Dummy question for clever people.

[Rupert Bear]

I think the original question was if a manifolded set was filled would the two mixes be the same and the answer to this one is yes, you are worrying too much!

[Finless]

Originally Posted by [b

Quote[/b] (Rupert Bear @ Nov. 12 2003,09:37)]I think the original question was if a manifolded set was filled would the two mixes be the same and the answer to this one is yes, you are worrying too much!

It is not a worry thing.

It really depends on how fast the mix "settles" - have you ever analysed your cylinders right after they have just been topped up with air (or even a quick burst of O2)?

In reality, of course, I will carry on diving just the same BUT if the variance post/pre "settlement" is of any significance then it MAY be worth taking into account.

For example, if I requested a 1.4 pp O2 mix and it has come out slightly rich and gives me 1.5 PPO2 at max depth. If there is then a further variance to be allowed for because the gas hadn't mixed thouroughly when analysed then it should be taken into account.

I guess it can't have caused much of a problem or else we would have heard about it as being the cause of more incidents. Or, maybe, it is because of the safety margins that are built into the limits set for recreational scuba diving (i.e. max PPO2 1.4 etc) that discrepancies in the mix analysis do not have any effect.

[Finless]

I received the below information from JB at Diver magazine (to which I am a subscriber). Of course, my thanks to JB at Diver for taking the time to answer a question from me Mr J Public.

Gases do not mix instantaneously The consensus of opinion is: If you leave the manifold isolator valve fully open the gases should be mixed after an overnight interval. (I guess that it does not have to be dark!)  Gas analysis expert David Tracy tells me that if you analyse a nitrox mix immediately after partial-pressure blending you will get a richer mix recorded than if you leave it for 8 hours. I hope this helps.

- yes I am happy now.

[Rupert Bear]

Originally Posted by [b

Quote[/b] (Finless @ Nov. 12 2003,12:08)]Gases mix almost instantaneously The consensus of opinion is: If you leave the manifold isolator valve fully open the gases should be mixed after an overnight interval.

I think the "should be mixed" and "consensus of opinion" means nobody is really sure about how long it would take  
 My experience of mixing is based on how fast O2 will mix in water so it is only partially relevant.
Based on the not quite completely certain answer I think I may try it next time I mix up trox in the twins and see what happens and how long it takes.

[Spike Jackson]

Originally Posted by [b

Quote[/b] (FionaB @ Nov. 10 2003,14:04)]

Spike

Did you want two separate mixes or did you ask for the same ?  If you wanted them both the same you should have asked them to be re-done.  They should be within +/- 1%.

Fiona

No - I wanted 2 33% mixes. The filling was delayed and only completed just as we arrived on site so it was either can the dive or make do with what I had.

[Finless]

Originally Posted by [b

Quote[/b] (Rupert Bear @ Nov. 12 2003,12:19)]Based on the not quite completely certain answer I think I may try it next time I mix up trox in the twins and see what happens and how long it takes.

Rupert,

I had to edit my quote from JB (ie gases do not mix instantaneously).

If you get round to it I would be interested in the results of your test. When I get my O2 analyser I will have a go myself.

Rgds
Bryan