Gas Management on Double-Tank Side-Mounting

[Aquabot]

Hello,

I am interested in getting into side mounting in the future. Thus, I have begun self-teaching myself side-mount by watching a lot of online video demonstrations, course outlines, and power point uploaded by scuba instructors. Of course, I will take a side mount course in the future.

However, I am currently having quite a bit of difficulty with Gas Management and the Rule of Thirds. I am learning these two skills from power points online. (Sidemount) (Padi Sidemount Diver Course Presentation)

I came up with my own test question.

Tank A: 3000 PSI
Tank B: 3200 PSI

Tank A's turn around time: 2000 PSI (3000 PSI divided by 3 = 1000 PSI. 1000 PSI x 2 = 2000 PSI)

I breathed 300 PSI to balance Tank A and Tank B. (A question through, on both power point examples, the power point taught me to breathe 600 PSI to balance both tanks. But wouldn't breathing 600 PSI make it unbalanced because Tank 1 would be 2700 PSI while Tank 2 would be 2600 PSI?)

2000 PSI - 300 PSI = 1700 PSI

1700 PSI / 2 = 850 PSI

3000 PSI - 850 PSI = 2150 PSI

So, 2150 PSI is my turn around point?

So, hypothetically speaking, I start with 3000 PSI. I use 850 PSI to get to my destination, leaving me 2150 PSI. I then use another 850 PSI to go back to wherever I came from, leaving me 1300 by the time I surface.

Another test question.

I came up with my own test question.

Tank A: 2800 PSI
Tank B: 3200 PSI

Tank A's turn around time: 1800 PSI (2700 PSI [2700 is divisible by 3] divided by 3 = 900 PSI. 900 PSI x 2 = 1800 PSI)

1800 PSI - 500 PSI = 1300 PSI

1300 PSI / 2 = 650 PSI

2800 PSI - 650 PSI = 2150 PSI

So 2150 PSI is my turn around pressure!? That just doesn't feel right. Ok, let's say I start the dive with 2800 PSI. I breathed 650 to get to my destination, leaving 2195 PSI. I then use another 650 PSI to go back to my starting point, leaving 1545 PSI by the time I surface. That is 1545 PSI. I can still go to my destination and back AGAIN and still have 245 PSI left.

Did I do something wrong or having that much gas by the end of the dive the whole point of Rules of Thirds.

I know I'm really late in replying to this and I'm sure somebody is already mentioned it, but working pressures in psi is a waste of time. You need to convert psi to cubic feet and do your math there for your gas management.

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[ffdiver]

I'd take a copy too if you don't mind. Goodlifedivers @ gmail.com

 

[ColdH2O]

Can you fire me a copy as well? dclerm@yahoo.com

Thanks

 

[rongoodman]

If the tanks are unequal, i'm going to use thirds based on the smaller of the two, so if one has 3000 and one has 3300, at the turn, one will have 2000 and the other will have 2300. If you lose the larger tank at the turn, you're only going to have that smaller third remaining to get out with, so if you used any of the extra gas in the larger tank to penetrate farther, you're screwed. (Realistically, I wouldn't use a full third to penetrate, it's just cutting things too close.) I don't find balancing tanks to be all that fussy, at least not with the Al80s used in Mexico, so I just switch about every 500 psi.

 

[Akimbo]

...I'd be glad to send this document to anyone if they want it. It's in PDF format and I keep it on both my tablet and phone for quick reference.

You can post an Acrobat/.pdf file just like an image file. For example:

 

[Doppler]

Has anybody gotten the PDF? I have not.

 

[victorzamora]

Has anybody gotten the PDF? I have not.

Nor have I

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[tracydr]

What gets fun is when your buddy has back mounted doubles, and you have sidemount tanks, and yours are larger/smaller than your buddy's tanks, and of course you have unbalanced volumes in your own side mount tanks. Then throw in buddy number 3 that has different volume sidemount tanks than yourself...

I wrote up simple to understand "how to" on tank matching, figuring thirds, etc that I had to present to someone for my Tech DM (NAUI TSL) course. I write in a way that even your 90 year old grandmother could figure it out. (I spend a lot of time writing very technical things for non-technical people at work)

I regularly dive with both sidemount and backmount divers, and almost never with the same volume tanks between myself and my buddy, so I end up doing a lot of math every dive.

I'd be glad to send this document to anyone if they want it. It's in PDF format and I keep it on both my tablet and phone for quick reference.

I would love to see a copy. Tracy@doctracy.org

 

[Doppler]

Well, my X-Ray magazine article is still up. X-Ray Mag #56 | X-Ray Magazine

 

[Scott]

Has anybody gotten the PDF? I have not.

I have a copy. I'll forward it on.