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Hi to all on Techdiver,
We have a small group of Trimix divers in the UK and were discussing how we
might determine the % of helium in a ready-mixed cylinder of trimix.
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
and nitrogen ? Then I discussed the possibility of finding the % He and N2 by
using the difference in density.
I had a quick go at the maths & it doesn't look so bad....These are all metric
units. My apologies to the Imperial unit divers in the USA, but the principle
doesn't change.
At STP (1 bar pressure & 20 degrees C) 22.4 litres of gas weighs :-
Oxygen = 32 grams
Helium = 4 grams
Nitrogen = 28 grams
So for a 10 litre cylinder at 200 bar, let's try out a typical 65m mix of 19/35
Trimix....
19% O2 weighs 543 grams
35% He weighs 125 grams
46% N2 weighs 1150 grams
The total = 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 = 1797 grams, ie a 21 gram difference.
Now if the steel cylinder weighs around 10 kg then we need to be able to weigh
around 12 kg in total to an accuracy of + or - 10 grams (ie around 0.1%
accuracy) to get the He % to an accuracy of 1%.
I've no idea of how available or expensive a weighing balance of this sort
of accuracy is...but it doesn't sound too difficult. For any given %O2, and
cylinder volume and pressure it would be easy to work out a chart giving the
%He
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
needs a compressability curve for each gas instead of the simple ratio I used.
Does anyone have access to non-linear gas compressablity curves in the 200 bar
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
get the cylinder wet on the outside or do anything other than just add gases
between weighing the cylinder empty & then full.
c) Decanting will heat the gases up & the temperature increase must
either be compensated for or removed by allowing full cooling.
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) *
* * 44 151 794 5218 *
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