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

[overthinking_diver]

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.

The 22lbs number I used in the OP is from the GUE materials. They claim a wetsuit has +22lbs of buoyancy at 10ft, not at the surface. I don't know if these numbers are right, I just took them at face value.

If you dive according to the rules, you will do your last deco stop with roughly 80 bar in your tank

Again, the definition for balanced rig I'm using says "nearly empty tanks for the last stop".

But in practice both you and @jadairiii are right - a simple recreational dive with a single AL80 seems possible as long as 1) you're perfectly weighted for the end of the dive with ~1000 PSI at 10ft and 2) you use very conservative gas planning.

I am still convinced that balancing a rig according to the GUE definition is impossible but I guess you can bend the rules a bit for simpler dives.

 

[tursiops]

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...

No improvements to this system after some 60 years? Remarkable.

 

[JoeTPhilly]

Hmm, based on some of the comments I think a video would help.

 

[BlueTrin]

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.

You should order one of these:

 

[JoeTPhilly]

You should order one of these:

View attachment 794983

Haha, is there enough room for a pee valve?...

 

[Angelo Farina]

No improvements to this system after some 60 years? Remarkable.

Bucher did show how to dive in air at more than 100m without BCD...
After decades of practice of this aspproach done by coral hunters, everyone here learned that a stupid plastic bag provides a life-saving buoyancy help subdstantially with no cost and no space - weight - problems.
So I find advisable to always have a plastic bag in my pocket.
It can also be useful for carrying something you have found on the bottom or for many other purposes...

 

[tursiops]

Bucher did show how to dive in air at more than 100m without BCD...
After decades of practice of this aspproach done by coral hunters, everyone here learned that a stupid plastic bag provides a life-saving buoyancy help subdstantially with no cost and no space - weight - problems.
So I find advisable to always have a plastic bag in my pocket.
It can also be useful for carrying something you have found on the bottom or for many other purposes...

I often have a plastic bag in my pocket too, but only because I found it drifting on the reef as garbage.
I'd rather pick stuff up into a mesh bag.

 

[Ayisha]

...I am still convinced that balancing a rig according to the GUE definition is impossible...

I believe that when you do the calculation, it is meant to show you that if you are using a wetsuit for "deeper" dives; a wetsuit with a steel tank or especially steel doubles, that you will not be diving a balanced rig. People are frequently surprised by those numbers in the class. It is a gentle way to help people come to the realization that a drysuit or AL doubles, etc., might be a better idea.

From that, comes the discussion of how you achieve a balanced rig or mitigate that imbalance.
What you do with that moving forward is up to you. People have done Fundies with a wetsuit and steel tank, and even a wetsuit and steel doubles.

I'm not sure if the numbers are entirely correct either. The first time I used a HP steel 80 cf tank with a wetsuit, I emptied the wing when I got to the bottom at 80 feet, and had no issues whatsoever initiating an ascent. By the time I ascended a few feet, the momentum started and I had to vent gas as usual.

If you're concerned, you can make sure you can swim your rig up - with caution - for peace of mind. The only thing my GUE instructors told me was that it might have been dangerous to test it from 80 feet, and that 40 feet would have been enough. I'm not sure if they thought I swam it up all the way. I only continued until I had to start deflating, then continued the dive.

...the definition for balanced rig I'm using says "nearly empty tanks for the last stop".

But in practice both you and @jadairiii are right - a simple recreational dive with a single AL80 seems possible as long as 1) you're perfectly weighted for the end of the dive with ~1000 PSI at 10ft and 2) you use very conservative gas planning.

If you're "perfectly weighted" at the end of the dive at the last stop, 10 feet, with 1000 psi, you will be *underweighted* if the crap hits the fan and you have less gas than that at the end. It will be more difficult to hold the stop, especially if you are task loaded.

The GUE definition of weighting yourself to hold the last stop with nearly empty tanks and making a controlled ascent to the surface is exactly right.

It might take purposely practicing with a tank that is getting lower to see how well you're holding a 10 foot stop. You can also reach back and feel if your wing is empty or ask a buddy. If you have much gas in the wing with a nearly empty tank at 10 feet, you might be overweighted or not venting fully.

Hope that helps...

 

[Nick_Radov]

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.

That is misinformation. Your wetsuit loses buoyancy at depth: basic Boyle's Law calculation. Most of the buoyancy (and warmth) of neoprene comes from the gas bubbles permanently trapped inside the rubber. As the ambient pressure increases with depth, those bubbles are compressed and suffer a proportional loss of buoyancy. This is true regardless of the type of neoprene used.

If you don't believe me then put an empty wetsuit and some weights into a mesh bag and take it down to 100 ft (30m). Use a fish scale to see how much lift it loses at depth.

To answer the original question, yes it is pretty much impossible to have a fully balanced rig with a thick wetsuit. With a single tank most of us can probably swim it up if we have to, but it's safer and more comfortable to use a proper shell drysuit.

 

[huwporter]

Just to add some real-world experience to all the theory, a number of years ago I experimented with swimming a twinset and wetsuit up from 30m while simulating complete catastrophic wing failure.

Wetsuit was a 6.5mm semi dry, the twins Faber 12 litres. With an Ali backplate, I needed 1kg of tail weight to be perfectly weighted just below the surface with almost empty tanks. At the start of the dive, with a little over 200 BAR in the tanks - and watched by my pre-briefed buddies - I found a nondescript flat rock at 30m, completely emptied the wing, and tried to swim it up.

From 30m it was just possible to make progress; I can't give you a figure for how negative it was, but it required serious and continuous effort - full strength continuous straight leg flutter kicks - to make slow progress, and any pause in kicking would lead to immediately sinking back down. It would get a bit easier as you got shallower - I only kicked up to 25m or so - but with fairly full tanks you'd still need to kick against something like 4-5kg of negative buoyancy from gas even at the surface (depending on how much gas you chugged through on the ascent), until you ditched your rig. CO2 retention would be a hazard.

Lower pressure at the start of the ascent would make it easier to make progress, and by safety stop depth you would be approaching neutral, but then you would also have less reserve gas to suck through kicking up to there. Ali twins, or a single tank, would allow more ditchable weight, but if you did ditch it then by safety stop depth with low tanks you'd be positive, so you'd better not be in deco.

The conclusion was that it should be survivable, even in the absence of a liftbag, buddies to assist or a downline to climb, but it sucked. Deeper than 30m, or in deco would be a no-go. In contrast, I've done the same thing in a drysuit using just the suit for the ascent and it was absolutely no big deal at all. This is all just physics - there is nothing here that is specific to "DIR - GUE".