To: "Ian"
From: Charles Read
Subject: Homebrew Nitrox/ O2 calcs.
Cc:
Bcc:
"Ian" wrote:
>I'd be very interested in seeing some tables and/or the calcs involved to
>work out mixes.
My pleasure. I'll give the formulae; let's say you want to end up with
X% Nitrox at pressure P in your tank. To keep it simple we assume temperature
changes in the tank are kept to a minimum (nice cool bath, slow fills)
and the tank starts empty, is partially filled with O2 and topped off with
air. What pressure (call it p) should the partial fill be? Let's write
x for X/100, i.e. X% written as a fraction.
The partial pressure of O2 at the end WILL be:
p atmospheres of pure oxygen I put in plus
0.21(P - p) atmospheres of pure oxygen present as part of the
air used to top off.
The partial pressure of O2 at the end SHOULD be just:
Px
so,
Px = p + 0.21(P-p)
0.79p = (x-0.21)P
(x-0.21)P
p= ---------
0.79
Trivial Examples to check the formula:
I want 200 bar of pure oxygen.
P=200, x=1.0 so formula gives
p= (1.0-0.21)*200
-------------- = 200.
0.79
Right! you fill the tank with pure O2 and don't top off at all.
I want 230 bar of air.
P=230, x=0.21 so formula gives
p= (0.21-0.21)*230
-------------- = 0
0.79
Right! Don't need any O2 to get pure British air.
Real examples:
I want 210 bar of 33% Nitrox.
P=210, x=0.33 so
p= (0.33-0.21)*210
-------------- = 31.9 bar initial fill.
0.79
I want 3300 PSI of 33% Nitrox (my "standard fill" cavediving in
Florida)
P=3300, x=0.33 so
p= (0.33-0.21)*3300
----------------- = 501 PSI initial fill.
0.79
Happy Mixing!!