Personally, I analyze on tank valve (or orifice or whatever the thing is called that you put the first stage on) if the regs are not installed. This is always the case when I pick up my tanks from the dive shop.
I analyze on a low pressure inflator hose if possible (backgas tanks) when I have regs installed.
In any case, I assume that the reading at the tank valve will be the same as the analysis on the second stage.
I wonder if anyone has any *actual* data or *real life* experience with this. The only way I could see a set of doubles having O2 in once cylinder and 100% He in the other is if someone filled them that way on purpose. There are too many steps involved for the doubles to wind up in this state for this to have been inadvertent.
Easy, start with a set of empty tanks and a PSI "recipe" for a particular TriMix blend, lets say 18/45
For the purpose of this example we will assume the calculated PSI numbers are all you need to use, no He or temp fudge factors.
Lets assume 3500 PSI tanks
you need 286 psi O2, 1575 psi He and 1640 PSI air.
Lets also assume you have 4 sets of dubs and 4 bottom stages and 8 deco bottles to fill also.
Normal practice would be to transfill / boost all the O2 first into all the tanks that need it. That means you are moving the din adapter and fill whip from tank to tank to tank, unless you have 16 din adapters. Doing the O2 first keeps the max O2 pressures down, and it means you aren't blowing the O2 out of the fill whips and booster after each tank, and don't have to keep swapping the inlet (inlet to the booster)whip back and forth between the O2 and He bank.
Then you repeat the process for He, i.e. fill all 8 of the tank /sets of dubs needing He in one setup.
Then top with air. Same deal, keep moving the din adapter from tank to tank to tank.
Let's say on one set of dubs you 1) have an un noticed Closed Iso, and you used the left post for O2 and the right post for He. Remember you are probably in a hot, cramped, crowded fill station and may be struggling to get the fill whips to reach all the tanks.
Then the phone rings just when you were going to do the air top on the dubs with the closed Iso.
Bingo you have set with 286 psi of 100% in one side and 1575 PSI of He in the other.
This requires a series of screwups, and a failure to analyze, but it's easily possible.
More likely would be 100% on one side and ~ 10/45 on the other (or the reverse) if the Air top was introduced to bring the tanks up to 3500 psi. In this case all that was required was a single filling error, leaving the ISO closed.
Worse if the diver skips analyzing and just looks at his SPG he may see a "full" set of dubs, and passes out on the surface.
Murphy never sleeps....
Tobin