too heavy...or too light?

[gomi_otaku]

OK, so on dive #7 (2nd dive in cold salt water) I had 30# total- 18 on a weight belt and 12 in my integrated, with a 7mm FJ & jacket. I really noticed trouble with my bouyancy more this time than last, especially under 20 ft. I would either start to ascend and have to dump any air in my wing, or I would be going along near the bottom and start to crash to the sand. Is this an indicator that I have *too much* weight? I've had trouble descending in the past (OW cert dives, and used 28# in fresh water with the same wetsuit but a BC instead of BP/W). I'm thinking the weight is causing me to crash to the bottom, and the amount of air I am keeping in the wing to counteract it is causing me to cork if I get a little too close to the surface. If someone has an opposite or different idea, let me know- I'd really like to start dropping some of this weight, and if that's my problem I have a good reason to dump a little next dive.
gomi_

 

[coachrenz]

Have you done the surface buoyancy test? Try it at the end of a dive with an empty tank, especially if you are using AL80's.

If you have a buddy who is willing and some of the smaller weights you can have the person had you some until you are trimmed just right. If you aren't familiar with the test I am talking about, you could check you OW text or PM me and I can give you the gist of it.

Welcome to the great sport. Hope the bug bites you as it has me.

Tim

 

[spectrum]

What you describe could be exacerbated by having too much weight but unless you are GROSSLY overweighted is not the cause. Let's look at what happens when you are diving...

Lets' cheat and go to the end of the dive first. The time when your correct weighting is exposed is at the end of your dive. If you had a nice long dive and your cylinder is down to something like 500 PSI you have released 5-6 pounds of compressed air in the course of your dive. This was real weight that helped you get down at the start of the dive.

A very important part of your dive is to make a safetly stop or shallow swim-in on a shore dive. You also need to be in control for a slow final ascent. This lets you off gass gracefully and evade that speedboat you suddenly hear buzzing at you. You want to end the dive just barely buoyant. This is where the bobbing plumb at eye level with 500 PSI in your cylinder and an average breath test comes nto play. By varying lung volume you mask should be able to rise clear of the water or go completely under. Adjust your weight accordingly.

Now being in Seatle I will assume you are in a 7mm wetsuit with perhaps a second layer of some sort on your core. Lets's assume you did the weight check above and your lead is just right. You swim out from the beach and have the air you will breathe in your cylinder so you are about 5 pounds negative so there is some air in your BC to let you snorkel out. Same deal on a boat dive while you make final in the water buddy checks.

Also keep in mind that your suit is positively buoyant at the surface and is the reason you have most of that weight strapped to your body. As soon as you begin to descend that neoprene will begin to compress (it full of gas bubbles like a cake) which reduces its displacement and hence your buoyancy. The must be compensated for and the change continues through your descent. (BC Buoyancy Compensator)

So you lay on the water(or bob vertically) and dump some air from your BC. Nothing happens. You exhale deeply... still floating. Dump some more and exhale and you slip below the waves. After a few feet your suit is already compressing so you begin to sink faster. Meanwhile you are equalizing which will just add a distraction and may force you to pause your descent.

So as you manage your ears you add short bursts of air to your BC so you remain slightly negative and continue to drift down. Not only is your suit compressing but that bubble of air in your BC is getting squished. Remember that it's the displacement of that air bag that gives you the lift so you must add air fast enought to counteract the squeezing of the BC and your suit, both of which are getting squeezed smaller with every foot of descent. All the while keeping your ears in the comfort zone.

Next you must realize that buoyancy change reactions are not instantaneous. You have momentum either upward or downward in the water column. As the bottom begins to come into view (assuming some significant visibility) you need to add even more air to your BC to halt your descent but not so much that you go positive and sling shot back to the surface. This takes practice and experience with your gear.

When you get it just right you can glide down to a stop a few feet from the bottom and just start swimming.

If you add too much you will start to rise and will need to dump air and if you make big moves in your air volume you mill be like a yoyo. Small changes are the key. For fine adjustments try dumping air with your inflator dump valve. Back and shoulder dumps are sometimes too effective for fine adjustments when you are new.

So if you consider what is happening to your buoyancy and what you are doing when you dive you can see that steady small adjustments are key. Give changes time to take hold and it should come together for you.

Pete

 

[The Kraken]

Gomi,
What you describe is the "damned if you do, damned if you don't" factor of thick wet suit diving.

With a 7mm FJ/J you have 14mm on your torso.

It takes a lot of weight to get that suit down from the surface. Once you do and then hit the neoprene compression zone, somewhere around 13 to 18 feet, you lose the inherent surface buoyancy of the wet suit and become, essentially speaking, overweighted at depth.

It takes some careful work to resolve this problem. Uncontrolled ascents in shallower water can occur because of this problem.

