Is sidemount gas planning really better for buddy diving?

[SuPrBuGmAn]

Thats gas you're losing off both tanks in either case until it IS isolated. *shrugs* Hopefully it ain't much, but if we're point fingers at who gets out with a little extra...

[DA Aquamaster]

The isolator does add the potential to mismanage the isolator, but the only time that creates a real risk is if you are isolated (by mistake) while on the primary and do not notice the lack of change on the SPG. But that is the same issue you would have with a roll-off with or without an isolator.

The potential risk with an isolator itself would be impacting the overhead hard enough to damage the manifold and cause a leak and at the same time damage the isolator so that the valve cannot be closed. In that case, you are no worse off than with a plain cross bar manifold.

In contrast, with sidemount or independent doubles, you tend to refer to the SPGs more often and have more opportunity to notice a static reading in the one you are actively using. If you miss that, you have far bigger issues that should keep you out of a cave in the first place.

In that regard you could argue that the only real benefit of an isolator is to be able to isolate the tanks prior to a problem and in effect dive them as independent doubles when the conditions warrant that approach. But nobody uses them that way.

That whole argument predates the isolator manifold itself, going back to the mid to late 60's. The strongest opposing argument is probably the decades of operational use of isolator manifolds that suggest that manifold failures as well as isolator failures are exceedingly rare.

Personally, I was a big fan of independent doubles in technical diving until I got into cave diving. Then I was more or less forced to a more "modern" isolator manifold configuration to satisfy my Cavern/Intro instructor.

[FW]

...In contrast, with sidemount or independent doubles, you tend to refer to the SPGs more often and have more opportunity to notice a static reading in the one you are actively using. If you miss that, you have far bigger issues that should keep you out of a cave in the first place.....

Just exactly what would cause a static SPG reading in sidemout, or independants?

You are also making a big assumption about how regs would be rigged with a manifold. Not everyone dives backmount the same.

I was on a dive, doing a circuit once. Just past halfway through, my buddy showed me his SPG. He had less than 1/3 left. No point in turning, it was closer finishing the circuit. I unwrapped my long hose, and watched his SPG. I didn't want him totally OOA, so I handed him my long hose, so he would still have gas, if we got separated in a restriction comming up.

None of this made sense to me. We had made that same circuit before, and he finished with over a third in the past (I can make it on sixths).

I remembered that he didn't dive "DIR" so I looked at his hose routing, and remembered that he breathes off the short hose, and has the SPG on the same reg. I also remembered that when I was gearing up, my isolator was closed. We got gas at the same place, so I reached over and opened his isolator. I heard a long hiss, while the tanks equalized. He was just about in panic mode, thinking he was still loosing gas. I showed him his SPG, and he pointed back at the isolator. I nodded, and reached to get my reg back. He wouldn't give it to me We swam along sharing air, until we got back to the mainline before he would go back on his own reg. FWIW, I don't dive with him anymore. I don't think he even dives anymore, which is probably good.

I went back to the fill station, and told the owner he needed to teach his new "Tank Monkey" how to fill doubles

[DA Aquamaster]

Just exactly what would cause a static SPG reading in sidemout, or independants?

Total gas loss would do it, so would would really excessive skip breathing or maybe being dead for a few minutes.

But mostly I was thinking a failure like a stuck needle on the gauge.

You are right about the assumptions - there are some bizarre odd configurations out there.