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Conscientious Objector
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1,269 Posts
Blending up to 300 Bar is perfectly possible, but a bit more tricky that your standard 232Bar fill.

The gas laws which we were all taught way back in school and/or OW / OD / Novice I (showing my age) are true for an ideal gas. Nitrogen comes close to being an ideal gas and following these laws, but oxygen is pretty poor. The difference between the theory and the reality is comparatively small at lower pressures and can easily be "lost in the noise" of human and/or gauge error when producing nitrox fills to 232Bar. This means that partial pressure blending is acceptable in this region.

Once you get up towards 300 bar, the theory/reality gap widens. The gas that starts behaving badly first is Oxygen, so the richer the mix, the greater the potential for error.

The good news is that there are other theories that DO track (more) accurately the behaviour of these less ideal gasses at high pressure and there are bits of software out there that, with practice, will allow you to carry on using the partial pressure method to create a pretty accurate blend.

If you want real accuracy, the thing to do is to go hardcore and blend by mass. The scales that you need in order to do this will be pretty expensive and I would suggest that in most cases aforementioned "pretty accurate" method should see you right.

This website explains it in more depth and using algebra... and is written by a chap called Nigel Hewitt, a YD regular, I believe!

Van der Waals and Nitrox
 

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Registered
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3,640 Posts
Discussion Starter · #7 ·
Cheers for that Mal.
Normally I don't need the stuff but I will be doing a few dives that I think would suit nitrox nicely but I have 300bar tanks. I know I can always fill the tanks to just 200 but was just doing a few calcs and I lose a fair bit of bottom time by dropping to 200.
Just a few things going rouns in my head.
 

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All hail the mighty ZOM
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26,851 Posts
If you leave your cylinders for a day or so at the fill shop rather than rushing it, decent accurate 300bar fills are more than possible. The trick is to leave it to cool after the O2 then top it off with the last few bar before adding the air, leaving to cool and topping that off too for the last few bar. Doing it all in one go and fast it will cool off and the accuracy won't be spot on.

And apologies for being a twat :D
 

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Muppet
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350 Posts
I fill my own nitrox. Never had a problem with a fill before. They are almost always within 1% of the targeted mix, unless I cock it up... You just have to top it off after it has cooled down.
 

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Doing it Richard
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149 Posts
I have spent the last hour reading Nigel Hewitt's website. Really intresting stuff on there.
Yes, I've just been on a trip diving round the Shetlands on Valkyrie and Nigel was on board. He's a very interesting guy, very knowledgeable.
 

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Off to T.D.F.
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695 Posts
If you leave your cylinders for a day or so at the fill shop rather than rushing it, decent accurate 300bar fills are more than possible. The trick is to leave it to cool after the O2 then top it off with the last few bar before adding the air, leaving to cool and topping that off too for the last few bar. Doing it all in one go and fast it will cool off and the accuracy won't be spot on.
To put it another way, assuming you're not doing it yourself call up your LDS (or whoever you need to get the fills off) and see what they say; if they don't seem to understand this stuff, then ask elsewhere! (A few will have banked nitrox, in which case it's easy peasy).
 
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