Just one of those things you have to deal with when diving thick neoprene.

the Kraken

 

[pasley]

Hard to say without knowing much more about you, height, weight, bodyfat, etc. But I believe Spectrum has it right on.

I will add to that one word: Momentum. If you are heading down, and add air to stop that it will take a few seconds to overcome the momentum. This creates the illusion of, “I added air, and nothing happened” so you add more, then when the downward momentum is overcome by the first burst of air; you have too much in the BC and start up. Upward momentum is more quickly overcome (gravity helps) so when you dump air the effect appears to be more instant. This is what causes the yo-yo cycle to start. So add air in a short burst, wait a minute then add more if required.

Frankly, I would gamble you are too heavy. When you descend do you go down slowly, or drop like a ton of overweighed bricks?

Another point I have noted. When newer divers are trying to descend, many are unconsciously moving their legs (fining). This creates lift, the opposite of what they want. Many also are waving their hands about, which also creates lift. Just keep everything still, and exhale, exhale, exhale, exhale, small sip of air, exhale, exhale, exhale, exhale until you are at 15 feet, then small breaths and you no longer need to exhale.

I dive a 7mm wetsuit with 20 pounds. I could dive it with 16-18, but I find 20 pounds to be more comfortable. In a drysuit I wear 26 pounds. I am 6 foot and 145 pounds. The farmer John adds more neoprean and that will take more lead. Figure 3 pounds of lead for every pound of wetsuit.

 

[Don Burke]

That thick wetsuit is going to have you chasing your tail on buoyancy, although you can do some things to make it easier on yourself.

Some divers add weight to allow them to submerge with a wetsuit with quite a bit of air in it. Preloading the suit or hanging out on the surface to let it fill will help avoid that.

As stated before, making sure you are not finning up while you try to submerge can make a big difference. Any weight you carry to counteract finning will make you more unstable.

In any case, once you are a few feet down, the suit starts to compress and you get heavy pretty quickly. At the beginning of a dive it can be really tough to get and stay neutral, especially shallow, due to the weight of gas in your cylinder.

 

[The Kraken]

The problem is exacerbated by the fact that you need to have enough weight to keep you neutral at your safety stop with, say, 500 psi in the tank.

The added positive buoyancy makes it necessary to have more weight to keep you neutral at the end of the dive. Needless to say, at depth and with the wet suit fully compressed, you'll be signigicantly overweighted.

Let me rephrase that . . . you'll be weighted properly based upon the rig you're diving, but you will be carrying more weight at depth than is optimally desired.

Once again, just a matter of physics.

the Kraken

 

[Salty Diver]

First off...diving is fun and should always be fun. So don`t worry about the weight problem you will figure it out soon enough.

I can give you may two cents worth if you like. I dive in the cold Bay Of Fundy in eastern Canada. We wear 7mm wetsuits or Dry suits so I understand your problem. Here is what we do.

Find a dive site that you are comfortable with. One that is only about 10 to 15 feet deep. Use a ALUM 80 tank with 300psi (empty) make sure that the kit you are wearing is the same kit you will be wearing all the time. Also take a few small lead weights with you.

Now go down to 10 feet and relax. What you want to do at this point is to keep adding or removing weight until you are neutral at 10 feet. You should be able to do a fin pivot without adding any air to your BC.

Now I know that someone will read this post and tell me that I am just worng. Well everyone has there opion. I hope this helps.


Now

 

[DaveDog]

If you you go down with 300 psi be damn careful. Even in 10-15 feet. I would just get enough weight to descend with 500 psi in water I cloud stand in with buddy present.

 

[Charlie99]

Find a dive site that you are comfortable with. One that is only about 10 to 15 feet deep. Use a ALUM 80 tank with 300psi (empty) make sure that the kit you are wearing is the same kit you will be wearing all the time. Also take a few small lead weights with you.

Now go down to 10 feet and relax. What you want to do at this point is to keep adding or removing weight until you are neutral at 10 feet. You should be able to do a fin pivot without adding any air to your BC.

Now I know that someone will read this post and tell me that I am just worng.

I won't say you're wrong, but 1) he should use whatever tank he normally dives with, whatever that is, and 2) another alternative to doing the test with a near empty tank is to do it with more air in the tank and then factor in the air weight. For example, the air in a 80 cubic foot tank is about 6 pounds. So a good method would be to do your described weight check, but with a couple thousand PSI so one can take their time. Then if, for example, neutral with no air in the BC is with 25 pounds at 2,000 psi in an AL80, then the right weight for a near empty tank is 29 pounds (6 pounds for 3000psi of air is 1 pound per 500psi).

Of course, one might want to be able to control the ascent to a bit closer to the surface and choose to be able to achieve neutral buoyancy at 6 or 8 feet. The weight penalty for this is negligible for tropical divers, but might be more than a double 7mm diver is willing to accept.