Gas Blending Math (He and Nitrox)

[Raywerner]

Thanks to the wonderful information Ihave gleaned from posts on this board, and in Vance Harlow's super Oxygen Hacker's Companion, I have built a continuous mixer (knew I could find a use for that OxySpy) to use with my Max-Air 35. Now, the question I have is where can I find the math that would allow me to blend tri-mix using pp blending of He, and topping it off with Nitrox?

Thanks!

[Stone]

Since you are already using continuous mixing for Nitrox, consider continuous mixing for Trimix.

Uncle Pug and Omar can help.

In the mean time, try a search using some of the key words in this thread to find some good information and pictures of some nice mixing stations.

Good Luck!

[Uncle Pug]

Raywerner once bubbled...
where can I find the math that would allow me to blend tri-mix using pp blending of He, and topping it off with Nitrox?
Ray Continuous Blending trimix efficiently would take a helium analyzer. Expensive but it sure makes topping up partially full tanks easy.

Doing what you propose is not difficult. GUE standard gases can be PP blended using EAN32 for topping.

However... if the math escapes you I am wondering if you have taken a blending course and/or a trimix course?

Still there are several free mix programs on the web that will do the figuring for you... but you really need to have a good understanding of gas blending anyway IMO.

[Scuba_Vixen]

if the math escapes you I am wondering if you have taken a blending course and/or a trimix course.....


Tripped the same trigger with me .....
I know by the time I got to blending for myself, what part of my brain the dive computer hadn't rotted, was pretty well smoked from crunching numbers learning mix.

I found my TDI blender course made me do all the math with just a calculator and the formulas. When I did the DSAT course, they just give you a program on a CD for the PC. (You'd be SOL if the PC went down and you had tanks to fill)... Unfortunately, both fail to account for real gas factors (z factors), so if you don't have one of the good excel programs to use to make some adjustments with, neither will get you spot on consistantly.

Ray, by the time you're really ready to do some blending, you'll be giving the "where to find the math" info to the next guy, lol


Darlene

[Raywerner]

if the math escapes you I am wondering if you have taken a blending course and/or a trimix course.....

Uncle Pug and Darlene,

I am in the "just playing with it" stage, and I will be taking the blending course this winter. I did follow Pug's advise and build in the ability to do continuous Tri-mix blending when I constructed the mixer, but I am just trying to gain an understanding of the process before I plop down the $90 per bottle of He that my gas supplier wants. I am fortunate, though, in that I do have an OxyCheq He analizer so I will be ready when the time comes to do the blending.

That's kinda why the question. Doing Nitrox is simple enough, adding the He is a new thing to me and I am interested in learning as much as I can before I take the course. (Old brain cells need a leg up, you know.)

Ray

[Uncle Pug]

Raywerner once bubbled...
I am interested in learning as much as I can before I take the course.
Check out the Atomox website.

[blacknet]

Hello,

You could use various blending packages available on the web. My package is at http://wetlands.simplyaquatics.com it does nitrox/heliox/trimix.

Ed

[Scuba_Vixen]

or is it still just based on ideal gas law equations. Would be nice to see both, so you can tweak in between if you need...

[blacknet]

Hello,

that's in the works and the palm version does account for He compression by adding 5%. To my understanding O2 compression is very minimal.

Ed

[Scuba_Vixen]

Actually, the Z-factor for O2 is less than 1, so you'll add a bit less than the ideal equation gives, same for the top off with air., The He is about 1.06 (these change relative to pressure, but those are approx at 3000)

It all translates to about +60# He and -8# of O2 and -50# air on a mix of 15/55 at 3200# .... The usual add 5% rule of thumb comes pretty close

Looking forward to the final product....

Darlene

[blacknet]

Scuba_Vixen,

So what % for O2 and Air?

Ed

[Scuba_Vixen]

They vary with gas percentage and pressures. Take a look at this spreadsheet as you plug in different mixes and see the change in z-factors and the percentage change they represent in the amount of gas to add or subtract from ideal.

http://www.atomox.com/documents/real_gas.xls

There's a pdf format page that has all the equations if you want to incorporate them into your program.

An interesting way to see the interplay of the z-factors is to see the values for heliox mixes, use the same mix at different pressures (like 2000# and again at 3200#...as if you made it without a booster and then with)

Then look at something like 18/35 and then 10/70 (both at 3200# or so, which is a normal fill range), again you'll see more change as a higher percentage of the mix is a more compressable gas.

