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Hi Kent,
On Wed, 28 May 1997 08:25:39 -0700 Kent Lind wrote:
> Interesting idea. Please post your conclusions when you sort out the
> responses.
Sure.
I can't find any fault in your logic but I have to wonder
> why you feel it necessary to go this route. Are you not confident of
> your ability to mix gases using partial pressures?
Mixing for the 1st time, using an empty cylinder is no problem. But if you
have
a 1/2 full cylinder of trimix & want to top it up with either helium or air (or
even O2 if you have the pressure in the bank) then the partial pressure
calculations get a bit more suspect. If the trimix has been stored for some
time then there can be gas loss. Is this just the helium of all the gas ?? I
had a 19/45 mix go down from 250 bar to 140 bar over a 6 month lay-off period.
An O2 analysis showed the O2 % was still 19% & so all the gas must have leaked
as a mix. I've heard however of helium differentially leaking.
These are the sort of problem we want to address.
I have not mixed
> trimix so I'm just shooting in the wind here but it would seem to me
> that if you start with a helium-oxygen blend you can confirm the O2
> percentage of the mix at this stage. Then if you add air, the change in
> O2 percentage will provide some independent confirmation of the amount
> of air (nitrogen) you have added. Of course, this would only work if
> your initial heliox mix is less than 21 percent O2.
See above.
>
> Secondly, Have you determined how precise you need to be in your helium
> and nitrogen percentages? O2 seems to be the critical one. If you are
> confident of your O2 percentage, how much difference will it make in
> your deco and MOD calculations if you change the helium/nitrogen ratio
> by a percentage point or two in either direction?
1% error in the assumed He % will make virtually no difference in the deco or
the equivalent narcotic depth. 5% or 10% wiil start to make a signficant
influence on both. I just used 1% to show the accuracy that might be obtained
by weighing to + or - 10 grams.
>
> Finally, what will you do if your weighing method produces a result that
> is different from what you expected it would be using partial
> pressures? Will you drain the tank and start over or will you just dive
> it anyway? I suspect you will ultimately have to compare the precision
> of your pressure gauges to the precision of your scale and determine
> which one gives you a more precise measurement of gas percentages.
> Personally, I'd be more inclined to trust the pressure gauges as I would
> be more confident that I was measuring gas and not some other foreign
> objects on the tank.
Indeed. It depends upon how careful one is & the sort of situation that may
have caused an uncertainty about the He % in the first place.
Regards, Steve M.
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* Dr. S. G. Millard, * E-Mail : ec96@li*.ac*.uk* *
* Senior Lecturer, * *
* Department of Civil Engineering * Tel : 0151 794 5224 (UK) *
* University of Liverpool, * 44 151 794 5224 *
* PO Box 147, * (International) *
* Liverpool L69 3BX, * *
* UK. * Fax : 0151 794 5218 (UK) *
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