Wet Suit Compression/Expansion

[scubasavvy]

When I was doing my checkout dives this July I ran into a slight problem. We dove down in to a rock mass with a bunch of lobsters and hung around there at about 35 feet for 15 or so minutes. When coming back up to about 20 feet, my compressed wetsuit expanded and I floated to the top with my BCD totally deflated and 33 pounds still attached. I was not overweighted, it took 33 pounds just to sink me, for those of you who had the notion. I was wearing a 7mm Farmer John, so I had about 14mm at the core.

To me, this could be very dangerous if I were diving at a deeper depth and I was on my way to my 15 ft safety stop. Are there any ways to prevent another accident like this from occurring?

[gibson1525]

the problem is your tank, not your wetsuit. in fact, your wetsuit should compress during the dive making it less buoyant. as you use your air, your tank becomes more buoyant.

probably the best thing to do is get in the water with a tank @500 psi (the lightest your tank will every be) and see how much lead it takes to get perfect buoyancy. this way you'll have plenty of weight when you come up to do your safety stop.

[quasimoto]

Sounds like the problem that I had when I first started diving. The instructor kept telling me that I had on too much lead for the suit that I had. But every time I got down to around 1000psi and anything above 20 feet..elevator going UP. Finally I did a check like gibson suggested and sure enough, I was 4 lbs too light.
Another thing that we also found out was that it takes 4 lbs just to sink me and my gear. That doesn't include the swing for air. So I am automatically diving 8lbs.

[scubasavvy]

I'm a New England diver, so I need so much weight it's crazy. Between the Farmer Johns, the boots, hood, etc there's so much that needs to be neutrally buoyant before weighting myself. I've been considering diving with a steel tank...

[WV Diver]

Originally Posted by gibson1525

the problem is your tank, not your wetsuit. in fact, your wetsuit should compress during the dive making it less buoyant. as you use your air, your tank becomes more buoyant.

probably the best thing to do is get in the water with a tank @500 psi (the lightest your tank will every be) and see how much lead it takes to get perfect buoyancy. this way you'll have plenty of weight when you come up to do your safety stop.

I think this says it pretty well for me. You can figure 1lb of weight per 500psi on a 3,000psi Al80.

[MSilvia]

Originally Posted by scubasavvy

I'm a New England diver, so I need so much weight it's crazy. Between the Farmer Johns, the boots, hood, etc there's so much that needs to be neutrally buoyant before weighting myself. I've been considering diving with a steel tank...

A steel tank will let you dive with less weight on your belt, but it won't address this problem. You say you had 33 lbs on your belt for the dive, and that that's how much it took to sink you. While that's true, you should realize that you're going to finish the dive with less weight than you started with, so you need to have as much weight as it took to sink you PLUS as much weight as the air you breathe during the dive. That compressed air is heavier than you think, and as you use it up you'll become more and more buoyant. With a heavy new england wetsuit, you'll have compression to help you at depth, but as you noticed that won't help you stay under near the surface.

What you need to do is figure your correct weighting out. It's not how much you need to sink at the beginning of the dive that's important, but how much weight it takes to keep you neutral for a safety stop with a near-empty tank. Huron Scuba has a great chart that will show you the buoyancy characteristics of different tanks. If you look at the row for Luxfer Aluminium 80, you'll see that it 1.4 lbs negative when full, and 4.4 lbs buoyant when empty... that's a loss of 5 lbs of weight between the start of a dive and the end (if you suck it dry, which you're too smart to do).

In other words, try putting yet another 4-5 lbs of weight on your belt, and THEN you should be correctly weighted. Since you mentioned steel tanks though, if you got a tank that was more negative, that's weight you don't need on your belt, but the weight of the air won't change. If, for example, you used a PST LP80, you could possibly even take a pound off of your belt and still be properly weighted.

Of course, you'll need to start the dive with some air in your BC to compensate for the slight overweighting you'll start the dive with.

[scubasavvy]

I thought that steel tanks are less buoyant when they empty?

[awap]

If you lost bouyancy control at 20 ft, the problem was probably more than just the change of bouyancy of your tank. 20 ft is about 1.6 atmospheres and if your wetsuit is approximatelu 30 lb positive on the surface it would be less than 20 lb bouyant at 20 ft. Unless you were diving fairly large doubles, it was not just the swing in tank bouyancy that put you on the surface.

BTW, the steel vs aluminum issue only effects how much lead you must carry. The swing in bouyancy as you consume gas is the same - about a pound per 16 cubic feet.

[scootermcfly]

I've had a similar problem here in upstate NY where at the end of my dive I have about 500-800 left in my AL 80 and I end up more bouyant. So, I found that I usually steal a nice rock, not a large one, but just something to add to my weight and once I hit the surface, I take it out of my pocket or just let go and let it go back to the bottom (must make sure all other divers are up first though so I don't drop on one of them)

Scooter McFly