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

[overthinking_diver]

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):

buoancy_loss_from_10_to_100_feet + weight_of_gas_in_tanks <= weight_we_can_swim_up + positive_buoyancy_at_10_feet_we_can_handle.

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?

 

[overthinking_diver]

You can try to balance the rig by dropping more weight at the start of the dive when you still have gas in your tanks and less near the end. If you could do this perfectly, i.e. drop weight equal to the weight of the remaining gas in your tanks, you could get rid of the *weight_of_gas_in_tanks* term from my condition.

However, having to think how much weight to drop during an emergency is not something I personally want to have to think about.

 

[tbone1004]

@overthinking_diver this is why you rarely see thick wetsuits being used by that contingent and why you'll see people like me say that 7mm farmer johns are dangerous when diving deep. Drysuits are standard for two reasons, minimizes the change in buoyancy with depth since you can control the volume, and serves as redundant buoyancy in the event of a wing failure.

Any normal person can kick up 10lbs without fins on and 16lbs with fins is somewhat annoying but you can get it up.

 

[JoeTPhilly]

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?

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.

 

[gmerick]

Discussion should consider vital capacity of the lungs also. Tidal volume is somewhere around .5 litre, but that half litre can take place in up to a five litre range. Meaning natural buoyancy of one's own body can easily vary by 10lbs on breath alone. Wing failure beginning of dive @100 ft. Drop 6 lbs lead, breath off top of lungs, (22lbs - 6 lead - 4 lungs) yes a diver can swim that 12 lbs up 30 feet, and at 70 feet the wetsuit has gained enough buoyancy.

 

[JoeTPhilly]

Discussion should consider vital capacity of the lungs also. Tidal volume is somewhere around .5 litre, but that half litre can take place in up to a five litre range. Meaning natural buoyancy of one's own body can easily vary by 10lbs on breath alone. Wing failure beginning of dive @100 ft. Drop 6 lbs lead, breath off top of lungs, (22lbs - 6 lead - 4 lungs) yes a diver can swim that 12 lbs up 30 feet, and at 70 feet the wetsuit has gained enough buoyancy.

Need to drop lead = non balanced rig.

 

[gmerick]

Need to drop lead = non balanced rig.

Same numbers, but no need to drop lead. I overlooked the wetsuit's 6 lb buoyancy @100ft. And in Minnesota, 7mm wetsuits are very common.

 

[lowwall]

I'm not a DIR diver, but isn't this exactly the reason they require drysuits for cold and deep dives?

 

[overthinking_diver]

Discussion should consider vital capacity of the lungs also

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.

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.

I'm not a DIR diver, but isn't this exactly the reason they require drysuits for cold and deep dives?

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.

 

[JoeTPhilly]

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.

Hi, he made the statement because of the problem you saw illustrated in the Fundies document; namely how one will be overweighted while diving heavy wetsuits.

I thought the purpose of a drysuit was so you can have a pee valve and then have a week long argument on balanced vs. unbalanced...