Hope that helps,


Darlene

[blacknet]

Hello,

I have used the the table on that site but would like to use the formula. The only problem I see is there are way to many variables that comes into play with the Z factor. I have talked to many blenders and everyone tells me they use +5% for He and use the idea number of O2 and top with air. Said their typical mixes of He falls between 40-60%, which is 1200 and 1800 psi. according to the chart that is in the 5% range.

What would be good is to use a basic Beattie-Bridgeman equasion instead of the chart.


Ed

[MikeFerrara]

If you take a couple days to mix it works well to use real gas laws. However if you're not going to take the time to let things ccol and the re-top temperature has a greater effect than compressability. In that case you do better by guessing and just adding a little extra He and maybe a little less O2.

My wife takes two days to mix and uses the Atomox spread sheet. I just dump in the gas, analyze and dive.

[Scuba_Vixen]

isn't going to be a major factor in the first mix or maybe two, By the time you get to the 5th or 6th fill without dumping and starting fresh, The accrued error can get to be significant, if you're not being as accurate as possible with each fill. If you live where He prices are reasonable, dumping every 2 to 3 fills is easy. At almost $200 a bottle, I try to use it efficiently. (though I believe saving money by using lower % is false economy).

All in all, 5% as a rule of thumb, probably accounts well for compressability and heating. It's just that some of us anal retentive engineering types have too many thumbs.

Darlene

[omar]

The Beattie-Bridgeman equation is a complicated fit to experimental data. It requires 5 specific parameter values in addition to the critical point values for each gas. The B-B is not a trivial matter to implement. A typical learning exercise for upper level Chem-E majors is to write a FORTRAN program that will calculate the Z for a selected gas at a specific temperature. Most don’t get it right.

The individual behavior of the gas is dependent upon the molecule size and its attractive forces. The Atomox spread sheet is a good general guide on the real gas behavior for high pressure and small (relative) volume interactions. More detailed specific needs go beyond typical scuba applications.

An important additional factor to consider when mixing is the accuracy of your gauges. It does not take long to for the errors to multiply.

omar

[blacknet]

Hello,

You are correct that it's not an easy process. I have been consulting my thermodynamics book on this and i'm not sure if it's worth investing the time and energy. For most blenders they use 40%-60%ish of He and o2 is close to 1 it's probably not worthwhile adding anything extra in.

Ed

[blacknet]

Scuba_Vixen once bubbled...
or is it still just based on ideal gas law equations. Would be nice to see both, so you can tweak in between if you need...

Hello,

I just added a zfactor lookup. The metric conversion is still abit on the buggy side but the imperial works great.

Ed

[Uncle Pug]

Your blender program file is huge! It would take me a loooong time to download on a dialup. How is it different than the Atomox blender... other than the later's compact size since it is an excel file?

[blacknet]

Hello,

The PC version which is rolled all inclusive install package is 11 megs, that includes MDAC 2.6. If you already have mdac then you can just get the exe which is 164k. The exe is at http://wetlands.simplyaquatics.com/software/blender-PC.exe

I'm not familar with the Atomox package. Mine does zfactor (still working on some bugs with this), cost of gas, metric/imperial (tho the metric has some slight issues) hypoxic, max end, po2, ead, mod@1.6 and 1.4, best mix @1.6 and 1.4, blowoff, tank stats and bank mix top off.

Ed

[jeepster0000]

Does anyone actually know the formula with the Z-factor included. I know the formula is a PITA but I am writing my own blending program so I want it for that. I have the TDI advanced gas blending and service manual but it only uses the ideal gas law. I should know this from my physical chemistry class, but I forgot and I have to go and find my book. Help me if you can.
Thanks
ADAM

[blacknet]

Hello,

Yes I have it, give me a few days and i'll post it. There happens to be 5 variables to work with and the main equasion with around 5 sub equasions to solve. I used the table pasted above and doing the lookup method. It's alot easier than doing the math.

Ed

[voidware]

The easy way to do this is with liquid gases.

1 cup liquid nitrogen
2 1/2 cups liquid helium
1 1/3 cups liquid oxygen
dry ice to taste
shake well. Serve chilled.

there no worries.

Brandon

[blacknet]

Hello,

Sorry it's taken so long to reply.

Beattie-Bridgeman equation of state
(Air from - 145 ¡ãC to + 200 ¡ãC)

p * V^2 = R * T * [ v + 0.04611 * (1 + (0.01101/V)) * (1(4.34*10^4)/(*T^3))

p = pressure, atm
v = molecular volume, lit/mol
R = gas constant 0,08205 lit¨Zatm/mol¨ZK
T = temperature, K.


And I prefer mine stirred not shaken.

Ed

[jeepster0000]

Awesome thank you so much. Now I will write a progam and see how it tests out.