Blending discrepancy

[colinicky]

Using 2 different blending programs to mix 18/45 trimix I get different amounts
According to Nigel Hewitts Van der waal's one :-
Add 16 bar O2 & 102 bar of He then air top to 235 bar

According to Proplanner version :-
Add 19.2 bar O2 & 105.8 bar He then air top to 235 bar .

I know there is not a lot in it but which one is correct ? I know if I switch Nigels to "Ideal gas laws " it's the same but which works in the real world ?
I used to use DSAT blending program but lost it in a pc crash

[nigelH]

Originally Posted by colinicky

I know there is not a lot in it but which one is correct ? I know if I switch Nigels to "Ideal gas laws " it's the same but which works in the real world ?

Well neither of them.

The ideal laws are an approximation that will always get you within a couple of percent and Prof. Van der Waals worked out what was wrong and came up with a more accurate formula (and cleaned up the 1910 Nobel prize for his trouble). Even then there are discrepancies around the triple point but most of the later formula only work over the small ranges where the fun physics is so VdW is the best for general blending work at the moment.

If you're not blending to 300 bar you can get quite reasonable results on the ideal laws. You get mixes that are close enough that it makes no odds and the day by day changes to you body chemistry have more impact on your decompression profile. I'm just an instrumentation physicist (you gotta earn a crust somehow) so I have the gas analysers and I like accurate numbers - plus the fact that I used 300bar cylinders on nitrox and trimix and the ideal laws do get a bit sloppy up there.

[Andy Kerslake]

You will find a few discrepencies, but the actual filling takes a bit of practice and a lot will depend on flow rates used.

Generally tho' if filling quickly 10% over is a good number for the Helium, with the O2 just a bit slower and no extra. This way if anything you always end up lighter on the O2 and more on the He which is the best option.