Dissimilar tank volume calculation....made easy???

[ScubaStingRay]

Awesome chart! I'll be printing that for diving my 85s with my brother's 108s (okay, my 108s).

[kensuf]

If you're mathematically challenged then use the chart. It's simple and quick.

In this graphic Diver A (red) is on 85's and Diver B (blue) is on 121's, both are filled to 2700 psi.

Diver A's usable thirds is 900 PSI. They can both look up on their charts to see how many cubic feet 900 PSI in double 85s is, and they see that it is 57 cubic feet.

This means that diver B can use NO MORE than 57 cubic feet for penetration, but he can use less than that. So he looks up on his chart to find the column in his row that matches up with NO MORE THAN 57 cubic feet. It turns out to be 600 psi.

So.. Diver A can use 900 PSI. His turn pressure ("Critical Air Supply") is 1800 psi.

Diver B can only use 600 PSI for penetration. His CAS is 2100 PSI.

And then, to make it very easy, they can write their CAS on their wrist slate so they do not forget it.

[kensuf]

And there's no shame in being mathematically challenged. I have a friend that has a hard time calculating thirds due to dyslexia, but otherwise he is a solid diver.

[ScubaStingRay]

Sometimes it's about ease, knowing that you don't have to double check your mental calculations. I can do mental math pretty well, but that won't stop me from pulling up excel every chance I get.

[Mark Vlahos]

I made a very comprehensive chart that I posted on ScubaBoard several years ago. The link over there is dead. I am also very mathematically challenged so making the chart was a lot easier, and a lot more accurate than trying to do it in my head.

I printed an edited copy of my chart. Use a laser printer, not inkjet. Laminated it and stuck it in my wetnotes.

My primary buddy dives the same size tanks as I do, so no problems there. Most others that I dive with use tanks a little smaller, and I know from experience that the pressure difference to match gas volume ends up at 100 psi fewer on my gauge. The other diver with the smaller tanks may be able to consume 1,100 psi, while I can only consume 1,000 psi from my larger tanks.

Mark Vlahos

[FW]

Sorry I missed this thread. I was out of town.

FWIW, when I put the warning about different size tanks in the first NSS-CDS Cave Diving Manual, I never intended it to be a math problem. It was mostly to let people know it is best for everyone on a team to wear the same size tanks. It was Sheck Exley, PhD in Mathematics, that added all the calculations, and Brent Booth continued the concept in the current manual.

[Brent]

Sorry I missed this thread. I was out of town.

FWIW, when I put the warning about different size tanks in the first NSS-CDS Cave Diving Manual, I never intended it to be a math problem. It was mostly to let people know it is best for everyone on a team to wear the same size tanks. It was Sheck Exley, PhD in Mathematics, that added all the calculations, and Brent Booth continued the concept in the current manual.

Now that my name has been mentioned, I'll jump in here with some comments. When I wrote the current process for tank matching, it was back in the stone age before we all had the electronic toys that are so common today. Mark Leonard from Dive Rite actually asked me to come up with a "talking paper" of sorts because in those days ( early 80's time frame ) there was little to no info on dissimilar tanks. We were diving at Ginnie and the guys beside us on the picnic table were trying to figure out how to match up big tanks and little tanks and that's how it all began! So to sum all this up; the process for tank matching is an integral part of your cave training ( or it should be ) and even though in this day and age, you can pull up dozens of spread sheets on the Web to figure out your turn pressure, I believe all divers should still be able to do the calculations at the dive site using some of the "stone age" methods that are in the Cave Diving Manual. One other point to ponder. Back in the day, most of us dove 104's or maybe 95's and as often as not the turn differential might not be much more that a couple of hundred PSI. Today, there is a multitude of different size tanks on the market and you just might meet up with someone at the dive site that wants to join in with you, and they have tanks that could be vastly different in capacity from what you're diving, and your PC is back in the hotel! There you have it!
One last item to mention. I HATE math and probably most of you guys and gals do as well. Hit those books!!

Brent

[stickydeco]

Just subtract 100psi off your penetration gas if you're diving 1 size up from your buddy. For instance, 104/108/112/130s at 3600psi would have 1200-100 psi usable (1100) if the buddy had 95/120's. The rounding error gets to be a bit much if you start comparing 130's to 100's, but for one size up or down it's conservative enough that unless you're diving straight 1/3rds it's a non issue.

Some good advice James. You can see from the attached that after a few runs of all that math crap, I came up with the same suggestion in the attached guide. A number of new folks have recently found it helpful so I am including it here: https://www.dropbox.com/sh/h4juiw0cr...mN6dxXGnUyR7ca

[PeterBuzzacott]

One of my sidemount tanks was not filled by accident but the other was. They are both the same size. When I got to the dive site I added the two pressures together, then divided by three. That was my turn pressure.

You can do the same for two buddies wearing doubles, like I did for two tanks. If they are not wearing the same size doubles then just convert what you've both got into Cubic feet, or litres, add them together, divide by three. That's the number you need to turn with (on thirds). First person to reach that volume calls the turn. Once you know the turn volume then you both work out what pressure that means for your particular doubles (if they are different sizes).

So, my suggestion is, if the tanks are all the same size then add the pressures together and divide by three. If they are not the same size then convert to cubic feet or litres, add them together and divide by three. Then convert that back to pressure for your particular tanks.

Please do not do this for multiple buddies, multiple tanks or for fourths or fifths. It works for two tanks, or two buddies in doubles, turning on thirds.

[Dominick Gheesling]

If you're mathematically challenged then use the chart. It's simple and quick.

In this graphic Diver A (red) is on 85's and Diver B (blue) is on 121's, both are filled to 2700 psi.

Diver A's usable thirds is 900 PSI. They can both look up on their charts to see how many cubic feet 900 PSI in double 85s is, and they see that it is 57 cubic feet.

This means that diver B can use NO MORE than 57 cubic feet for penetration, but he can use less than that. So he looks up on his chart to find the column in his row that matches up with NO MORE THAN 57 cubic feet. It turns out to be 600 psi.

So.. Diver A can use 900 PSI. His turn pressure ("Critical Air Supply") is 1800 psi.

Diver B can only use 600 PSI for penetration. His CAS is 2100 PSI.

And then, to make it very easy, they can write their CAS on their wrist slate so they do not forget it.

Struggling through this as we speak I didn't take enough notes to explain how to use the chart. Glad I found your post!