Are there formulas for Blending Nitrox and how to achieve a certain percentages?

[Kevrumbo]

It is true that my response was intended as Ray took it--there is no more need for the average nitrox user to know these formulas than it is for the average automobile driver to know how the blend of gasoline is made.

On the other hand, a little learning can be a dangerous thing. If someone looks up the formulas, rents an oxygen supply bottle, and attempts to make his own nitrox without understanding the need for oxygen clean equipment and proper technique for handling the oxygen, the results can be quite unpleasant.

What a patently cynical, patronizing and epistemological "Fob-off" response --so disappointingly ironic coming from an "Educator" (and Moderator).

Shame on you again John. . .

 

[rx7diver]

There is no reason to know how to do this unless you plan to blend your own nitrox.

Basking Ridge Diver,

I just began reading through this thread. Maybe there's a post further down that addresses this. Meanwhile, let me state that this statement is the antithesis of how I and many people I know value education. I encourage anyone to learn whatever catches his/her interest, as thoroughly as he will, for whatever reason. FWIW, quite a while ago I taught myself quite a bit about blending (it's not terribly difficult stuff, as others posted), without apology, even though I have no interest in ever blending my own diving gas.

EDIT: I just finished reading the entire thread. I see that others, too, have posted their disagreement with the statement quoted above.

FINAL EDIT: For the sake of full disclosure, let me add: After I was comfortable with my understanding of blending, I actually "blended" my own gases. I owned (own) three oxygen-service OMS 46 cylinders, an oxygen-service transfer whip, an oxygen analyzer, and two oxygen-service PST HP (3,500 psig) 100's partially filled with oxygen-compatible air.

As I recall, one of the OMS 46's was completely full, containing 100% oxygen. The other two OMS 46's were partially filled, each with different blends of EAN at different psig. I calculated that if I titrated so much oxygen from the oxygen 46, to the other 46's, I would end up with this EAN mix and that EAN mix. I put an album on the turntable, took everything out onto the patio, titrated, waited, and then tested. The results were as predicted.

Then I calculated that if I titrated so much oxygen compatible air (from my PST 100's) to the EAN 46's, I would end up with this EAN mix and that EAN mix. I titrated, waited, and then tested. The results were as predicted.

Then I took my EAN 46's to the dive shop where I had purchased my OMS 46's and PST 100's and the original gases that were in them. I explained to the owner (my late friend and first technical diving instructor) what I had done, and showed him calculations regarding the mixes I expected to have once my EAN 46's were topped off with oxygen-compatible air from his banks. He wasn't aware that I had been teaching myself about blending and was as giddy and excited as I was to complete this demonstration, which we did. Again, the results were as predicted. A wager was involved. I won!

And that, gentle readers, completes this post!

Safe Diving,

rx7diver

 

[lowviz]

Jeez, guys. A little hard on John, today?

Kev, it is awfully goddam rare that you leave enough of an opening to bust on you for your calcs. I had a moral obligation to do so. FWIW, I need the exact equations or I can't do dynamic modeling. It is fun to spin the dials and do "what iffs".

Yeah, pure O2 demands respect. 99% of that is just using clean equipment and *S*L*O*W*L*Y* opening valves. That procedure also applies to deco bottles.

 

[Basking Ridge Diver]

I looked at the Oxygen Hackers Companion and just in the this little snippet I found value -
So I may have found a book that I can understand and gain further information but I am not looking to become a blender just understand what is really going on.

[FONT=Verdana, Arial, Helvetica, sans-serif]On Nitrox Mixing Calculations:[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif] "The place where a lot of home mixers get into trouble in partial pressure mixing is figuring out how much O2 to prime the tank with to get a desired percentage of O2. The really dumb ones say "let's see, I want 32% nitrox, and it's a 3000 psi tank, so I'll add - let's see, 32% of 3000 is 960 - 960 psi of O2!"
The slightly smarter ones figure, "I want to make 32% nitrox, but since there¹s already 21% O2 in air, that means I should add - uhmmm, 21 from 32 is .... 11% more O2! 11% of 3000 is 330 psi, so I'l l add 330 psi of O2. "
The first method is obviously totally bogus since it does not take into account the O2 already in the air. That diver would end up with something like 46% nitrox, and, very likely, CNS poisoning.


[/FONT]

 

[Slamfire]

It also doesn't take much recreational nitrox diving for you to be able to estimate "If I have 1000psi of 32% and have the shop top off to 3000psi at 21% I'll end up with about 25%" off the top of your head.

The math required to directly solve the OPs question is really simple. Using Ray's example:

1000 psi x 32% = 320 psi O2

You will add 2000 psi of 21% for a final pressure of 3000 psi. So...

