The concept of heliar was to eliminate the need to handle pure oxygen in
the field, nick named poor mans trimix  the method of mixing works its
resulting diving gas is less than favorable. One can still use this mixing
methods and still get a more appropriate mix by using premix enriched air
nitrox and helium. 

For example: 
to make an 18/50/32 trimix which has a very good working depth range 

one would fill the scuba cylinder with 50% of the final pressure with helium
then top off with 36% enriched air from either a premix bank or a
denitrogenation (DNAx) system to final pressure, or from an ANX system
(this uses oxygen). 

Poof -- you have 18/50 trimix ------ no oxygen handling 

take it a step further and the premix of 36% is now you intermediate 
110 (120) decompression gas

trans fill some oxygen in the o2 deco bottles and you are set ... 

The primary problem with heliar is it will ALWAYS give you a hypoxic (on
the surface) mix simply due to the fact that the oxygen in the air gets
diluted by the helium.  If you want 50% helium you will have 11-12% oxygen
--- its direct proportion to the helium added. 

25% helium would yield .1575 oxygen fraction in the mix.  The o2 is low 
enough for a relatively deep dive but the helium just is not enough. 

I've got some charts on the premix / helium method I will put on the web
site in a few days to make mixing trimix very simple with premix OEA. 

Happy New Year. 

Joel Silverstein
http://www.trimixdiver.com






At 12:12 AM 12/31/99 -0500, dmdalton wrote:
>Kevin,
>
>I think you are quite right that Heliair isn't the "perfect" gas because it
>locks you into a very rigid set of mixtures as you pointed out in your
>chart. In some cases, however it might be a reasonable way to go. Try this
>on for size, unless my math is wrong (wouldn't be the first time :) I think
>the gas you suggested (12/40) would actually be quite suitable for use at
>200 fsw.
>
>Correct me if I am wrong but I think the formula is:
>
>Atmospheres Absolute (ATA)  X   %  (Percent of Gas in Mixture)   =  PP
>(Partial Pressure)
>
>If we want an equivalent nitrogen depth max of 100 fsw then we would be
>looking at a PP of 3.16.
>
>4 (ATA) X .79 (%) = 3.16 (PP)
>
>Breathing 12.6 / 40 / 47.4 at 200 fsw you would have a N2 PP of  3.318 which
>would result in an  END of 105.6 fsw.
>
> 7 (ATA)  X  47.4 (%)  =  3.318  (PP)
> 3.318 / .79 = 4.2 (ATA)
>( 4.2 -1)  X  33 = 105.6 fsw  (END)
>
>You would have an O2 PP at this depth of  .882 which would be fine. Remember
>your O2 PP at the surface breathing air is only .21 .
>
>On the other hand the 18/40 you mentioned would give you an O2 PP of  1.27
>and if you strayed down to 220 it wold take you right to the edge for a
>working dive with a PP of 1.379. Remember air maxes out at 218 fsw as far as
>O2 PP goes.
>
>7.6 (ATA) X .21 (%)  = 1.59 PP  ( O2 PP of 1.6 at rest considered the max )
>
>One down side to the 12/40 mix is that this mix is hypoxic from the surface
>where your O2 PP would be only .126 until you got to about 20 fsw where it
>would increase to .20 .  Consequently you need something else to breathe
>from 0 to 20 and from 20 to 0.
>
>Comments anyone???
>
>Dave Dalton
>
>----- Original Message -----
>From: Kevin Connell <kevin@nw*.co*>
>To: <tgunther@co*.co*>
>Cc: <techdiver@aquanaut.com>
>Sent: Thursday, December 30, 1999 5:32 PM
>Subject: Re: HELIAIR
>
>
>> Heliar is DIW.  There is only one depth that the heliar mix curve matches
>> PPO2 and END, and it's not anywhere under 200fsw.
>>
>> Here is a mix chart for heliar for a 3000psi tank (I think)
>> (ftp://ftp.nwls.com/pub/kevin/mixing.xls)
>>
>> He psi  he%      n2%    o2%
>> 100             3.3%    76.4%   20.3%
>> 200             6.7%    73.7%   19.6%
>> 300             10.0%   71.1%   18.9%
>> 400             13.3%   68.5%   18.2%
>> 500             16.7%   65.8%   17.5%
>> 600             20.0%   63.2%   16.8%
>> 700             23.3%   60.6%   16.1%
>> 800             26.7%   57.9%   15.4%
>> 900             30.0%   55.3%   14.7%
>> 1000            33.3%   52.7%   14.0%
>> 1100            36.7%   50.0%   13.3%
>> 1200            40.0%   47.4%   12.6%
>> 1300            43.3%   44.8%   11.9%
>> 1400            46.7%   42.1%   11.2%
>> 1500            50.0%   39.5%   10.5%
>>
>> as you can see, none of these mixes make any sense.
>>
>> For example, a mix for 200fsw would be something like 18/40, and you can't
>> get the right combination here.  You could dive 12/40, but that is a
>little
>> low on the O2, and unnecessarily hypoxic.
>>
>>
>>
>> At 01:26 PM 12/30/1999 -0800, you wrote:
>> >Team:
>> >
>> >I am told that many members create their own mixes using personal cascade
>> >systems.
>> >
>> >Because heliair would probably be the easiest introduction to this
>method, and
>> >also increase the safety of ones mix (only interested in < 200fsw at this
>> >point), I was wondering if anyone would like to share their experiences
>in
>> >this
>> >area.   Pros, cons, concerns, applicability, etc.
>> >
>> >Thanks,
>> >Tod
>> >
>> >
>> >--
>> >Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'.
>> >Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
>>
>>
>> ----------------------------------
>>   Kevin Connell <kevin@nw*.co*>
>>
>>   NW Labor Systems, Inc
>>   http://www.nwls.com
>>
>>
>> ----------------------------------
>>
>> --
>> Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'.
>> Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
>>
>
>--
>Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'.
>Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
>
Joel Silverstein
http://www.nitroxdiver.com
--
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