Weighting - Converting to Fresh water and Steel

[Conor]

I had my first UK and fresh water Dive courtesy of the UK Wreck Ferrets on Saturday and made a surprise discovery. Previously when I used my standard kit in the Red Sea and 12L Al I have needed about 8kg. So I needed to figure out how much weight I would need in Fresh water and with a steel tank but otherwise exactly the same gear. I was thinking -2kg for Fresh and -2kg for Steel giving me around 4Kg as a starting point.

When I went there to hire the weights I thought I would ask the guy in the rental area what his guess would be based on the above, he reccomended starting with 6Kg. So I went along with this thinking I would do a check after the first dive with a light tank. First dive, begin Descent, air out jacket, bottom rushing towards me, I end up struggling to equalise and get air into my wing fast enough.

After I the dive I drain my tank down to about 60Bar (was getting too many strange looks due to the jacuzzi effect to go to 50, especially as Clootie was doing the same) and start doing a buoyancy check.

I start off by dropping 1 kg, then another, then two, then another one, then the remaining one and much to the surprise of Clootie, Rage and TX100 not to mention the group from the school standing near by I discover that even with no weight in a 5mm Semi I sink quick.

Is a Dumpy 12L Steel considerably less buoyant than a Long 12L Steel ? I could belive losing 4 or 5 kg moving to fresh with steel or even 6 if you subscribe to the Red Sea needing more lead theory......but losing all 8 ?

Any solutions would be appreciated.

Cheers

Conor

 

[Walter]

Take the dry weight of you and all your gear (including weights & tank) and multiply by .025. That is the amount of weight you'll need to subtractwhen moving from sea to fresh water assuming all gear remains the same (including tank).

To compensate for your change in tanks, you'll need to know the buoyancy characteristics of the two different tanks. Look here for tank info.

 

[Conor]

Thanks for that Walter.

The salt to fresh bit is easy 125Kg X 0.025 = 3.125kg max so 8 drops to about 5.

As for the tank, although I don't know the specific make of the tank (it was a rental), it looks like the most buoyant empty Al tank is about +3.8 lbs, the least buoyant Steel is -6 lbs, therefore about a 10lbs or 5Kg difference.

So this would give a max difference of about 8 kg which I guess would make 0 weight within reach (depending on the tank). It still seems weird to me though. being slightly negative with no weight in a 5mm Semi and 60Bar in the tank.

If nothing else it means I would have air in my wing at all times during a dive, including at the safety stop with a near empty tank.

Is it normal to have an unweighted diver like this ? I can't say I like the idea of having no ditchable weight.

 

[Walter]

I can't say how usual it is for others, but I usually dive with no weight.

 

[Epinephelus]

Tank buoyancy charts available on the internet are notoriously inaccurate. Measure 'em yourself. Just as an example, the difference in my AL80 and my Steel 95 is twelve pounds at 500 psi. (+1 / -11, regulator included)
E.