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To Steve Millard and other Trimix enthusiasts,=20
Just for fun...take a deep breath of the mystery mix and if you sound =
like a duck you have more than 20% He and if you sound near normal you =
have less than 20% He. If you label and use your trimix like you're =
supposed to you can always calculate what a top-up will create. If you =
are guessing with weights I recommend dumping the puzzle and getting a =
fresh start. Too easy for most. =20
Sempre Deep,
Capt. Jim=20
-----Original Message-----
From: Steve Millard [SMTP:ec96@li*.ac*.uk*]
Sent: Wednesday, May 28, 1997 10:32 AM
To: Techdiver
Subject: Analysing the helium in Trimix ??
Hi to all on Techdiver,
We have a small group of Trimix divers in the UK and were discussing how =
we=20
might determine the % of helium in a ready-mixed cylinder of trimix. =20
Gas chromatography is out of the question for all practical purposes.
The %O2 is easy to find out..but how to distinguish between the =
remaining helium=20
and nitrogen ? Then I discussed the possibility of finding the % He and =
N2 by=20
using the difference in density.
I had a quick go at the maths & it doesn't look so bad....These are all =
metric=20
units. My apologies to the Imperial unit divers in the USA, but the =
principle=20
doesn't change.
At STP (1 bar pressure & 20 degrees C) 22.4 litres of gas weighs :-
Oxygen =3D 32 grams
Helium =3D 4 grams
Nitrogen =3D 28 grams
So for a 10 litre cylinder at 200 bar, let's try out a typical 65m mix =
of 19/35=20
Trimix....
19% O2 weighs 543 grams
35% He weighs 125 grams
46% N2 weighs 1150 grams
The total =3D 1818 grams
but if we change the He by just 1% then...
19% O2 weighs 543 grams
36% He weighs 129 grams
45% N2 weighs 1125 grams
The new total =3D 1797 grams, ie a 21 gram difference.
Now if the steel cylinder weighs around 10 kg then we need to be able to =
weigh=20
around 12 kg in total to an accuracy of + or - 10 grams (ie around 0.1%=20
accuracy) to get the He % to an accuracy of 1%. =20
I've no idea of how available or expensive a weighing balance of this =
sort=20
of accuracy is...but it doesn't sound too difficult. For any given %O2, =
and=20
cylinder volume and pressure it would be easy to work out a chart giving =
the %He=20
and %N2, if you know the total weight of gas mix.
Any comments/thoughts ?
Some problems might be
a) Pressure x Volume isn't a perfect constant at high pressures, =
so it=20
needs a compressability curve for each gas instead of the simple ratio I =
used. =20
Does anyone have access to non-linear gas compressablity curves in the =
200 bar=20
to 300 bar range for O2, He and N2 ?
b) You would have to be very careful not to add any extra clips =
or to=20
get the cylinder wet on the outside or do anything other than just add =
gases=20
between weighing the cylinder empty & then full.
c) Decanting will heat the gases up & the temperature increase =
must=20
either be compensated for or removed by allowing full cooling.
Regards, Steve M.
*************************************************************************=
*
* * =
*
* 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) =
*
* * 44 151 794 5218 =
*
* * (International) =
*
* * =
*
*************************************************************************=
*
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