HOW MUCH LIFT FOR COLD WATER

[cool_hardware52]

So let's say it's the beginning of your dive, and you've just jumped off the boat. You're "carrying" 6lbs of air on your rig. And you're saying someone told you it's OK to have to swim down at this point to "get negative"? OK. So somehow you manage to swim down (Not sure how this is done as I would imagine your fins would pop out of the water and you wouldn't get any traction...but anyways...). So you do your dive. Let's say you use 1000 psi going away from the boat. That's 2lbs of negative buoyancy you've lost. And then let's say you use another 1000 psi meandering back to the boat. You're now 4lbs "lighter" than at the beginning of the dive. You've lost 4lbs of air "weight". So you ascend, and in the process you use another 500 psi. And you're at your Safety Stop, 15 ft below the surface. So how are you going to hold your safety stop? You're now 5lbs "lighter" than when you started the dive. It's as if you've ditched 5 lbs of lead during the dive. And to start the dive, you had to swim down to get negative? You were 5 lbs heavier when you had to swim down.

rwestfall stated that this was a known technique. But it doesn't sound like a good idea and I've never seen anyone practice it. That's just my $0.02 cents on the subject.

I've done exactly this countless times. It's exactly how divers managed to, and still mange to dive without BC's at all, i.e. careful weighting, not oversized BC's. With thick suits and modest sized cylinders it's easy, and I would argue safer than being over weighted.

The reality is safety stops are conducted at ~ 1.5 ATA vs 1 ATA at the surface. Most of the change in suit volume happens pretty shallow. That's why it's waay more difficult to hold depth at 5 ft vs 100 ft. I have yet to encounter a 7mm wetsuit that doesn't lose buoyancy from the surface to 15 ft approximately equal to the weight of the gas in a "normal" single cylinder. (No Heiser 190's)

My definition of minimum ballast is the least ballast required to hold a shallow stop with a near empty tank.

If a diver is weighted that way and is using a thick wetsuit (this is a thread about cold water after all) I'd love learn when they would need more wing lift than the initial, surface buoyancy of their suit.

Tobin

 

[sarahlee]

I agree. I merely posted what I personally use. It wasn't intended as a general rule

i like simple answers that help you to get a general idea of what might apply to you - dont tell anyone dr. bill - but i thought your answer was on of the most helpful

 

[84CJ7]

I had a single dive when I was new where I both sank down while swimming up at full power at depth with too little air in the bcd (bcd failure would have meant a weight dump which isn't the best way to dive) and then couldn't hold a safety stop with a low (but safe) tank level when I drifted from 15 feet to 10 feet by accident and then just took off as the suit rapidly expanded.
A lot of that was poorly fitted regular crappy neoprene 6.5mm farmer john wetsuit that trapped air, made movement difficult and diving miserable.

I prefer to carry too much weight, but I find the proper suit does more for correct weighting than anything.
I actually have no idea how much weight I carry at the moment, I haven't adjusted it in a fair while.

I now only dive stretchy technology wetsuits, brand doesn't matter but I use a henderson hyperstrech (now sold as thermoprene) 7mm (7/5?) with hyperstretch hood and gloves. I can't even feel the wetsuit and no more wild buoyancy swings. Dropped a ton of weight too.
You couldn't get me to dive in regular neoprene or double layers ever again even if I had to cancel a dive vacation because of it.

I don't know what suit you have but they do make an 8/7 semi dry in the thermoprene if you are prone to getting cold, though that's treading on drysuit territory in my opinion. But normally your hands and feet and face and then your air go before your core temp in my experience.