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From: "David Shimell (shimell)" <shimell@se*.co*>
To: Kevin Connell <kevin@nw*.co*>
Cc: techdiver@aquanaut.com
Subject: RE: Trimix Question
Date: Tue, 20 Apr 1999 15:51:06 +0100
Kevin

I hot fill, O2 first, Helium then air.  By hot fill I mean add the O2
slowly, then the Helium at a moderate rate.  Two quick blasts of air to
"swirl" the gas and moderate fill rate for the rest of the air.  I over pump
each of the gasses by 10% to allow for cooling and compressibility.

Often I remix and use a booster.

The example I gave below was extreme and happened once.  I have seen several
other occasions when the analysis has changed over time - tens of minutes,
though not hours.

My analyser has always read consistently.  In the case below I used two
other analysers to double check.

BTW, the 1% accuracy you mention of analysers I understand to be correct.
However, consistency of analysis reading is another matter.  My analyser is
normally very consistent for any given gas when the readings stabilise,
though it may be out by 1%.  I recall that the instruction leaflet with my
Van Der Graff analyser indicates that it is temperature compensated.

David Shimell
Project Manager, Sequent Computer Systems Ltd., Sandton, South Africa.
Email: shimell@se*.co* <mailto:shimell@se*.co*> 

-----Original Message-----
From:	Kevin Connell [SMTP:kevin@nw*.co*]
Sent:	Monday, April 19, 1999 4:23 PM
To:	David Shimell (shimell)
Cc:	techdiver@aquanaut.com
Subject:	RE: Trimix Question

David:  What procedure do you use to fill your trimix?  I don't experience
these problems.  Have you used more than one O2 analyser to see if this
happens with consistently?  I've seen variations of 1% or less, but that is
normal given the accuracy of the sensor, varying flow rates, and
temperatures.

