Weight adjustment when changing tanks

[john-the-mason]

Hi
I often dive in the Red Sea. Generally speaking, the centres i use have 12L and 15L alloy tanks (this is a designation, I know internal volumes may not reflect this exactly). Sometimes on a more demanding dive or due to low fill or tank problem it may be necessary to switch to a 15L tank without the opportunity to do a buoyancy check. I do not change my weights yet I hear people advising form one extreme - take off 2 Kg because it is heavier and PUT ON 2Kg because it is lighter when nearly empty. The best informative site I have found on the subject is :-

weight adjustment when changing from 15 litre to 12 litre scuba diving tanks

It seems to reinforce my argument - does anybody else have any advice or comments an the article

Thanks all

 

[spectrum]

Holy crap, talk about strangling an issue.

Your total air supply, consumption and most of that feature are irrelevant.

I think towards the end he has calculated his way to EMPTY BUOYANCY which is the property you need to compare ending with the statement "If you empty the tank the 15L tank is 0.6Kg heavier than the 12 Lt. " with .6 kg being the adjustment if you care to make it. If your weight is based on the lighter cylinder I would not bother removing. If you are going to the lighter cylinder it all depends how close you cut things.

However..... I think the queastion is oversimplified as I'm sure those volume cylinders can come from numerous suppliers with varying properties. You need to find empty buoyancy specifications for any cylinder considered compare that. Otherwise take a guess or do a weight check. Unfortunately as you mention the best check is done after the fact.

Pete

 

[john-the-mason]

Hi
Many thanks for your feedback - however - 'Your total air supply, consumption and most of that feature are irrelevant.'
I find this a strange statement as

'Your total air supply' needed based on 'consumption' is what dictates the use of a larger tank
'consumption' will dictate how much air you have left and therefore the weight of the tank/buoyancy at the end ow the dive
'You need to find empty buoyancy specifications for any cylinder considered and compare that' that is what this article does for Luxfer
?????????????

 

[hellboy888]

Never add weight extra weight in order to balance Trim,better switching to thinner booties.
check your fin weight and patch..consider not using any other accessories.

 

[john-the-mason]

Never add weight extra weight in order to balance Trim,better switching to thinner booties.
check your fin weight and patch..consider not using any other accessories.

Hi Hellboy888
Thanks for your reply - but not really sure how relevant it is

 

[Croomrider]

I would not bother changing my weight for those two tanks since the buoyancy when empty is only .6 kg difference. If it was 2-3 kg difference like in switching from some steels to aluminum, then I would adjust.

Mike

 

[Steve_C]

Hi
Many thanks for your feedback - however - 'Your total air supply, consumption and most of that feature are irrelevant.'
I find this a strange statement as

'Your total air supply' needed based on 'consumption' is what dictates the use of a larger tank
'consumption' will dictate how much air you have left and therefore the weight of the tank/buoyancy at the end ow the dive
'You need to find empty buoyancy specifications for any cylinder considered and compare that' that is what this article does for Luxfer
?????????????

Question was not about why you choose a different tank. Maybe you like the color, it does not matter. Question was about weighting.
The point is for a given tank, the dive may not go as you plan. No matter what your consumption rate, reason for choosing the tank, etc., the key thing is to make sure you are not positive when the tank is nearly empty and you are at a shallow depth. Note this is not just the weight difference of the tanks since they have different volumes. So the question is simply when empty, in the water, what is the change of buoyancy of the two tanks.

 

[drinksfromtap]

In my short time diving I've used AL-80s from an unknown manufacturer, Worthington HP-100s, Worthington HP-120s, and a Luxfer HP 120, all of which have different buoyancy characteristics. I've used this chart Scuba Cylinder Specifications from Tech Diving Limited - 928-855-9400 and adjusted my weighting by the difference in empty buoyancy between a tank I've done a buoyancy check with and an "unknown" tank - for example, switching from a Luxfer Al-80 to a Faber HP-120 I'd subtract the buoyancy of the known tank (+4.4 lb) from the buoyancy of the HP-120 (-7.2 lb) to get the total change in weight (take off about 11.6 lbs). In other words,

(tank i'm using buoyancy) -- (tank I know buoyancy) = (change in lead).
-7.2 lbs -- 4.4 lbs = -11.6 lbs

In practice, I've found that I've been able to shed a few extra pounds going from aluminum to steel than I would have estimated, which could be because of my not really having optimized weight placement to get the best trim (particularly with the aluminum tanks).

 

[john-the-mason]

Many thanks, in particular:-

Croomrider - succinct, to the point - brief - cheers

Steve_C yes - exactly my question - and consensus seems to be with alloy tanks in general and Luxfer in particular - leave your weight alone

Drinbksfrom tap - honestly I would avoid any change that involved altering my weights over 11 pounds. And TAKE off 11+ pounds! I only dive with 8 pounds in the summer !! Do I need to diet between dives Are you sure this is correct

Thanks again guys

 

[drinksfromtap]

I'm sure it's not taking off too much because I've dived it but to be fair, the tank I use in my example is really negative compared to most others and it wasn't really a good example for your situation. If I was using that chart I linked to for you switching between an 80 cf (close to 12L - about 2 kg positive when near empty) aluminum tank and a 100 cf (close to 15 L, about 1.4 kg positive when near empty) aluminum tank, I'd calculate you could drop maybe half a kilo, which may not be enough to bother with depending on how tuned in your weighting is.

You'd need to put on a bunch of body fat instead of dieting if you'd want to drop 11 lbs