Gas blending help

[bcooper67]

I just recently took my Basic Nitrox course and really got into it...maybe too into it. I'm starting my TDI advanced Nitrox and Deco Procedures, but since basic nitrox, I've become obsessed with learning more and more about gas laws and calculating Fractions of gas. On my own, I've figured out how to calculate tri mix based on planned depth and desired END of N2. This really just made me want to know more. So, based on some talk of our LDS considering carrying banked 40% Nitrox, I got to wondering how would I manually calculate certain fills. This should be simple, but it's just not turning out that way.

Say I have a partially filled tank and i want to fill it using 40% Nitrox and air as my top off gas. How would I go about calculating it? btw...I have a software program that figures it all out, but my goal is to totally understand it and be able to do it all manually.

Problem: HAVE: 80 ft3 cylinder filled with 400 psi of 32% Nitrox
Desire fill of 3000 psi of 34% Nitrox
Using 40% Nitrox(banked) and air as a top off gas

Can somebody walk me through these calculations?

Thanks

Coop

 

[sea2summit]

This is best learned from a certified instructor not over the internet.
I don't know your certs just like you don't know mine.

 

[Thalassamania]

Nitrox Fill Calculator

 

[TSandM]

It's actually pretty simple.

You want 3000 psi of 34% Nitrox. So you want .34 x 3000 = 1020 psi of O2. You have 400 psi of 32% Nitrox, which is .32 x 400 = 128 psi of O2. So you need 892 psi of O2.

Now, because you are using two fill gases with both oxygen and nitrogen in them, you have to solve simultaneous equations in two unknowns. You have 400 psi, so you need 2600 more.

2600 (added psi) = x (amount of 40%) + y (amount of air)
892 (psi of O2) = .4x + .21y

x = 2600 - y

892 = .4 (2600 - y) + .21 y
892 = 1040 - .4y + .21y
892 = 1040 - .19y
.19y = 148
y = 779 psi (air)
x = 1821 psi (40%)

These calculations become trivially easy if you use 100% O2 instead of 40% as your O2 source.

 

[Spimon]

*EDIT* Just saw you have a program to do it but want to do it manually. I'll leave my post here for future searches anyway.


I've made a spreadsheet to calculate things like this.

There's 2 things mine does;

1. Calculates the final mix if you start with X% @ x bar and add Y% to end up at y bar.
2. Calculates the required mix to add to X% @ x bar in order to achieve a particular final mix at a particular final pressure.

It's available on my website in the 'tools' section called 'Nitrox Blending Calculator'. Much more basic than some Nitrox calculators out there. It is all in metric so won't be useful to a lot of SB'ers.
Simon Yachting - YA/RYA Skipper

 

[bcooper67]

Thanks guys, especially TSandM. Simple algebra! I should have been able to figure it out on my own, but sometimes you just can't see the forest for the trees. This is the same type of equations we went through in high school and college Chemistry...obviously I didn't retain it all.

side note: It's quite amazing how many people CANT do the above calculation manually. I asked my instructor (Padi MSI) and he couldnt do it. He said that's what they have programs for. What would we do if we lost our computers or iPhones? It would be a DARK day : )

 

[TSandM]

I majored in math . . .

Actually, a funny story. When I took my RecTriox class, we did blending calculations. The instructor was taking us through doing calculations for blending Nitrox, using this simultaneous equations method, IIRC. I looked at the problem and finally couldn't stand it any longer, and asked if I could show him another way. I got up, ran through the problem in a few seconds (using 100% O2, you can simply solve for nitrogen) and he looked at me in complete befuddlement. Smartass students

 

[kanonfodr]

side note: It's quite amazing how many people CANT do the above calculation manually. I asked my instructor (Padi MSI) and he couldnt do it. He said that's what they have programs for. What would we do if we lost our computers or iPhones? It would be a DARK day : )

Hey, don't hate on us lazy divers .

Peace,
Greg

 

[EGad]

Thanks guys, especially TSandM. Simple algebra! I should have been able to figure it out on my own, but sometimes you just can't see the forest for the trees. This is the same type of equations we went through in high school and college Chemistry...obviously I didn't retain it all.

side note: It's quite amazing how many people CANT do the above calculation manually. I asked my instructor (Padi MSI) and he couldnt do it. He said that's what they have programs for. What would we do if we lost our computers or iPhones? It would be a DARK day : )

That reminds me of one of my high school algebra teachers. He was convinced that calculators were a Communist plot to make us dependent and then take them all back to screw with our engineers and scientists. Pretty creative and thought provoking idea, but he stuck with it into the 1990's, so I don't think the threat was as serious as he made it out to be. Although he was a great teacher and very entertaining. (For a math teacher.)

Also, tried that speech with my nephews. Didn't work. Solidified my reputation as the crazy uncle.