2000 psi x 21% = 420 psi O2

420 psi + 320 psi = 740 psi O2 in your final mix.

740 psi O2 / 3000 psi total pressure = 24.66667% O2 final mix.

 

[lowviz]

... I am not looking to become a blender just understand what is really going on. ...

Oxygen, Nitrogen, and Helium all compress differently. To help understand the significance of this: Take a given volume of each pure gas at standard temperature and pressure and compress same to a new, much smaller given volume. Let them all come back to standard temperature. The final pressures will all be different.

Oxygen and Nitrogen will be close to each other, Helium will be further off. These gasses don't behave as ideal gasses. There is a deviation that causes problems with your concentrations when you use helium.

You can pretty much ignore the whole issue for oxygen and nitrogen.

 

[Kevrumbo]

Jeez, guys. A little hard on John, today?

Kev, it is awfully goddam rare that you leave enough of an opening to bust on you for your calcs. I had a moral obligation to do so. FWIW, I need the exact equations or I can't do dynamic modeling. It is fun to spin the dials and do "what iffs".

Yeah, pure O2 demands respect. 99% of that is just using clean equipment and *S*L*O*W*L*Y* opening valves. That procedure also applies to deco bottles.

You don't need a course in Quantum Mechanics if all you need to figure out is your car's gas mileage.

If compressibility is an important issue for you, then great! Objectively enlighten us on why you think this is not a trivial matter for yourself --but not by dubiously claiming "moral obligations" rhetoric as motivation to do so.

You don't need a Physical Chemistry & Thermodynamics Course to practice this --just Nitrox Diver and a Blender Class to learn the basics and caveats of working with pure O2.

 

[dumpsterDiver]

I used to mix my own tanks in the garage. I first took a nitrox course and then just did the math. I used to hate dumping left over nitrox since it is a waste of money and oxygen. I figured the ratios out and wrote the formula on a spreadsheet to figure out the OP's question. I see there are lots of off the shelf solutions available now.

I'm surprised that people would think that understanding ratios.. that all this is... is too hard. we can assume prefect gas laws and get good results.

Having a working knowledge of this stuff can be useful to any diver. I often use an equilizer whip between nitrox tanks on the boat. What if someone is presented with the option of whipping tanks back and forth and they have no analyzer on the boat? Having a working knowledge of this stuff can be very useful.

When I used to commercial dive (far off shore) I figured the recipe to make a 50% bottle by mixing pure oxygen and 32%.. (the gases we normally had on hand) just in case I got really bent and wanted the guys to mix me something I could go deeper on and start the in-water recompression.

The only reason the dive shops who use partial pressure blending dump the tanks to zero is because it is easier and less chance for error by the shop monkeys.

About 20 years ago,, the local shop bought a membrane system and they banked two mixes.. I made up a whole set of tables for them using spreadsheets, so they could dilute their bank mix down super fast and easy by just topping off with air. I had a chart for each bank mix and for each possible tank pressure.. Once again, understanding ratios and how to do the arithmetic is not going to be detrimental to the casual diver.

 

[lowviz]

If compressibility is an important issue for you, then great! Objectively enlighten us on why you think this is not a trivial matter for yourself ...

Well stated, nice jab. I'll entertain your question. But first, lose the "us". Nobody cares but you.

Mathematical modeling is an interesting subject to me. I had the good fortune to be educated by a math teacher (in my far distant past) that truly understood math. He passed on to me the fundamental idea of what math really is and how to use it. That remains with me today.

If I don't model compressibility my real-time simulation gives me the wrong answer and that gives me an incorrect understanding of how to blend accurately. So then why bother? Just pump the tanks, analyze, and adjust. Diving is my recreation and I enjoy the math in it too. Probably the best answer to be had to your challenge.

--but not by dubiously claiming "moral obligations" rhetoric as motivation to do so.

It's called "humor", dude.

You don't need a Physical Chemistry & Thermodynamics Course to practice this --just Nitrox Diver and a Blender Class to learn the basics and caveats of working with pure O2.

Yep, but you get into problems with helium. I'm planning on going there shortly. Now there is the REAL "geezer gas".

- D

 

[Storker]

we can assume prefect gas laws and get good results.

Provided your tanks aren't rated to higher pressures than some 230bar/3300psi, that is.

On this side of the pond, we've got 300bar/4350psi rated tanks, and compressibility/non-ideal behavior quickly becomes an issue when you pass 230bar/3300psi pressure. A 10Lx300bar tank holds about 90% the amount of gas compared to a 15Lx200bar tank, even though the nominal capacity is the same. That can give some... interesting results if you PP blend and don't analyze afterwards.



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