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Discussion Starter #1
The gas blender at my LDS is off sick at the mo..... So I can't get a nitrox fill for tomorrow.
I have twin 12's (mainfolded) with 220bar of 36%. I need 30%. Is there a formula I can use to achieve this mix?

i.e. bleed down to X?bar, then top up with air....?

Yup I know I should go on a gas blending course etc.... But this is a one off, I do regularly top up my cylinders with air after a dive with nitrox, but never paid any real attention to the percentages, only checking just before the dive to reset the computer. However in this case I can not go over 30%!!

Help and thank you in advance


Cheers marcus
247 (my first 3)
 

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Fill instruction:
Drain tank to 138.0 Bar
Add air to 230.0 Bar

Plenty of gas mixing programs knocking about on the t'internet......
 

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You have 220bar of 36%.

You want to remove x bar of the 36% from the 220 bar, then top back up to 220 with 21% to get 30%, which would be 66bar of O2. So,

((220 - x) * 0.36) + (x * 0.21) = 66, or

79.2 - 0.36x + 0.21x = 66, or

79.2 - 0.15x = 66, or

x = ((66-79.2)/-0.15), or

x = 88bar.


In reverse that would be ((220 - 88) * 0.36) + (88 * 0.21) which would be 66 bar of O2, or 30%.

I think.
 

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As a matter of interest, the formula for a complete fill to 232 would be,

((220 - x)*0.36)+((x+12)*.21 = 69.6

The differences being the 'x+12' and the total bar of O2 on the RHS of the equation. Or drain 81 bar.
 

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You have 42 bar of O2 -> ((0.36-0.21)/0.79)*220. The rest (178) is air.

You want 30% -> ((0.30-0.21)/0.79)*220 -> 25 bar, the balance air.

((0.36-0.21)/0.79)*x = 25
x = 25 / [ ((0.36-0.21)/0.79 ]
x = 131

Reduce to 131 and top with Air to 220.

Matt.
 

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Meh...

Go buy v planner or forget nitrox diving.

Seriously, calculating pp top up blends ain't rocket science, to the extent where if you have to ask, you shouldn't be diving it, let alone mixing it.
 
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Nigel Hewitt
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Go buy v planner or forget nitrox diving.
Seriously, calculating pp top up blends ain't rocket science, to the extent where if you have to ask, you shouldn't be diving it, let alone mixing it.
I started asking questions and look where it got me...
Questions are good.
 
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