Topping off nitrox to get a desired mix

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Bowers

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Scuba Instructor
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So i have some doubles that have 32% and only 1300psi left. I can fill air for free at my fire dept. So I usually just top off then check O2 if i dont need another nitrox fill. However this week im planning a dive that would be ideal to have 29% and doesnt need that many cubic feet, so i thought it would be nice to just fill until i was at 29% and stop regardless of final pressure. I am comfortable with the math to partial pressure blend using 100% 02 topped off with air, and i can easily figure what a resulting mix would be if topping off a nitrox mix to a known pressure. But for whatever reason i cant seem to figure out a direct equation to tell me at what pressure to stop when diluting existing nitrox to a desired mix. I have come to an answer after "plugging and chugging" and know i need a fill to 1800psi, but would love it if someone could explain a straightforward mathematical way to get directly to this number. Thanks in advance.
 
Take the equation to find an ending nitrox blend given the proportions of gas added--The one you used if you were to top a half full tank with air.

(Starting nitrox mix) x (% of total gas) + Air x (1-% of total gas) = Desired O2 % ----(is technically Desired O2% x 100%)

.32 x (X) + .21 x (1-X) = .29 x 1
.32X + .21 - .21X = .29
add/subtract to simplify
.11X = .08

X=.7272 = 72.72%

This means that to get an ending nitrox blend of 29%, your initial gas must make up 72.72% of the NEW total gas volume

(New Total Gas Volume) x .7272 = 1300 (Current fill pressure of tanks)

New Total Gas Volume = 1300/.7272

New Total Gas Volume = 1787 PSI

So that means adding 487 PSI to your tanks
 
The math is above - adding 500psi gets you ~29%, 1000psi gets you 27% (adding air)

Is the air you're adding ok for oxygen clean? If you ever PP blend nitrox with pure O2........
 
The math is above - adding 500psi gets you ~29%, 1000psi gets you 27% (adding air)

Is the air you're adding ok for oxygen clean? If you ever PP blend nitrox with pure O2........
its just fire dept air, but i dont partial pressure blend with high ppO2 in these tanks, they only get banked 32% or air.

and thank you cpttightpants21 for the math. I had been trying to simplify it down but never made the connection that it was 72% of the new volume.
 
Download ccr mixer pro on to your sell phone. Makes life much easier.
 
I just took the gas blender classes, sort of a what the heck, I'm here wouldn't be bad to learn kinda thing - class was good though. I lost count of how many tri mix fills I did, like 7 or 8 big bottles of helium.

Book is all about the formulas, class explains em but the apps or spreadsheets makes the math easy.

Or if you have multideco....
 
EDIT: nevermind. Missed that you were using doubles.

Was going to ask about the details of a dive for which 29% is ideal, but a half-full cylinder would work.
 
I can fill air for free at my fire dept.

I would check the CO and CO2 levels in your tank(s) after the fire department fill. The fire department may not be as careful where they put the air compressor or what standards they follow. Fireman can tolerate small amounts of CO and CO2 at the surface that might be more toxic for you at depth.
 

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