Wet Suit Compression/Expansion

[muddiver]

I think the statement that you were over weighted is inncorrect. But, that's my opinion. It sounds to me like you didn't have enough weight to be neatural at 15 feet with an empty tank. I find that there is a combonation of things that cause one to be buoyant on ascent. Work with your BC and make sure you get all of the air out of it when you dump. I have a back wing type BC and it will tend to hold a small amount of air if I do not tilt my self just right to get the dump valve to the highest point. Also if you were using an aluminum tank it will become buoyant as you use up the air in it and that can make a big difference at 20 to 15 feet. I don't know how much you personally weigh, but if you had less than 10% of your body weight plus four pounds on your belt / in pouch, than you might have been too light.

Personally I do my stop at 20 feet because the change in buoyancy at 15 feet is to difficult to deal with.

 

[Charlie99]

As I stated before, I did the weight check and was neutrally buoyant. My instructor made us all do it while hanging on to the ladder on the back of the boat.

Two observations ---- first, if the weight check was done at the beginning of the dive with a full tank, then you were carrying the extra weight of the air at that time. Air weighs about 6 pounds per 80 cubic feet.

The second thing is that your lung volume makes several pounds difference in buoyancy. A natural tendency is to suck in a big breath of air if you are having some sort of difficulty. Combine this with a bit of air still left in your BCD or trapped in your wetsuit and sucking in a big breath can make you pop to the surface. You need to drill yourself so that a super full exhale is your automatic response to beginning to pop upward.

 

[Hickdive]

Sounds like the problem is due to your weighting being adjusted for a full cylinder and an empty BC at the start of the dive.

A proper weight check is done with a cylinder on reserve air and an empty BC to simulate your buoyancy at the end of a dive.

This has nothing to do with your wetsuit and everything to do with the weight of the air consumed during the dive.

 

[Blackwood]

This has nothing to do with your wetsuit and everything to do with the weight of the air consumed during the dive.

Agreed. The wetsuit theory makes no sense. If the wetsuit is pushing a diver up, the same wetsuit wouldn't allow the diver to get down.

Doing a weight check with a full tank doesn't work unless you add some for consumption.

 

[SteveAD]

Agreed. The wetsuit theory makes no sense. If the wetsuit is pushing a diver up, the same wetsuit wouldn't allow the diver to get down.

Doing a weight check with a full tank doesn't work unless you add some for consumption.

You're missing a point here, it is both the weight of the air used AND the compression of the suit. Assuming the diver is "perfectly" weighted at the start of the dive and is just able to get down, a 7mm wetsuit will compress at depth and lose bouyancy, allowing the diver to stay down while the tank weight decreases. Upon ascent the suit re-expands and regains it's bouyancy only now the diver is too light to stay down and suddenly goes "cork" from about 10 or 15 feet.

 

[Blackwood]

You're missing a point here, it is both the weight of the air used AND the compression of the suit. Assuming the diver is "perfectly" weighted at the start of the dive and is just able to get down, a 7mm wetsuit will compress at depth and lose bouyancy, allowing the diver to stay down while the tank weight decreases. Upon ascent the suit re-expands and regains it's bouyancy only now the diver is too light to stay down and suddenly goes "cork" from about 10 or 15 feet.

If a diver is "just able to get down" at the beginning of a dive, said diver isn't "perfectly weighted." He's underweighted (CCR divers notwithstanding).

The buoyancy of the wetsuit is more-or-less the same at the beginning of a dive as it is at the end. The difference (what caused the corking) was the weight lost to expired air.

The same thing would have occurred had the OP been wearing no suit.

 

[SteveAD]

You might have noticed that I put perfectly in quotes. With no suit the diver will begin to have difficulty staying down as soon as air consumption reduces total mass below a critical point. With a thick wetsuit, the loss of bouyancy due to suit compression is the primary reason we use BCs, and the diver is able to consume air at depth well past the point of proper weighting in shallow water. When they ascend and regain the suits full bouyancy, that is the trigger. Most tanks "bouyancy swing" is 6 to 8 pounds, the bouyancy on a 7mm farmer john can easily exceed 20 pounds, and by 30 feet a significant percentage of the suit bouyancy is lost to compression.

 

[Daner]

I'm guessing that you are not underweighted considering my buddy is a 230 lb male who uses a Al80 and 2 piece 7mm wetsuit uses 34lbs. Judging by you profile picture you don't appear to be a very large woman so your weighting is probably OK for now. I would guess that there was air trapped in your BC or you started your ascent too quickly and dumping air wasn't able to stop your momentum. It just takes experience.

 

[Rob9000]

- Like Blackwood said, I like to be more than just barely negative when doing a buoyancy check with a full tank

- In an uncontrolled ascent, you can try going vertical/heads up in the water to help get trapped air out of the BC. Depending on the BC, if you are prone in the water you can have the deflater wide open with no air coming out, yet still have air in the BC that is "above" the location of the valve. Just make sure you are not sculling/kicking while doing this as it will propel you in the direction you don't want to go.

- Also, exhale when dumping air to catch yourself. Like others here mentioned, full lungs of air account for several pounds of lift.

 

[boat sju]

shott,

I'm just getting back into diving and am learning to cope with the same problem as you. I can do my safety stop just fine at 15' but 12' and less I call my "cork" zone. Dealing with the decompression of a new 7mm john/jacket and trapped air pockets in a new bc that I wasn't completely used too, caused big problems when I start for the surface after my stop. There's not enough time to correct anything at that point. What's helping me is to be properly weighted with 500 psi left in my tank - maybe even floating at top of mask (about a pound heavy). Making sure I get all the air out of my bc and my hood at the ss. A more experience diver told me to leave the ss by completely exhaling before I start up and then breathe shallow. This all seems to be helping.