Please help me with my buoyancy :)

[spectrum]

I'll also probably be at redondo next wed, and am glad to help out then too.

(Piranha -- PM me if you want to meet up and see if I can be any help)

Gee I'm glad to see this post since it was going to be my suggestion.

It sounds like im_a_piranha is the victim of a multi-varriant science experiment in buoyancy and had an intructor that did not delve into the physics of a good configuration or helping her zero in on a good personal set-up. An afternoon with a good mentor should do wonders. If that can't happen the follow the above advice.

Please believe that it can become effortless. Also accept that in cold water diving you will be adding considerable air to your BC to compensate for the loss of buoyancy as you take your neoprene into the depths. there is nothing wrong with that. Needless extra weight however is evil and will result in an unstable condition.

Pete

 

[namabiru]

I had the same problem, but after doing AOW I found it was because i was overweighted.

Another problem I had was the BCD. My first dives, the BCD was a bit too big, so it actually acted more like a life preserver and almost made me want to float! Having a lot of room at the neck is kinda a bad sign... after i dove with another DS and got a smaller BCD (and better quality too), I sunk like a rock.

 

[onfloat]

Air has weight. It doesn't matter if you are using Aluminum or Steel, you will lose weight during the dive and as such you must compensate for it. Take a look at the bottom of this page:

http://www.njscuba.net/gear/eqpt_05_tanks.html

Your right air does have weight, but if you look at the chart you posted, most of the steel tanks are neutral or negative bouyancy when empty, therefore you do not have to compensate for them. The goal is to be neutral at the end of the dive. Aluminum tanks float when empty or close to it and that is what you would have to compensate for.

 

[Azza]

Your right air does have weight, but if you look at the chart you posted, most of the steel tanks are neutral or negative bouyancy when empty, therefore you do not have to compensate for them. The goal is to be neutral at the end of the dive. Aluminum tanks float when empty or close to it and that is what you would have to compensate for.

You are right on the money.
An AL80 is about 2kg POSITIVELY buoyant when empty. An AL95 (the most common tank here in NZ) is 3 kg positively buoyant when empty. A steel tank is often a little negative leaning toward the neutral side of things when empty.
Have you ever seen an empty AL80 dumped at depth? They float right up into the arms of your surface crew

 

[TxHockeyGuy]

Your right air does have weight, but if you look at the chart you posted, most of the steel tanks are neutral or negative bouyancy when empty, therefore you do not have to compensate for them. The goal is to be neutral at the end of the dive. Aluminum tanks float when empty or close to it and that is what you would have to compensate for.

I realize that the tank will be neutral at the end of the dive, but you need to be neutral at the end of the dive. If you start your dive perfectly neutral then at the end of the dive you are going to be underweighted, regardless if you are using a steel or aluminum tank.

For example, lets say a we have a diver who for is perfectly neutral in the water with no weight. Now lets say they have to wear a 7mm which has 15 lbs of buoyancy (note that I just made that number up, I have no idea what the buoyancy is for a 7mm). Now lets say the diver is diving an OMS 85 per the site I referenced earlier. The OMS is 6.7 lbs negative when full, we'll round that to 7. So ignoring the rest of the gear, we'll just say it's neutral, the diver needs 8 lbs of weight to be neutral at the beginning of the dive. If the diver went in like this at the end of the dive they would be underweighted by 7 lbs. Of course that's assuming they sucked the tank dry which isn't realistic so they would probably be 5 lbs underweighted. That is why you have to account for loss of weight due to air useage during the dive.

The advantage of the steel over the aluminum is that your tank is always more negatively buoyant than an aluminum, both at the start and ending of the dive. That means you can wear less weight with a steel, but you still have to account for the loss of weight due to air useage during the dive.

 

[Storm]

I had the same problem, but after doing AOW I found it was because i was overweighted.

Another problem I had was the BCD. My first dives, the BCD was a bit too big, so it actually acted more like a life preserver and almost made me want to float! Having a lot of room at the neck is kinda a bad sign... after i dove with another DS and got a smaller BCD (and better quality too), I sunk like a rock.

That's why I asked what typr of BC she was using.

This was just one pf the issues my wife had...even the xsmall mens BC that the LDS were tained woth used for women students, fit poorly and actually contrinuted to her trim and buoyancy woes. Once we got her into a compensator that could be properly fit to her body type (in our case we went to a BP/Wing, but there are Jacket BCs made for the female form.) her buoyancy control and trim improvement was remarkable.

One thing, and I hesitate to mentioned this as I do not want to kick off a war of words, but if im_a_piranha, can find some mentors in her area who have both recreational and technical training in their backgound, she might get the opportunity to practice with both type of divers, and see both types of gear configutations. It might help her dial in a bit better.

 

[onfloat]

I realize that the tank will be neutral at the end of the dive, but you need to be neutral at the end of the dive. If you start your dive perfectly neutral then at the end of the dive you are going to be underweighted, regardless if you are using a steel or aluminum tank.

For example, lets say a we have a diver who for is perfectly neutral in the water with no weight. Now lets say they have to wear a 7mm which has 15 lbs of buoyancy (note that I just made that number up, I have no idea what the buoyancy is for a 7mm). Now lets say the diver is diving an OMS 85 per the site I referenced earlier. The OMS is 6.7 lbs negative when full, we'll round that to 7. So ignoring the rest of the gear, we'll just say it's neutral, the diver needs 8 lbs of weight to be neutral at the beginning of the dive. If the diver went in like this at the end of the dive they would be underweighted by 7 lbs. Of course that's assuming they sucked the tank dry which isn't realistic so they would probably be 5 lbs underweighted. That is why you have to account for loss of weight due to air useage during the dive.

The advantage of the steel over the aluminum is that your tank is always more negatively buoyant than an aluminum, both at the start and ending of the dive. That means you can wear less weight with a steel, but you still have to account for the loss of weight due to air useage during the dive.

I thought we might be discussing this from the same side of the fence.

 

[Lamnid]

You weigh the same as me (55kg) and I have 11kgs (24lbs) in the red sea with an aluminium tank and (they told me but it doesn't sound right) 4kgs (8.8lbs) in the mediterranean with a steel tank. But i think there is more to your weight and buoyance than just your belt. How you hold yourself in the water and how you breathe will also affect your vertical movement in the water column. I am much more confident with my buoyance now but i still find myself adjusting the air in my BCD fromm time to time.

 

[limeyx]

Air has weight. It doesn't matter if you are using Aluminum or Steel, you will lose weight during the dive and as such you must compensate for it. Take a look at the bottom of this page:

...

No, it is not so that you are just moving lead around (necessarily) -- it all depends on the dry weight of the tank.

Look at the empty dry weight of an AL 80 (31.7 pounds) and then a steel 85 (31.0 punds on your chart)

So you are essentially carrying the same weight on land, but since the steel tank will be neutral or negative in the water, you need less lead on the belt and thus carry less weight overall.

And yes, of course you still need extra lead to compensate for the air that you breathe during the dive, but the AL tanks you need to add 4 or so pounds of *extra* lead that you dont need with the steel.

 

[GDCB]

Limeyx,

Thanks for that break down. I've heard this explained before and missed something in the translation. That clairified it for me. I don't quite get the physics other than that the steel 72 is physically smaller than the Al 80 and it can be overfilled to hold as much air, but I know what you're saying is true.

Carl