At 02:17 PM 4/19/1999 +0100, "David Shimell (shimell)"
<shimell@se*.co*> wrote:
>Jim
>
>Ah, Jim, I have a good opportunity to disagree with you!
>
>Firstly, my experience: I've never had a problem mixing Nitrox mixtures -
>they always analyse out to their eventual percentage.  However, on several
>occasions I've had a problem with Trimixes (usually 16/44).  On one
>occasion, I repeatedly analysed a mix over 4 hours and watched it rise from
>around 13.X% to stabilise to 15.2 ish and then rise again in the morning to
>an accurate 16%.
>
>We are not talking about the bullshit of stratification i.e. the gases over
>time separate into layers, merely the slow process of diffusion via
Brownian
>motion.
>
>I have also found that rolling does help Trimixes but rarely bother as I
>analyse next morning where possible.
>
>I can't explain this but if the gas is indeed viscous at these pressures,
it
>would go some way to explaining why in your most eloquent language,
>"blasting high pressure gas through a tiny oriface in a violent fashion
into
>your tank and the goddamn molecules are flying all over the fucking place"
>does not cause instantaneous mixing.
>
>David Shimell
>Project Manager, Sequent Computer Systems Ltd., Sandton, South Africa.
>Email: shimell@se*.co* <mailto:shimell@se*.co*> 
>
>-----Original Message-----
>From:	Jim Cobb [SMTP:cobber@ci*.co*]
>Sent:	Friday, April 09, 1999 5:25 PM
>To:	Steve Hogan; pdisler@io*.ne*; silent@cu*.ne*
>Cc:	Tech Diver
>Subject:	RE: Trimix Question
>
>I really have a hard time getting my brain wrapped around this 
>stratification thing. When you are adding gas to a cylinder it not like 
>you are decanting it in like you are mixing a black and tan. You are 
>blasting high pressure gas through a tiny oriface in a violent fashion 
>into your tank and the goddamn molecules are flying all over the fucking 
>place. They hardly have a chance to delicately float down like a bunch of 
>snowflakes into a nice stack of different gas densities.
>
>When you are in a fire what do you do to avoid the smoke? Crawl on the 
>floor. The smoke/air mixture is up on the ceiling because of its 
>temperature, not because the house is not being rolled. As the 
>temperatures mix and even out the smoke is more than happy to fill the 
>room top to bottom.
>
>This rolling tank BS is an old wifes tail and cannot be explained via 
>physics. The inside of a tank is smooth, there are no vanes sticking out 
>to "mix" the gases, so how could rolling possibly help?
>
>The temp differences do not have to be measured in 100's of degress for 
>inversion layers to take place but eventually the temps even out and you 
>get a good mix measurement. You don't have to roll the goddamn tanks. 
>Can't you knuckleheads see this?
>
> Jim
>
>Sender: Steve Hogan  Date: 4/8/99 8:17 PM
>
>>Jim,
>>
>>Since I fill the cylinders slowly there is not much heating going
>>on. I suppose you could test this by doing a fast fill (with the O2)
>>and let us know. If we don't here from you we will pretty much know 
>>what happened (wink).
>>
>>If temperature were an issue then warming a tank after there is a known 
>>good mix should throw off the measurement. I always measure my mixes 
>>in my garage (my safe haven) so it is a bit on the cold side. I 
>>do not wait until I am on the boat to check the mix (that comes under
>>the heading of preparation).
>>
>>Comments.
>>
>>Regards
>>
>>Steve
>>
>>-----Original Message-----
>>From: Jim Cobb [mailto:cobber@ci*.co*]
>>Sent: Thursday, April 08, 1999 4:55 PM
>>To: Steve Hogan; pdisler@io*.ne*; silent@cu*.ne*
>>Cc: Tech Diver
>>Subject: RE: Trimix Question
>>
>>
>>I don't know about you guys but I think the problem we are dealing with 
>>here is the temperature of the mix rather than stratification. As the mix 
>>cools off the reading becomes more accurate.
>>
>>   Jim
>>
>>Sender: Steve Hogan  Date: 4/8/99 6:53 PM
>>
>>>
>>>I have run into this and have modified my mixing procedures 
>>>so that after I add the O2 to the helium I wait 1 day (I 
>>>usually mix at night so waiting around is not appealing)
>>>before measuring the O2 % (O2 & He). It is not hard to figure
>>>out what percentage I expect after mixing O2 with He.
>>>Next step is to add air, wait a few hours (usually doing 
>>>something else) and then measure again. 
>>>I fill about 50 psi/min so I expect to not get much mixing from 
>>>Brownian motion.
>>>
>>>Steve
>>>
>>>-----Original Message-----
>>>From: Patricia Disler [mailto:pdisler@io*.ne*]
>>>Sent: Friday, April 09, 1999 1:14 PM
>>>To: S I L E N T I M M E R S I O N
>>>Cc: techdiver@aquanaut.com
>>>Subject: Re: Trimix Question
>>>
>>>
>>>
>>>
>>>
>>>At 09:16 AM 4/8/99 -0700, you wrote:
>>>>Frank,
>>>>
>>>>>Not me, but I do let the filled tanks rest a day before analyzing. An
if
>>>>>you want to tell me
>>>>>thats not neseccary I recommend that you try this: Fill oxygen, top
with
>>>>>air, wait a minute,
>>>>>analyze mix, turn the tank so that the valve faces ground, wait another
>>>>>minute and than
>>>>>re-analyze the mix.
>>>>
>>>>And it's the same... or not?
>>>>When filling a tank the temperature and pressure are high which in turn
>>>>gives a high diffusion coefficient. Makes the mix homogeneous almost
>>>>instantaneoulsy (couple of seconds).
>>>>What kind of differences do you get?
>>>>What kind of analyser do you use, are you sure the hoses are drained
when
>>>>using a regulator or something?
>>>>
>>>>Regards,>
>>>>THOMAS
>>>
>>>I have witnessed Nitrox mixtures take a while to fully homogenize.
Several
>>>years ago while mixing 80/20, (Yeah I know don't start) it came out 63%
O2
>>>approx 30 min after the very s-l-o-w top off to fill pressure. Yes I
>>>checked the calibration of the aynalizer (Teledyne TD-50). Each time I
>>>would check the O2 reading would climb. I did this over a period of
>several
>>>hours. Later it was right on 80%.
>>>I'd have to say stratification can and does occur, esp. if there is not
>>>enough turbulence during top off.
>>>I have never seen this with Trimix as the fill rates are much higher.
>>>However I suppose it could occur if they too were topped slowly.
>>>At any rate I never seem to dive my gas until they show the correct
>>mixture.
>>>
>>>
>>>
>>>"SILT HAPPENS"JD   JEFF DISLER
>>>  SAFE CAVING       NSS 26000
>>>--
>>>Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'.
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>>>
>>
>>
>> -------------------------------------------------------------------
>> Learn About Trimix at http://www.cisatlantic.com/trimix/trimix.html
>>
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>>
>
>
> -------------------------------------------------------------------
> Learn About Trimix at http://www.cisatlantic.com/trimix/trimix.html
>
>
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