Yet another Buoyancy question?

[BB1]

Hi,

Can anyone suggest where I am going wrong in my calculations/weighting please?

Me: 6'4" and 240lb's and a 5mm semi-dry wetsuit using 21lbs lead with an AL80 tank.

PADI calc says 10% of body weight for a 5mm which is 24lb but doesn't take into account body composition.

At start of dive I breathe out for all I'm worth and cannot sink below eye level.

If I duck dive the first few metres it is then not a problem due to suit compression.

At the end of the dive as I am coming up I let all the air out of the BCD and am slightly negative and swim the rig up (which is not a bad thing and not hard work).

Question is why do I seem underweighted at the start but not at the end??

Am I still to tense and not relaxing enough?? I have 16 dives so far and love it!
Is this kind of problem normal?!?
Is my weighting out?

Any suggestions appreciatted - even a don't sweat it you're close enough!

 

[TSandM]

Well, at the beginning of the dive, there may be some air trapped in your suit, or in the padding of your BC, or even in the bladder of the BC. In addition, Tobin George says that neoprene does not rebound fully during ascent, and therefore, without any other factors, you're a little less buoyant at the end of the dive than at the beginning.

 

[BB1]

Thanks!

Last to dives where with a Hollis Alu plate and S25 wing (distributor tryout day) and so no air in BCD Maybe a little air trapped in the semi-dry as its not skin tight until submerged - I lost weight!

So do you think my weighting is correct or close to correct?

I am trying to zero in correct current weight so I can talk to Tobin about what BP/W I need!

 

[pasley]

...Me: 6'4" and 240lb's and a 5mm semi-dry wetsuit using 21lbs lead with an AL80 tank....
At start of dive I breathe out for all I'm worth and cannot sink below eye level. ....At the end of the dive as I am coming up I let all the air out of the BCD and am slightly negative and swim the rig up (which is not a bad thing and not hard work).

Question is why do I seem underweighted at the start but not at the end??

Money on the table is you are kicking your legs back and forth unconsciously while on the surface. So your legs are swimming you up, while you are trying to go down. This fits your description of can't go down at start of dive, but when you point your fins at the sky, you can and at the end of the dive you are negative.

I have observed this in many newer divers including myself. When we are in the water we absent mindfully move our feet back and forth in a swimming motion. Cross your ankles then deflate your BC then Exhale hard and see if that does not fix it.

 

[BB1]

Wow, never thought of that - will give it a try next time.

Thanks for a really helpful suggestion - sounds "right" if you know what I mean!

 

[Bubbletrubble]

So do you think my weighting is correct or close to correct?

How are you doing your weight check? If you aren't using an empty tank to do your weight check, then are you taking into account the remaining gas in the tank?

FYI, you seem to be using a reasonable amount of weight...but the only way to really tell if it's the appropriate amount of weight for you is to do a proper weight check. Part of doing a good weight check is ensuring that you aren't kicking during the weight check and that your wetsuit and BCD aren't hiding any bubbles.

 

[idocsteve]

Sounds like you have enough weight, and you're exhaling completely, all your numbers are good, calculations fine..

Yet you jump in and you cannot sink below eye level..hmm..

I'm going to think outside the box on this one.

Is it possible that the water is very shallow?

 

[BB1]

I last did a proper weight check with an empty tank and a 3mm wetsuit and needed 18lbs (6 weights) Figured 1 more for a 5mm.

Next trip out I'll use some clip weights instead of all on the belt and do a weight check. Trouble is that will be a check against a rental jacket BCD.

The example I used in the OP is with 50 bar left in the tank which is what I should be ending the dive on.

I haven't bought my own gear yet but have some $$'s burning a BIG hole in my pocket - thankfully sensible enough to slow down and make sure I try and get the right good gear first time.

 

[BB1]

idocsteve



Thanks for the

 

[Bubbletrubble]

I last did a proper weight check with an empty tank and a 3mm wetsuit and needed 18lbs (6 weights) Figured 1 more for a 5mm.

Why guess?
If you change your wetsuit, do a weight check.
If you change your BCD, do a weight check.
If you change your tank...well...you get the picture.
It only takes a couple of minutes to a proper weight check.

You can do a good weight check with a full tank. Figure out your weight requirements with the full tank and then add enough extra lead to compensate for the loss of gas at the end of the dive. For a full AL80, this is about 5-6 lbs.

Hope this helps...