Weighting

[olphart]

This is one of those questions that I think I know the answer, and may sound like a muppet even posing it, but here goes:

Nomad LTZ, Bare XCD2, Thermal fusion undies, lp85's, 10lb in the spine pockets, 6lb in shoulder pouches, fresh water shore dive.
At the beginning of the dive, empty the suit, close the valve, empty the BC, exhale and sink..VERY slowly. I was wondering if I even had enough weight. Felt I might have had too much weight at 30 or 40ft. Trim sux, foot heavy even when giving the rig a yank to move things up.
That aside, once I brought the tanks down to about 700lb, I had plenty of weight to hold at 15, even tried at 8ft, no problem, lots of weight. HMMM, how the hell can you be under weight, (almost) with full tanks and probably slightly over weight when empty?

 

[Razorista]

It is a normal 'rule of thump' that for the weighting to be correct you should have to fight a bit to get submerged innitially.
At the end of the dive every hidden air pocket should have dissolved and every compressable material has been squeezed enought to completely fill with water.
Weigths should never 'sink' you, you should do that purposefully with controlled propusion and breathing.

 

[Bennno]

It is a normal 'rule of thump' that for the weighting to be correct you should have to fight a bit to get submerged innitially..

No, it's not a rule of thumb or any rule. It makes no sense. Due to the weight of the gas, you will sink without a 'fight'. With full lp85s you start the dive 'overweight' as you do with any other tank. The more gas you carry, the heavier you will be at the beginning of the dive.
If you have to fight to decent, you have too little weight. You need to be able to stay at your last stop depth with almost empty tanks.

@olphart
Sound like you didn't really empty your suit at the beginning of the dive.

 

[olphart]

That has to be the case. I was pretty sure I had emptied both suit and BC, but nothing else makes sense. My descent rate does accelerate pretty quickly if I don't get gas in there smartly.
Thanks to both of you.

 

[rongoodman]

That might be a "normal rule of thumb' for a single Al 80(not for me, it isn't), but the author is discussing double LP85s. That's nearly 14 lb of gas. If something happens and he ends up having to finish a dive sharing gas in a cave or on deco, being underweighted to that extent could be a disaster.

 

[Razorista]

No, it's not a rule of thumb or any rule. It makes no sense. ...

Of course it is!
That's one of the reasons many people experienced in frequent equipment change or sidemount tryouts recommend doing the weight check 'at the end of the dive' instead of saying 'enter the water with an empty tank'.

In a wetsuit it may be knowlege of debatable value, in a drysuit knowing this is essential to even approach correct weighting.

It also does not depend (much) on tank weight or gas-volume, except in the margin of error involved - it is still garanteed to be slightly off almost every time with almost every tank.

In sidemount another factor is added to the already complicated situation:
The weight of gas will pull the tanks down, not the diver.
Only in a well trimmed and weighted setup this will transfer completey, otherwise you get low hanging tanks dangling beside or below a floaty body.
Instinctive fin movement will negate a lot of weight then, much more than 4+ lbs of gas weight occasionally.

Thanks to both of you.

Welcome

 

[olphart]

Time for me to leave.

 

[MMM]

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[MMM]

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