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Darthwader
08-30-2007, 09:42
I've got an issue I'm trying to work out that I could use some experienced opinions on. I dive with a type II 7m O'Niell wetsuit. at depth, I'm fine. boyancy still isn't where I want it, but I'm improving. my problem is this. If I'm weighted with a full tank and appropriate weight (about 28 lbs) and neutrally bouyant at depth, how do I keep from popping up like a cork when my 7m decompresses at 15'? is this "sudden bouyancy" as sudden as I'm afraid it is or is this one of those areas that you must acquire a feel for? I realize that a gradual release of air from the bladder will compensate, but is there a dynamic between the now much lighter cylinder and my newly decompressed neoprene that I need to be aware of?

MEL-DC Diver
08-30-2007, 10:00
I don't think the issue is the wetsuit, but 28lbs of lead sounds like you are overweighted to me. At depth you end up compensating for this extra weight by putting a lot of air in your BC to get neutral. As you ascend, all of that extra air in your BC will expand quickly while you are trying to dump it, and in that 15' zone you become the cork.

Again, I am speculating about the overweighting, but the only people I have see with that much weight are really big guys diving dry.

ScubaToys Larry
08-30-2007, 10:21
And keep in mind that 80 cubic feet of air weighs about 6 lbs. So if you are fine at the beginning of the dive - but light at the end... you might have to add that extra 6 lbs of lead, then compensate at the beginning with more air.

But as pointed out, depending on your size, gear, etc, adding 6 to the 28 would be a lot of lead. Remember you physics that the closer you get to the surface, the faster the expansion on your suit, your bc, etc - so you couple that with the light tank, and it is something to keep an eye out for.

I tell my students, always ascend slightly negative. Keep letting air out so that you have to do a gentle kick to go up... if you start kicking, you should slowly descend. And that will mean you have to dump even more air faster as you get to the shallower depths and the expansion is happening more rapidly.

Just the fact you are noticing and asking about this lets me know you are on your way to be a very good conscious diver. Trimming out Buoyancy is something that takes some divers a bit of time to get down - especially in heavy neoprene.

Keep working on it and let us know how it goes!

Darthwader
08-30-2007, 13:35
I don't think the issue is the wetsuit, but 28lbs of lead sounds like you are overweighted to me. . .


And keep in mind that 80 cubic feet of air weighs about 6 lbs. So if you are fine at the beginning of the dive - but light at the end... you might have to add that extra 6 lbs of lead, then compensate at the beginning with more air.

But as pointed out, depending on your size, gear, etc, adding 6 to the 28 would be a lot of lead. Remember you physics that the closer you get to the surface, the faster the expansion on your suit, your bc, etc - so you couple that with the light tank, and it is something to keep an eye out for.

I tell my students, always ascend slightly negative. . . Trimming out Buoyancy is something that takes some divers a bit of time to get down - especially in heavy neoprene.
Keep working on it and let us know how it goes!

Thanks for the input guys. I've given much thought about the weight issue. I was diving an OMS comfort harness, Al BP, steel 80 (IIRC) and a 19cf pony. I was starting to feel like I actually needed more weight to compensate for the lightened cylinder, but I see that there's a lot of variable at play here that I need to consider.
Thanks for the encouragement Larry. I'll definately keep everyone posted,

Cheers!

medic001918
08-30-2007, 13:44
For diving an AL backplate and a steel tank (if that's what you were using), you're definatly heavy. When diving a 7mm suit with a steel plate and wing, and a steel tank, I wore about 12 lbs or so and I'm 6'2" and about 220 lbs. It sounds like you're considerably overweighted.

Keep working on trim and bouyancy. It will get better. And play with your rig to get it where you want it as far as trim and weighting go. I've shed a lot of weight as I've done more dives and gotten more comfortable. I don't have the nervous finning any more and descending is effortless. I've actually managed to drop enough weight that when diving a drysuit in freshwater with my rig, I don't use a weight belt. And when diving saltwater, there's only a few pounds on it (and soon I'll probably be able to shave those too).

Time and practice.

Shane

ReefHound
08-30-2007, 13:51
You don't weight yourself to be neutrally buoyant at depth, you weight yourself to be neutral at your safety stop at the end of the dive. And the change in buoyancy or wetsuit decompression is not "sudden" at 15'.

