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Steve, From discussions with Dr. Bill Hamilton a few years ago, the helium = content is not as critical as the oxygen content. Indeed, he actually = stated that the nitrogen/helium ratio could vary by as much as 10% if = the O2 content was within 1% tolerance. Respectfully, Jay K. Jeffries CoolWater Design, Inc. ---- From: Steve Millard <ec96@li*.ac*.uk*> To: Techdiver <techdiver@aquanaut.com> Date: Wednesday, May 28, 1997 9:49 AM 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 >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 >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 =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 >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 >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%.=20 > >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.=20 >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. > >************************************************************************= ** >* * = * >* 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) = * >* * = * >************************************************************************= ** > > > >-- >Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. >Send list subscription requests to `techdiver-request@aquanaut.com'.=20 -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send list subscription requests to `techdiver-request@aquanaut.com'.
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