Gas Planning and Dissimilar Tanks

[loquat149]

Gas Planning and Dissimilar Tanks

Several years ago someone posted a spreadsheet table of SCUBA cylinder specifications to The Deco Stop. Unfortunately I can't remember who that was, but I'd like to thank them now for sharing it with the technical diving community. I've modified their table a little and keep a laminated version of it in my wetnotes for planning purposes while we go over our in-water pre-dive checklists. Please feel free to modify this version as you see fit (e.g., add or delete cylinder sizes) and/or print out the PDF and add it to your underwater notebook.

Here's an example of how I use it to calculate turn pressures when diving in teams with dissimilar cylinder sizes and/or pressures. Let's say you're diving double steel LP-85's with 3600 PSI and your buddy has double steel LP-104's with 2900 PSI. (He just drove up from Miami where he couldn't get a proper cave fill). Refer to the Steel-85 column and note you have 232 cu ft of gas when your doubles are filled to 3600 PSI. Refer to the Steel-104 column and note your buddy has 228 cu ft of gas when his doubles are filled to 2900 PSI. Your buddy has less volume of gas than you, so his gas supply will determine the "team third". (Note I cleverly avoided using the phrases "my third" and "his third"). Since your buddy has 2900 PSI, he's going to round down to the next pressure that's evenly divisible by 3, which is 2700 PSI. He's going to take 1/3 of 2700 PSI (i.e., 900 PSI) off of his total cylinder pressure (i.e., 2900 PSI) to determine his turn pressure, which is 2900 - 900 = 2000 PSI.

Refer back to the table and note that 900 PSI yields 71 cu ft of gas in his double Steel-104's. Now find your turn pressure by reading the Steel-85 column. Note that 71 cu ft of gas corresponds to 1100 PSI in your double Steel-85's. So, take 1100 PSI off of your total gas supply to determine your turn pressure: 3600 - 1100 = 2500.

Your buddy's turn pressure is 2000 PSI.
Your turn pressure is 2500 PSI.
The team third is 71 cu ft of gas.

Do this a few times and you'll notice the following pattern holds for teams of divers using LP-85's, LP-95's, and LP-104's. All things else being equal, if the 85's diver needs to take, say, 1200 PSI off, the 95's and 104's divers would need to take 1100 and 1000 PSI off, respectively to match the same volume of gas.

Obviously there are several alternative methods of doing this, some involving actual math. Use the one YOU'RE most comfortable with.


http://dl.dropbox.com/u/54067345/Dissimilar_Tanks.pdf
http://dl.dropbox.com/u/54067345/Dissimilar_Tanks.xls

Dave

[Angry Turtle]

right on! Thank you

[rddvet]

Thanks Dave. I've been using tank factors but honestly i suck at it and sometimes forget how to do it till i realize i screwed it up a couple of times.

I think im going to stick that in my wetnotes too.


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[bpcfire1]

http://www.cavediveflorida.com/gas_match.htm

Is that the spreadsheets you were talking about?

[loquat149]

http://www.cavediveflorida.com/gas_match.htm

Is that the spreadsheets you were talking about?

Nope, it was an *.xls file. Looking at the original file's properties I see the author was Harry Averill. Thanks Harry!

Dave

[RN]

Or a real simple method would be what I call my rule of 10. This can only be done LP to LP or HP to HP. But if you know the equivalent tanks you can do LP to HP. For example, LP85s and HP100s are pretty close in volume when filled to 3600 psi so they are essentially equal. And LP95s and HP130s are pretty close in volume when filled to 3600 psi as well.

My rule - take the difference in rated cubic feet between tanks and multiply by 10. That's the difference in turn pressures. You subtract that much from the larger tanks.

Using your example. 85s and 104s are 19 cf difference. 19 x 10 = 190. Round up to 200 psi. The 85s can use 1200 psi which means the 104s can use 1000 psi. Using 1000 psi violated 1/3s though so drop both by 100 to get 1100 and 900 as penetration pressures. No table needed.

[Squirrel Girl]

Or a real simple method would be what I call my rule of 10. This can only be done LP to LP or HP to HP. But if you know the equivalent tanks you can do LP to HP. For example, LP85s and HP100s are pretty close in volume when filled to 3600 psi so they are essentially equal. And LP95s and HP130s are pretty close in volume when filled to 3600 psi as well.

My rule - take the difference in rated cubic feet between tanks and multiply by 10. That's the difference in turn pressures. You subtract that much from the larger tanks.

Using your example. 85s and 104s are 19 cf difference. 19 x 10 = 190. Round up to 200 psi. The 85s can use 1200 psi which means the 104s can use 1000 psi. Using 1000 psi violated 1/3s though so drop both by 100 to get 1100 and 900 as penetration pressures. No table needed.

I've lost something in your logic. Why does using 1000 psi violate 1/3rds?

[loquat149]

I've lost something in your logic. Why does using 1000 psi violate 1/3rds?

The 104's were only filled to 2900.

Dave

[Squirrel Girl]

The 104's were only filled to 2900.

Dave

Ah, there you go. It was in an earlier post. I've got too many things cramming for attention in my brain to remember everything in all the posts!

[loquat149]

My rule - take the difference in rated cubic feet between tanks and multiply by 10. That's the difference in turn pressures. You subtract that much from the larger tanks.

Thanks Rob, I like that.

Dave