No Misses
08-30-2007, 13:57
The biggest thing that I have noticed when people complain of too much bouyancy near the surface (<20 fsw). They are either finning too much and/or they have not let enough air out of thier bc. With some bc's you need to roll around a little bit to get all of the air out. For the excess finning, try crossing your fins {I got this one from Larry :smiley20:}. Give it a try next time you are coming up. Good luck, let us know how it goes.

P.S. The heaviest suit that I dive is a 3m farmer john (w/AL80). With it, I have a total of 12 lbs of ballast (5 lbs plate+7 lbs lead).

ScubaGir1
08-30-2007, 14:36
From what my instructor told me, as you descend down deeper and deeper, you have to put little bursts of air into your BC. That way when you get to the bottom you're not just falling and crash LOL. So, while you're down there you have some air in your BC, which would make sense that if you DON'T release it as you ascend you will start to shoot up. Are you releasing air as you ascend?

I've seen my boyfriend suddenly be at the surface (we were practicing buoyancy in about 20-30' of water). He said that he just floated up, so he's working on that (and getting MUCH better). It might have to do with the way he's breathing....
Oh yeah, and we dive in 7mm suits.

Good luck, I'm sure practice makes perfect. I know I'm still perfecting my buoyancy :smiley29:

thor
08-30-2007, 14:54
I had the same problem when I bought a new wetsuit. I was overweighted at the beginning of the dive, due to the brand new, never been compressed neoprene. Next dive I dropped 8 lbs.

Also, you are using up air during the safety stop which might affect your bouyancy. So even though you are neutral at the beginning of the 3 minutes, you may have to let out more air during the stop.

ertechsg
08-30-2007, 14:58
Try to accend with no air in the BC just using kick then at ss add a little

ReefHound
08-30-2007, 16:08
Also, you are using up air during the safety stop which might affect your bouyancy. So even though you are neutral at the beginning of the 3 minutes, you may have to let out more air during the stop.

Unless you are hyperventilating I have a hard time imagining breathing off more than a few ounces worth of air on a safety stop, well within range of breath control.

Darthwader
08-30-2007, 21:04
The biggest thing that I have noticed when people complain of too much bouyancy near the surface (<20 fsw). They are either finning too much and/or they have not let enough air out of thier bc. With some bc's you need to roll around a little bit to get all of the air out. For the excess finning, try crossing your fins {I got this one from Larry :smiley20:}. Give it a try next time you are coming up. Good luck, let us know how it goes.

P.S. The heaviest suit that I dive is a 3m farmer john (w/AL80). With it, I have a total of 12 lbs of ballast (5 lbs plate+7 lbs lead).
Thanks, I'll definately give that a try!:smiley20:
after reading the posts, I'm starting to certainly doubt myself about the weight issue. I'm definatly going to play with the pounds too to see how that works.
It sounds from the consensus of replies that the neoprene's compression doesn't factor in too much, but I'm still not convinced yet. I still think that an empty cylinder and an fully expanded 7m will upset the applecart at the shallower depths. unfortunately, at my level, there are just too m,any other variables to work out (like that nervous finning habit I didn't realize I might have:scratchchin:)
anyway, this is why I consulted the board. the Neoprene was just an idea that I wanted to bounce off you folks.
Cheers for now.

DivingsInMyBlood
08-30-2007, 21:50
I don't think the issue is the wetsuit, but 28lbs of lead sounds like you are overweighted to me. At depth you end up compensating for this extra weight by putting a lot of air in your BC to get neutral. As you ascend, all of that extra air in your BC will expand quickly while you are trying to dump it, and in that 15' zone you become the cork.

Again, I am speculating about the overweighting, but the only people I have see with that much weight are really big guys diving dry.

I used 26 pounds in a 7mm wetsuit when i was doing my open water (fresh water) and it felt fine (i weigh 225 pounds). I keep hearing you can drop a little weight when you become a better diver.

Charlotte Smith
08-31-2007, 06:18
I don't think the issue is the wetsuit, but 28lbs of lead sounds like you are overweighted to me. At depth you end up compensating for this extra weight by putting a lot of air in your BC to get neutral. As you ascend, all of that extra air in your BC will expand quickly while you are trying to dump it, and in that 15' zone you become the cork.

Again, I am speculating about the overweighting, but the only people I have see with that much weight are really big guys diving dry.

I used 26 pounds in a 7mm wetsuit when i was doing my open water (fresh water) and it felt fine (i weigh 225 pounds). I keep hearing you can drop a little weight when you become a better diver.

I used to dive w/ 12 lbs and went to a steel tank and don't use any weight, as you dive more and get better w/ your bouyancy you WILL be able to trim weight...at least I did.