnoticeable but because of team and also filling places I choose steel 12l doubles

My wetsuit is from 2 parts and is in the middle 10mm . Actual i have around 12 kilo in ''bioprene '' so i dive very comfort with these tanks in shallow water cca 10- 15m

 

[mlevsky]

That calculator is neat, what's the URL for it?

 

[lucca brassi]

Scuba tank size and buoyancy calculator it is accurate special to new bottom flat eurocylinders 8,5l , 12l

another one :

UK Scuba - Cylinder Buoyancy Calculation