One to make you think.

[Kim]

So lets see how you do! First off, I'm using metric values as that's what I dive - the principles are the same however.

1st Dive. I dive a 14 liter steel tank. I have 5 kilos on my weightbelt which makes me completely neutral at the safety with wing completely empty.

2nd Dive. I switch to a 10 liter steel tank. It weighs 4 kilos less than the first tank when full.

Question. What change do I need to make to my weights? If there is no clear answer from the information above, why not?

 

[Halthron]

I'll take a stab at it.

The second tank weighs four kilos less than the first tank, when full. The second tank displaces four liters less water as well. I'm used to imperial but I think these pretty much wash, 1 liter of seawater should be about a kilo.

The only thing left is to compare the remaining air in the tank. 500 PSI in a larger tank is more air than 500 PSI in a smaller tank. With air weighing about 1/2 kg per liter at 3000 PSI (yes, I'm mixing standards), it'll weigh about 1/12 kg at 500 psi per liter. Difference of four liters = 1/3 kg.

The answer would be to remove 1/2 kg.

Now, to find out where I screwed up

 

[jonnythan]

We don't have enough information. We either need the direct buoyancy figures on the tank (or use the displacement difference to calculate it).

 

[onfloat]

What's the bouyancy of the tanks when they are empty?

 

[Kim]

What's the bouyancy of the tanks when they are empty?

I didn't know. I had to make a rough estimate as to what the difference would be with only the above information.

 

[Jamdiver]

This might sound simplistic, but I think all that's necessary is for you to add 4 kilos on the second dive to compensate for the weight difference of a full 14 liter steel and 10 liter steel tank.

In essence you would be starting the second dive with 4 kilos less, so I think it would be necessary to compensate for this with the added (4kg) weight, providing that all your other kit stays the same.

Obviously the volume of the 10 liter tank is less than the 14 L tank and hence less air would be available for consumption during the 2nd dive. So the swing in buoyancy would be less.

Is this a real world situation Kim or just a test? If it were real world I'd add between 3-4 kg on my weightbelt to compensate..

I'm sure i've missed something here, but really everything else is constant just the change in weights of the tanks and displacement of the tanks, which I admit I haven't fully accounted for.
But like I said if it were real world i'd compensate as I noted above.

Kim, have you seen the mauling that Australia has been giving the West Indies?
An even more thoroughly depressed fan now

Oh and post #500 for me, hehe

 

[jonnythan]

The theoretical answer and the real answer are two very different things, so this exercise is almost exclusively academic, particularly if you don't know the specs of the tanks in question. It's total guesswork unless you have accurate information... and even then it's not very good.

The real answer is to wear the new tank and do a weight check.

 

[do it easy]

I'm not familiar with metric units or tanks, but I do know that they are measured in water volume. So if you assume that the wall thickness is negligible/proportional then the 10l tank has 4 liters less volume than the 14l, this will make you less buoyant by 4 liters.

Now the tank weighs 4 kgs less, so in essense, you have less weight which increases your buoyancy by 4 kgs.

So far, this approximately cancels each other out (1 liter water ~ 1 kg).

The remaining factor is the weight of the gas in the tank- if you assume a minimal amount of gas at the safety stop, then the weighting should be about the same. If you did one safety stop with a nearly empty tank and the next dive with a nearly full one, then the difference will be the weight of 10 liters of compressed gas, whatever that is. You have to add the weight of 10 liters of gas to be able to hold your stop.

The one approximation that I'm uneasy about is the 1 liter water ~ 1 kg. In the real world, it might be close enough to weight people, but within the scope of this problem it could make the difference between adding/removing weight. If the 1 liter of water weighs more than 1 kg, then you'll have to remove weight, and vice versa.

So how does that sound so far, and what does 10 liters of gas weigh? It will be the difference between the empty weight of the 14 liter tank and the empty weight of the 10 liter tank minus the 4 kgs (assuming that the 4 kgs difference is between an empty 14 l and a full 10 l). Now all we need to know is the specs on the tank. Does all this sound reasonable?

I edited this to make this academic problem more applicable to the real world. Pretend that you do the first dive with a 14 L tank and then have to change tanks and reweight for the second dive. And I also botched the part about the weight of the gas.

 

[Kim]

OK. First off this actually happened to me two weekends ago in Amakusa in Japan.

Basically those that have noticed that 1 liter volume of tank is roughly 1 kilo are on the mark. There can be slight discrepancies with different makes of tank - although in Japan they all tend to be from Asahi - but a 10 liter tank will displace about 4 kilos less than a 14 liter therefore making it less bouyant by that amount. As the real weight difference is 4 kilos these cancel each other out. Most places in Asia use 50 bar as their end dive amount of gas. That's 500lt in a 10lt tank (at 1ATA) - and 700lt in a 14lt. The difference of 200lt of air weighs 0.2408Kg. Basically you don't need to change a thing and you have a good chance of being close to the correct weight, and at a weight you can control with your lungs.

I was doing the dives off a boat in reasonably bouncy conditions. Doing a weight check wasn't really an option. Actually I didn't really think it through properly at the time - added 2 Kg - and dropped like a stone! :11: I was rather surprised at the time and sat pondering my miscalculation for a while afterwards. When I realized how I should have thought about it I felt a bit silly! Still - this one won't catch me out again.

By the way - in case you are wondering why there were two different tanks......the 14lt is mine....the 10lt was from the shop (it saved a little bit on the price as I can get cheap air fills).
Actually my tank also allowed us to dive a wreck that we wouldn't have be able to do otherwise. It gave me enough gas to search for and tie the boat into it so that the other divers didn't have to waste any time or gas with the smaller tanks and still have enough gas over to do the dive with them with plenty to spare.

 

[CIBDiving]

If there is no clear answer from the information above, why not?

because I never was any good at L3 -> ft3 conversion!