buoyancy problems

[Gator Diver]

Hey,
Any advice for me. I am having
problems with my buoyancy.
I dive with 3mm jumpsuit, 14 lbs wt., AL80.
I either seem to be hitting the bottom or
going back up to the top. Is it improper
use with my BC or just that I need some
more practice.

 

[MikeFerrara]

There's lots of past threads on the board on the subject but since you're in cave country you might look for a cave diver or cave instructor who teaches OW on the side and get them to give you some pointers.

Do a search on trim or buoyancy control and if you still have questions let us know. I'm not trying to avoid giving you an answer just limit typing.

 

[FredT]

Symptoms indiate you're badly overweighted.

The "bounce" indicates you are hauling a lot of "extra" air in your BC to offset too much lead on the belt. Small changes in depth under those conditions translate into big changes in buoyancy, thus the bounce.

BCs work best if you're ballasted to not need to put air into them AT ALL on a shallow warm water dive to be neutral coming up. The ideal would be to hit the water with about 2 breaths in the BC to compensate for the gas load, then VENT excess air as the dive progresses so the bag is empty and you are neutral at 300-500 psi in the tank towards the end of the dive. This is not always possible (cold water & heavy wetsuit means serious suit compression) but that is the "strive for" goal.

The ideal is to dive without a BC, but that went out of fashion in the 70's. :box:

FT

 

[Windwalker]

Aluminum tanks vary the weight by a great deal.

When I dive steel 120's I need 27 pounds of weight to sink me in a 7MM, when I use a al80 I need 36 pounds to sink me.

what is your body weight??

You might need a tad more lead to offset the Aluminum tank. it can change as much as 5 pounds(or more) during the dive.

Once more expierience is gained, then you might find that you will not need as much lead to keep you level.

your internal breath has a lot to do with your boyancy, if you are sinking and your lungs are full, then you need more air in your BC.. Ideally you should sink a little when you exhale, and rise a little when you inhale. That will even out as you get more expierence.

What type of BC do you have? (jacket style, Back inflate, Backplate?)

 

[diverlady]

Buoyancy is probably the most difficult skill to master when you're starting out. Try doing a search for it on this board. A lot of discussion has been done on the subject.

As you become more experienced you will be more relaxed underwater which will help your buoyancy tremendously. There will come a point when just your inhaling and exhaling will alter your position in the water by about, say, a foot up or down. Hang in there. You'll get it with practice.

Diverlady

By the way, when I first started I was wearing 28lbs with a 7mil wetsuit in freshwater. I wear 18lbs now.

 

[pasley]

newbie-in-fl once bubbled...
Hey,
I either seem to be hitting the bottom or
going back up to the top. Is it improper
use with my BC or just that I need some
more practice.

I suspect operator error. It sounds like you are going down and then adding too much air and so bouncing up. There may be other complications like too much weight as well (very likely).

Common errors made by new divers.
1st. When adding air to the BC, just give it a very quick press on the button for a short burst of air. It should sound like a short "bussh" and not like "buuuusssssssssh". When you hear sound release the button.

2nd common error: Now comes the hard part, WAIT. Yes wait. Contrary to what I (and I guess most people) first thought, the change in buoyancy is NOT immediate. It takes time for the air to effect your buoyancy. Why? I don't know, it just does. Wait for at least 2 minutes (by your dive timer), some would say 5 minutes before you add another very short burst of air. This should allow you to find neutral, before you over compensate and add another short burst and go positive and head for the roof.

3d Changes in depth and the loss of air from your tank will change your buoyancy during the dive, making a neutral diver positive. This is normal. A short release of a bubble or two from the BC will off set that.

 

[Windwalker]

FredT,
I was taught that when diving AL to add an extra 5lbs to my system to account for the boyancy near the end of the dive. If I were to hop in the water with only a swimsuit, I am one of those lucky people to actually be neutrally boyant. After adding a Jacket Style BC and a 2mm wetsuit I have to add 13 pounds to offset the inherant boyancy of the Wetsuit and the Jacket. What tends to happen is I use a large steel tank and a 8 pound weight belt to compensate. That seems to be a good weight for me.
Please, correct me if I am wrong. Newbie-in-fl is diving AL tanks in a 3mm suite, wouldn't he need more weight not less?

This is not a challenge, I know you know WAAAYYY more about this stuff then me, but I am curious if my figures are somehow skewed.

Thanks!

 

[MikeFerrara]

pasley once bubbled...


2nd common error: Now comes the hard part, WAIT. Yes wait. Contrary to what I (and I guess most people) first thought, the change in buoyancy is NOT immediate. It takes time for the air to effect your buoyancy. Why? I don't know, it just does. Wait for at least 2 minutes (by your dive timer), some would say 5 minutes before you add another very short burst of air. This should allow you to find neutral, before you over compensate and add another short burst and go positive and head for the roof.

2 minutes is enough time to rise or drop a couple hundred feet. You're correct that there is a delay in the effect of adding or removing air from the bladder (or your lungs). It's because of momentum and resistance. the key to compensating for it is anticipation. You make adjustments as they're needed not after they're needed. Once you find yourself reacting to buoyancy changes rather than proactively controling them you'll be somewhat out of control (cought behing) untill you catch up again.

 

[Charlie99]

Windwalker once bubbled...
FredT,
After adding a Jacket Style BC and a 2mm wetsuit I have to add 13 pounds to offset the inherant boyancy of the Wetsuit and the Jacket.

Please, correct me if I am wrong. Newbie-in-fl is diving AL tanks in a 3mm suite, wouldn't he need more weight not less?

This is not a challenge, I know you know WAAAYYY more about this stuff then me, but I am curious if my figures are somehow skewed.

Newbie-in-Fl is in "cave country". Perhaps he is diving in fresh water. If so, my guess is that at 14#, he is 5 to 10 pounds overweighted. This would make it difficult to control buoyancy.

Charlie

Nitty gritty on my guess:

For saltwater:
3mm wetsuit --- 6#
AL80 -- 4#
body 0-4#
total in saltwater= 10-14 pounds.

Subtract about 6# for fresh, and get 4-8# estimated lead needed, which is 6-10# less than what he has.

 

[FredT]

Windwalker

The net buoyancy difference between a "normal" steel tank (whatever that is, I consider an old 72 "normal") and an alumibomb is about 5 pounds. Therefore the difference in your net buoyancy between a steel and alumibomb is about 5#.

A 3mil jump is about 3-5 pounds positive MAX. Much less if it's well used and been crushed by going below 80' a few times.

The fact that he's bouncing is the flag here.

Suit and tank says this guy needs about 8 pounds max assuming he's somewhere close to neutral in his skin. At 14 pounds he's carrying about 3/4 gallon of air he doesn't need to just get even with his belt! Depth changes (especially when shallow) will multiply this error and give the bounce effect.

Tank construction isn't a hard & fast thing as I have an assortment of steel tanks that are from 8 pounds negative empty to 2 pounds positive empty and alumibombs from 5 positive to 2 negative empty. Please remember that in the abbreviated courses now available the instructor HAS to simplify things to get even the minimum information across that's needed to keep you alive in the class. Regretably not all will make that clear so "rules of thumb" like the 5#/aluminum thing come out as gospel. They are NOT gospel, but are an attempt to give you a working hint. The gospel about how to figure buoyancy takes a lot more time than they have available.

I did write up a letter to somone who had similar issues and saved it. If you want a copy in pdf format shoot me your e-mail address either in a PM or just mail me at the address below and I'll respond with the document (and a few others you may find useful) as an attachment.

FT