Gas Management With Sidemount

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On UTD Z-system sidemount, we turn on total remaining bottom gas after consuming usable Third's Turn Volume, just like in conventional Long Hose Primary backmount manifolded doubles DIR/Hogarthian diving. Notwithstanding its detractors & criticisms of the Z-system Sidemount Manifold, it still makes gas management & planning easy diving in the mixed team UTD concept --and especially in switching configuration from backmount to sidemount.

So for instance, on double 11L tanks (twin AL80's), if my total remaining volume after consuming usable third's is 2860 liters, I turn the dive when my SPG reads 130 bar if I were using backmount manifolded tanks. Similarly, if I were on Z-system sidemount doubles, I would turn the dive when both SPG's read 130 bar. It might turn out one tank has 150 bar remaining while the other shows 110 bar, but the total remaining volume after usable third's is still 2860 liters. Out of gas & emergency gas sharing contingencies are the same --donate the Long Hose.
 
Similarly, if I were on Z-system sidemount doubles, I would turn the dive when both SPG's read 130 bar. It might turn out one tank has 150 bar remaining while the other shows 110 bar, but the total remaining volume after usable third's is still 2860 liters. Out of gas & emergency gas sharing contingencies are the same --donate the Long Hose.

What about 60 bar and 200 bar? I still average 130 bar. That should be fine right?
 
What about 60 bar and 200 bar? I still average 130 bar. That should be fine right?
Ideally, I wouldn't intentionally be that out-of-balance between the two cylinders. . .
 
One of the things that popped up was when to switch 2nd stages. One suggestion was to breath a 1/3 of one tank, then 1/3 of the other.

Suboptimal. You could loose a tank with 3/3 and be left with one with 2/3, when with another strategy you could have achieved, asymptotically, 2.5/3 lost and 2.5/3 remaining.

Another one was to breath 30 bar/500 psi from one tank, and then 30 bar / 500 psi from the other.

Better.

Now, I was thinking was why not first breath 30 bar / 500 psi from the one tank, and then be breathing 60 bar / 1000 psi to reduce the number of switches.

Even better as this halves the pressure difference.

The optimal strategy is to keep the cylinder pressures as close to each other as possible because then you never loose a tank with significantly more gas. Your 30-60-60-60-... or a 20-40-40-40-... strategy is good. 10-20-20-20-... or 5-10-10-10-... is of course more gas-efficient but then you need to switch a lot and the purge vs. exhale risk assessment and other regulator switch risks start to become relevant.
 
. 10-20-20-20-... or 5-10-10-10-... is of course more gas-efficient but then you need to switch a lot and the purge vs. exhale risk assessment and other regulator switch risks start to become relevant.
While I agree that more switches=more risk, this is a necessary skill that needs to be practiced until it is second nature and practically risk free. Achieve that level of comfort and your ready for Sudden High Intensity Training to happen. If the switch isn't necessary it's still practice! Stay safe, be prepared.
Happy diving!
 
While I agree that more switches=more risk, this is a necessary skill that needs to be practiced until it is second nature and practically risk free

If the switch is done properly it's certainly no risk at all. The worst case is that the reg doesn't work for whatever reason (something got caught in the exhaust mushroom valve or such) and then it's better to catch it sooner than later.
Switching the regs often is good.
 
If the switch is done properly it's certainly no risk at all.

I've heard conflicting opinions on this topic. Some people seem to think that there's always a risk of freeflow, however, small, when depressing a purge button. Or is that only in near-freezing water? Others would suggest that there's a risk putting a reg that's flooded or not working in your mouth, so one should always purge. I always purge, and in my negligible experience thus far, I've never run into problems of either nature, so to me that just sounds theoretical. Thoughts?
 
I've heard conflicting opinions on this topic. Some people seem to think that there's always a risk of freeflow, however, small, when depressing a purge button. Or is that only in near-freezing water? Others would suggest that there's a risk putting a reg that's flooded or not working in your mouth, so one should always purge. I always purge, and in my negligible experience thus far, I've never run into problems of either nature, so to me that just sounds theoretical. Thoughts?
Especially on DPV/Scooter, switching out regs on the fly -there's always a chance of a freeflow. Also contaminated water -i.e. Engine room bunker oil; Cargo Hold aviation fuel --in the Truk Lagoon wrecks . . .it would be nasty to get a mouthful of that on clearing a switched reg.
 
Especially on DPV/Scooter, switching out regs on the fly -there's always a chance of a freeflow.

Well you can get off the trigger for the 5 to 10 seconds it takes to make the gas switch or opt to delay it to your next stop. Most of the time when scootering in caves I'll be breathing a stage anyway and gas switches aren't going to happen nearly as often.

Also contaminated water -i.e. Engine room bunker oil; Cargo Hold aviation fuel --in the Truk Lagoon wrecks . . .it would be nasty to get a mouthful of that on clearing a switched reg.

We heard that argument from you already. It's not an issue for 99.9% of diving applications. And I'd be worried about my skin, suit and the rest of the gear when swimming around in something as aggressive as aviation fuel.
 
Some people seem to think that there's always a risk of freeflow

Some do. But.

I don't think a freeflow is a risk (for a mentally prepared diver). If it happens, shut the tank, and open it only when breathing. You will lose no gas.
(start inhaling before opening the valve, stop inhaling after shutting it; needs practice; easy after that)

I believe that "Regulators only fail to deliver gas when you have just exhaled". It is more likely for the new regulator to be filled with mud or fish or water than it is for the one you already had in your mouth. The difference may be small enough to be irrelevant, but in theory...
 

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