DIR- GUE Balanced rig with a thick wetsuit - mathematically impossible?

[overthinking_diver]

This is why dual bladder wings exist.

Not in this subforum, they don't

Before people yell at you for suggesting this in the DIR forum - the whole point of this balancing act exercise is to avoid the need for one. Dual bladders present their own problems so avoiding them by balancing your rig is clearly better. I'm sure you know this. By your profile picture it looks like you know 100x more about diving than I do.

 

[Celt]

There’s a lot of misinformation about wetsuits. A wetsuit of high density neoprene 7mm long John + 5mm hood attached jacket does not lose its buoyancy. Used them for years with twin steel tanks and no bcd deep. My new MTM Elios Sub also does not lose its buoyancy. You need the right neoprene for scuba.

 

[Beau Holden]

Yes, you can't have a balanced rig in a 7mm wetsuit as covered in Fundamentals. As my instructor said, if you need more than a 3mm wetsuit dive dry.

reason I only two suits dry and 3mm. Have not been in the 3mm in over 7 years.

 

[ELCAM]

I am considering it. It's baked in into my weight_we_can_swim_up and positive_buoyancy_at_10_feet_we_can_handle numbers.




I thought a drysuit is recommended but not required. The standards just say "Exposure suit appropriate for the duration of exposure". In any case, I'm getting one mostly for this reason. I think GUE has ruined wetsuit diving for me.

It is if you’re seeking a tech pass. Otherwise a Rec pass is the best you can hope for.

 

[Ayisha]

It is if you’re seeking a tech pass. Otherwise a Rec pass is the best you can hope for.

The above is not true.

I thought a drysuit is recommended but not required. The standards just say "Exposure suit appropriate for the duration of exposure".

That is correct.

 

[Angelo Farina]

I think balancing a rig according to the GUE definition with a thick wetsuit is mathematically impossible regardless of tank(s) used and weight distribution.

I'm using the numbers from my GUE materials - a wetsuit gives you +22 lbs of buoyancy at 10 feet and just 6lbs at 100ft. I also assume the gas in an AL80 cylinder weighs 6lbs.

One condition for a balanced rig is being able to hold a 10ft stop with near empty tanks. Let's make our diver neutral at 10ft with empty tanks. His wetsuit gives him +22lbs so he needs 22lbs of weight to counteract that. This includes the weight of the tank(s), backplate, regulators, light, fins, lead. Notably, this does not include the weight of the gas in the tank(s).

Let's check our buoyancy at 100 feet with full tanks. We'll be negative of course, but how negative? We lose 22-6 = 16 lbs of buoyancy from the wetsuit. We also need to add 6lbs for the weight of the gas, assuming we're diving a single AL80. If we're diving more/bigger tanks, the math gets worse. So overall, we're at least 22lbs negative at 100 feet.

We can't swim 22lbs up so we need to drop some weights. The problem is we're neutral at 10feet so any weight we drop will make us positive. So if we drop 10lbs of weight, we'll be 10lbs positive at 10 feet and we can't hold our last stop.

This gives a hand-wavy proof of a more general necessary condition for a balanced rig (I won't bore you with the full proof but the idea is the same):

Where:

• buoancy_loss_from_10_to_100_feet (lbs) - how much buoyancy our exposure protection loses at depth
• weight_of_gas_in_tanks (lbs) - total weight of gas in our tanks
• weight_we_can_swim_up (lbs) - amount of negative buoyancy we can swim up from 100 feet with a failed wing. Note: this is different from amount of lead weight we can swim up. This means we're neutral at 100 feet and we add weight_we_can_swim_up lbs more.
• positive_buoyancy_at_10_feet_we_can_handle (lbs) - how much positive buoyancy we can handle at 10ft and still hold a somewhat comfortable stop.

Am I missing something? Is my math wrong?

Your math can be improved.
First, when you are at 3m for the last deco stop, you are at an absolute pressure of 1.3 bar. So your wet suit is already significantly compressed.
If it is true that it contains 10 liters of gas (22 lb buoyancy), at 3m depth they will become 22/1.3=16.9 lb of buoyancy.
At 100 feet the pressure is 4 bars, so the wet suit buoyancy will be 22/4=5.5 lb. You have lost just 16.9-5.5=11.4 lb.
Regarding the weight of air in the tank: an AL80 is roughly 11 liters at 200 bar, so it contains 2200 liters of air. With a density of 1.2 kg/m3, it weights 2.64 kg. If you dive according to the rules, you will do your last deco stop with roughly 80 bar in your tank, so the weight will be 11×80×1.2/1000=1.05 kg. So you have lost just 2.64-1.05=1.59 kg (3 lb).
So at 30m with almost full tank you will be negative by 11.4+3=14.4 lb.
You can compensate partly this by filling your longs, which provides something as 6 lb of additional buoyancy.
So your fins must counteract a negative buoyancy of 9 lb, which is quite feasible if you have decent fins and use a powerful double-effect flutter kick.
So a balanced rig is possible if using just an AL80 and limiting depth to 30m.
Of course with a twin tank at 50m you will become too much negative, so you should carry an additional buoyancy device, if your BCD fails.
Here in Italy we employ just a plastic shopper bag stored in one of your pockets (this was the approach introduced by Bucher in the sixties).
Said that, I always have a couple of shopper bags with me, wathever the rig, the depth and the suit...

 

[mariosx]

This is why dual bladder wings exist.

If I am not very wrong, dual bladder wings is against standards.

 

[Lorenzoid]

I thought a drysuit is recommended but not required. The standards just say "Exposure suit appropriate for the duration of exposure". In any case, I'm getting one mostly for this reason. I think GUE has ruined wetsuit diving for me.

I think GUE ruined wetsuit diving for me, too. But in a good way.

I’m diving a 3mm wetsuit today because the water is 84F. But I kinda miss my drysuit.

 

[jadairiii]

We can't swim 22lbs up so we need to drop some weights. The problem is we're neutral at 10feet so any weight we drop will make us positive. So if we drop 10lbs of weight, we'll be 10lbs positive at 10 feet and we can't hold our last stop.

Am I missing something? Is my math wrong?

This is an example of creating a problem that you should never encounter (if you are really diving DIR) in the real world.

To balance the rig, you have drop weights. No problem, right? Now your concern is that if you drop your weights you wont be able to hold at 10'. For this to happen, it means you violated all of the GUE minimum gas rules for the dive. You are on the bottom with near empty tanks when your wing fails. Why are you on the bottom with near empty tanks?

If your wing fails at minimum gas turn pressure, you should still be in pretty good shape to hold at 10'. If this is a recreational dive in 40', who cares, just go to the freaking surface and blow off your safety stop.

You can always create a scenario that the answer is "double wing", but there is a good chance you violated about 5 other rules before the double wing becomes an option.

 

[overthinking_diver]

This is an example of creating a problem that you should never encounter (if you are really diving DIR) in the real world.

For this to happen, it means you violated all of the GUE minimum gas rules for the dive. You are on the bottom with near empty tanks when your wing fails. Why are you on the bottom with near empty tanks?

I'm just following the definition of a balanced rig. It says you should be "able to hold shallowest stop with nearly empty tanks". It doesn't say "able to hold shallowest stop with minimum gas in your tanks" or "...with amount of gas you'll normally have at the end of the dive unless you do something dumb".

Maybe it is overly conservative (a lot of GUE dive planning is) and being "able to hold shallowest stop with minimum gas in your tanks" is enough. I don't know, I'm just trying to follow the standards as they're written.