Buoyancy challenges in fresh water vs salt. Why ?

[BRT]

You are overweighted if you are adding air at the safety stop with 500 psi in the tank. Empty the wing and start chucking weights, until you are neutral. Shore diving works best for this. You may have to bleed off air from the tank to get to 500 psi.

All good until you try to do a safety stop when you really need one....been down too long, had some problems, high on nitrogen and only have 300lb in the tank.

---------- Post added August 4th, 2014 at 05:53 PM ----------

Thanks everyone for the replies. As Tigerman pointed out, we are not diving the same configuration at all. I'd say we use roughly the same weights but that is to deliteberately overweight us a tiny bit to help out with the camera work. So

Warm saltwater: al80, 3mm suit, 14 lbs, slightly overweight (probably 2 lbs too much)
Cold fresh water: steel HP 100, 7mm with hood+thick gloves, 14 lbs

In the end doing a proper weight check is a must and I'll do that as soon as I can (on a charter boat with a group in crazy current, not so easy). That being said I was wondering from your experiences if the environment had an impact (fresh, cold, drift etc... other ?) on buoyancy challenges that went beyond proper weighting.

I think Martigan really highlighted a great point here : "When you dive, your wetsuit is being compressed when you change depth. The thicker your wetsuit, the more air bubbles inside, the bigger its volume, and the bigger the impact of pressure change induced by depth changes are.
You need to compensate for it with your BC more with a thick suit than a thin one. Your lungs are not enough in that conditions. " That definitely would explain the larger variations at least a bit.

Thanks again

More insulation equals more buoyancy change. Insulation is air. Air compresses. If you get your buoyancy figured out in cold water in a drysuit then diving is effortless in a 3 mil suit in warm water. Going the other way is tougher.

 

[Peter_C]

All good until you try to do a safety stop when you really need one....been down too long, had some problems, high on nitrogen and only have 300lb in the tank.

Everyone that I have seen start at 500 psi winds up at 300 psi by the time they are done

 

[BRT]

Everyone that I have seen start at 500 psi winds up at 300 psi by the time they are done

Good to have low enough air consumption and good enough planning to have done what you came to do and be out of the water by 500psi. Everything went well and you ascended slowly while enjoying the dive. Your breathing is slow and easy and 100 psi lasts forever at 15-20 ft. That's how it should be. But planning your weighting for that says you never figure on needing that last 500lb. Then comes the day when things didn't go that way. The current is running. You came up faster than you really wanted to. You are stressed and your computer is on the edge of the red zone and your SPG reads 275. Oh, and the weights they gave you were a little high in zinc and low in lead and you didn't pull out the scales you always have with you (right?) to make sure they were 16lb so they were actually 14lb. Now make that 275psi last while swimming head down. Been there...not going back.