High SAC rate and low SAC rate

[Pavão]

Sorry, didn't see Sabado's response before I posted

[steve2281]

Yes it does eliminate this as an issue.

As noted above, with proper gas matching, big tanks or small tanks does not matter, everyone on the dive will turn the dive when/if they arrive at the predetermined pressure equal to the "third" in the smallest set of tanks on the dive. That ensures everyone is diving the same size third and that any third will provide an adequate reserve for anyone else on the team.

Personally, I think thirds is pushing it too far in a low flow cave, so I'd scale it back another 100-200 psi from true "thirds" anyway.

Agreed. To state it another way:

Say the small doubles are filled to 240 cu ft
The larger doubles are filled to 400, 500, 600, or 1000 cu'. It doesn't matter.

Each diver has no more that 80 cu' to use before turning. Convert volume to PSI. The diver with the higher SAC will always turn the dive if proper gas matching was done.

[gschaut]

Gary, exactly how high are you filling these cylinders?

Just trying to keep the math simple to understand. The example I quoted would be 2640psi tanks filled to 4000. Have never done this myself, but have heard tails of 4K cave fills. Just be careful not to hit too many potholes on the way to the dive.

My basic point being: we do not "breath" PSI of tank pressure! We breath CUBIC FEET of breathing gas. Don't get the idea that "I have 2/3psi and my partner has 2/3psi, so we are safe".

You have to allow sufficent CUBIC FEET of breathing gas, in order to safely exit. NOT P.S.I.!!! Cubic Feet.

[Sludge]

By definition, a cave fill is 3600psi. That's our story and we're sticking to it.

[DA Aquamaster]

Just trying to keep the math simple to understand. The example I quoted would be 2640psi tanks filled to 4000. Have never done this myself, but have heard tails of 4K cave fills. Just be careful not to hit too many potholes on the way to the dive.

My basic point being: we do not "breath" PSI of tank pressure! We breath CUBIC FEET of breathing gas. Don't get the idea that "I have 2/3psi and my partner has 2/3psi, so we are safe".

You have to allow sufficent CUBIC FEET of breathing gas, in order to safely exit. NOT P.S.I.!!! Cubic Feet.

Agreed. That's why we use tank factors in gas matching.

For example, double LP 95s holds cu ft at 2640 psi. 190/2640=.07196. This means 1 psi is equal to .07196 cu ft per psi in double 95s. It's not real useful until you take that times 100 and and round a bit and now you have 7.2 cu cu ft per 100 psi.

So if my double LP 95s have 3700 psi in them, I know I have about 266 cu ft (7.2x37). Or I can round down and use 7x27=259 cu ft. I can do the latter in my head in three steps: 7x40=280, 3x7=21 so 280-21=259

at 3700 psi I will be using a 1200 psi third (or 1100 or 1200 if I want to add some conservatism). Let's say I pick 1100. 11 x 7. = 79.2, so 79 cu ft becomes my "third". (Or if I am match challenged, I can just use 7 and use 77 cu ft for the third).

My buddy has X7-100s with a tank factor of 5.8 (as a review that means 100 psi = 5.8 cu ft.

Since he has the smaller tanks, his tanks control the size of the "third". He's got a 3700 psi fill and rounding down, he'll use an 1100 psi third. 11 x 5.8 = 63.8 cu ft, so 63 cu ft "thirds".

So to ensure that I don't use a larger third than he does, and that his reserve would be enough for me to exit, I take his 63 cu ft third and divide it by my tank factor of 7.2 to get 8.75 - or 875 psi for my new "third" rather than my original 1100 psi.

If on the other hand we both dove the same 1100 psi thirds and we were both at turn pressure when we turned the dive, his reserve would be 16 cu ft less than we'd need to exit if I lost all my gas at max p.

[RN]

Here's an article I wrote on this topic a few months ago - http://chipoladivers.com/category/gas-management/. Basically, it doesn't matter who has the larger tanks or the smaller tanks, the person with the smaller tanks determines the volume used by both divers. In an ideal world everyone would dive the same size tanks and it would be simple to determine turn pressure/volume.

[DA Aquamaster]

Now there's an argument for you:

In a perfect world what tank size should we all (be mandated to) dive?

[Pavão]

Now there's an argument for you:

In a perfect world what tank size should we all (be mandated to) dive?

3.0 spare air?
And if you bring another one as back up, then you would have 4/3's just as reserve

[Jax]

Now there's an argument for you:

In a perfect world what tank size should we all (be mandated to) dive?

You're begging the question - assuming the perfect world would mandate the size tank to dive.

[aainslie]

Oh what the hell. I'll weigh in. It's been a while.

It's simple. Give the smaller tanks to the high sac diver. They'll be easier to swim. That way hopefully you'll close the gap between the two as the low sac guy has to work a little harder and air hog guy gets to goof off a little.

This is assuming the divers do what everyone suggests and calc thirds based on smallest tanks.

This is of course the exact opposite of what most people do. Go figure.