Gas Management on Double-Tank Side-Mounting

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:hijack:

So working on a Min Gas scenario, which for GUE (unlikely sidemounters, I know) is 1200 litres for a 30m dive, or 50bar in twinset, based on 2 divers breathing 30lpm. What is the correct protocol for having the gas available in Sidemount? 50 bar min in each cylinder (assuming 12l)? Although this is the same amount of gas, when all gas is accessible you can have one diver breathing 25lpm and the other 35lpm and it is balanced out. With isolated cylinders you lose this benefit, so do you push up the min gas in each cylinder or leave as is?

The hijack seems more fun... If you know one diver has an especially high SAC rate, you'd use that as the basis for your rock bottom calculation and that would decide your reserve for you, surely? If you're positing that someone might breathe even more than the breathing-more-than-expected amount, you can't really assume that'll be evened out by someone else breathing a little less in the event of diving doubles, so your rock bottom calculations are blown and it's all gone horribly wrong anyway. The whole point of doubling usual SAC rates when working out rock bottom is that it's a worst-case scenario - if you believe a team-mate will breathe faster than that then I guess your options are (a) plan reserves accordingly or (b) find a team-mate who keeps better control of their breathing when it all goes to custard :)

I tend to work out what I need for the dive, including contingencies and rock bottom, and then just carry a lot more than that. Seems to work.


GUE use 30lpm as the stressed SAC rate and 20lpm as normal. With min gas based on the stressed SAC. Now stressed SAC rates can massively spike, which is an issue in either manifold or independent tanks. I just wondered what the SOP is for sidemount.
 
GUE use 30lpm as the stressed SAC rate and 20lpm as normal. With min gas based on the stressed SAC. Now stressed SAC rates can massively spike, which is an issue in either manifold or independent tanks. I just wondered what the SOP is for sidemount.

Same... the initials on your cert card do not effect basic physiology... only affected psychology.
 
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'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'd like to read it if you wouldn't mind! Thx
 
You know, I'm a little late to the game, and I'm not sure I'm answering the right question, but here goes:

Calculating this all out in psi is a little hard on my brain so I usually go by cu ft when I get lost in the math. I'm in sidemount double 40cuft tanks, and since it's doubles, the average of the psi left both of the tanks is how much you have left if you're calculating your gas needs with the assumption that all gas is to be used in your calculations; the sum of the tanks as your total available gas. My buddy who dives one backmount 100cuft has roughly the same sac rate as me, so regardless that our psis will end up being different, our amount of cuft of gas will remain the same, vs remembering any conversion factors between us two.

So if an hour long dive leaves me with 500psi in each tank in a shallow open water environment, out of two 40cuft 3000psi tanks.... since 3000psi/40cu ft = 75psi per cubic foot of gas, I will have used all but about 7cu ft (ok 6.66 but lets not get into fractions) of gas per tank (500psi/ 75 psi) , which comes out to roughly (40-7) * 2 [for doubles!] = 64cuft of gas. What's 64cuft of gas out of an 80 in psi, because my double 40s equal an 80? 3000 / 80 = 37.5 psi per cuft, so 64*37.5 = 2400psi which is about 500psi remaining in an 80cuft tank (remember the 6.66 that we turned into a 7?). So I had 500psi left in an 80cuft equaling 500psi in each 40cu ft.

My buddy using the 100cuft tank, having the same sac rate, will have used also the same 64cuft gas, but will just be at a different ending psi than me (depending on his 100cuft/rated pressure conversion factor). Were his sac rate different, that # cu ft would change - if it's currently 0.5cu ft/min, and we're down 60min at 2ata, he'd go through 60cuft, right? Then just converting using that psi/cuft conversion. You'd just go through your regular pre-dive gas planning like usual, but it does really seem to work much easier for me if I use cuft all the way up until the end.

As to how I manage gas consumption in my tanks, I was taught to:
-futz around on the surface on right
-begin dive on left and bring down to 2500psi out of a 3kpsi tank
-switch to right and bring that down to 2kpsi
-switch to left and bring down to 1.5kpsi
ad nauseam.

This process is regardless of beginning psi, just to always have the same round numbers so you don't get lost and start wondering which tank you're breathing off of or how much you should have breathed out of it.
 
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'd like to see it too... doppler at techdivertraining dot org
 
GUE use 30lpm as the stressed SAC rate and 20lpm as normal. With min gas based on the stressed SAC. Now stressed SAC rates can massively spike, which is an issue in either manifold or independent tanks. I just wondered what the SOP is for sidemount.

Same... the initials on your cert card do not effect basic physiology... only affected psychology.


I'm quoting you in the next pub argument Mr Lewis.:cheers:
 
https://www.shearwater.com/products/swift/

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