New - At what depth and with how much air do you weigh yourself neutral

[The Iceni]

Liquid once bubbled...
Everybody knows that at 1 M you need more weight than on 5 M.

I am afraid I do not follow, liquid. Granted the pressure changes are greatest close to the surface so a diver has more difficulty in controlling the bouyancy in his air cell(s). However, as I see it, this does not mean he needs significantly less weight at increasing depths even when diving with a neoprene wetsuit or semi-dry.

In any case, like Genesis, I would not wish to be at all bouyant at any depth (particularly above 10 feet/3 metres at the end of a dive) which is why, I believe, the diver should aim for neutral bouyancy just under the surface with an almost empty cylinder/set, whether he dives wet or dry.

To my mind this is planning for the worse case scenario;-

The diver may need to do an extended 10 foot/ 3 m stop. This is very likely if he inadvertently overstays his sojourn on the bottom consequently using more gas than he planned. He will be unable to do any (or all) of the stop if he has anything less than 500 psi/50 bar in his cylinder at 10 feet/3 m if he has weighted himself to be exactly neutral there with 500 psi/50 bar. The weight of 50 bar of 32% Nitrox in a 12 litre cylinder is over a pound (0.6 Kg).

If the diver's SAC is an excessive 20 litres per minute, 50 bar in a 12 litre cylinder (600 litres) is enough gas for a diver to breath at 10 feet/3 m for 23 minutes (600/(1.3 * 20)). However, the diver will be unable to use this reserve for its intended purpose at such a stop if he is at all bouyant, and does not have an anchor line on which to hold.

A diver can always add bouyancy "on the fly".

He cannot add ballast!

 

[chrispete]

...otherwise you risk an uncontrollable ascent from your safety stop.

 

[nickjb]

With an empty tank at 5m. If you are getting very low on air the last thing you want is to be fighting to stay at your stop depth. I always plan to finish with 50 bar but if there is a problem its good to be prepared.

Its best to use maths rather than an empty tank to calculate the weight required. Do a buoyancy check with air in the tank then add weight equivalent to the weight of the air.

Does anyone have a good formula for the mass of air (preferably in metric)? I only have 'ball park' figures for a 12l tank.

 

[The Iceni]

nickjb once bubbled...
Does anyone have a good formula for the mass of air (preferably in metric)?

Hi nickjb,

How about The universal gas equation ? which is

PV = nKT

Where n is the number of mols, K is the universal gas constant (0.0831) and T is the absolute temperature.

Thus T = 273 + t degrees C. At 20 degrees C, T = 293

So at 20 degrees C one mol of air (n =1) at one bar (P=1) occupies;-

V = KT = 0.0831 x 293 = 24.35 litres.

Molecular weight of oxygen = 31.99 g/mol
Molecular weight of nitrogen = 28.01 g/mol

So the effective molecular weight of air is (0.791 x 28.01)+(0.209 x 31.99) = 28.85 g/mol and it weighs about 1.2 g/litre (at 1 bar).

Back to the original equation;

PV = n x 24.35, so n = PV/24.35

When P = 50 bar and V = 12 litres n = 50 x 12/ 24.35 = 24.64 mols

each of which will weigh 28.85 g

The weight of air in a 12 litre cylinder at 50 bar is 24.64 x 28.85 = 711 grams. ut:

And the weight of 100% oxygen in a 12 litre cylinder at 50 bar is 24.64 x 31.99 = 788 grams,

From this you can estimate the weight of air in a 12 litre cylinder at 232 bar (2,785 litrs) to be about 3.3 Kg. (100% Oxygen 3.7 Kg and pure helium 460 grams).

easy peasy!

(Note that the universal gas equation becomes increasingly inaccurate at high pressures and predicts more gas (and more weight) that there actually is. This is particularly so with helium, which will be about 75% of predicted in a 300 bar cylinder. In fact oxgen does the opposite, you get more for you money!

In consequence Trimix blending by partial pressures is a real headache. )

 

[nickjb]

Thanks doc.

I found this link:

http://www.subaqua.co.uk/cgi-bin/cylinder-buoyancy.cgi

which seems to do it all for me.

I might write a Palm app using your maths, though, so I can calculate on the fly.

 

[JeffG]

I do my weight check on the surface with a full tank to find the point of neutral bouy, then add 4-5 lbs to compensate for the air that I will "consume" during the dive.

To me, this is the easiest because you can do it on every dive before you have weight problems during the dive.

 

[DiverBuoy]

Ah the PADI way