I think Tobin and others have adequately explained the physical processes involved with filling tanks, discussed the likely affects on the mixing of not having the isolator open during filling, and the theoretical fact that eventually, (and that is the key word), the gasses will mix. BUT for anyone who doesn't dive manifolded doubles, partial pressure blend gasses and so on, who is reading this thread EVENTUALLY is not good enough.
Potentially if you have had a incorrect done fill with the iso closed, and just open it and then go diving, there is a potential to be breathing unmixed gasses, and that could mean you are breathing either very high ppo2 oxygen, or very high ppo2 helium, and either of those situations could get you dead in very short order.
So regardless of the semantics some people are arguing here, this is bad for you, could get you killed and it is much better to learn how to do it right, then analyse the mix and dive it safely.
The uncertainties are what have got quite a few people killed, along with wrong markings on tanks, incorrect mods, and so on. In my view this is one case where I couldn't give a hoot about "eventually" or the niceties of gas laws or physics. I want to know the fill has been done correctly, with the valves all in the right position, so I know what I am diving and won't get any nasty surprises.
I also have to say that in Victors example above I would't dive doubles with a different mix in them, if all my intended diving was within the mod of the higher gas, then I would have opened the iso, as soon as I realised, analysed both posts before diving, and if they hadn't mixed evenly by the time I dived I would have used the richer gas to determine mod, and the leaner gas to determine ndl's and learn't from experience. - Phil.
