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

[Basking Ridge Diver]

I am not Nitrox Certified currently - I am learning the formulas for FO2, PO2, MOD and EAD - they are somewhat straight forward and I believe I understand them.

The question is are there formulas (I assume in volume) that could tell you - I have an LP 85 with 500psi clean air so 21% - so I should fill X volume of O2 the rest with clean air to attain a 32% mix? I know you can test the tank as you proceed but that does not seem very efficient - so I looked in the Compressor threads but did not see this as a question or formula.

It seems like Blending should have formulas or is that just something you don't even need to know - leave it to the LDS?

 

[rhwestfall]

yes there are, google it (Gas Blending Software for SCUBA).

 

[tbone1004]

what Bob said, but you won't have to worry about any of that in your nitrox course. It is all fractional algebra, nothing complicated, but unless you are planning really specific dives where you have to have that nitrox percentage spot on and thus require PP blending, most of the time you'll be topping off with premixed nitrox from the banks and it will get you close enough to just analyze when done.

Technical Diving and Recreational - Nitrox Calculator will do it for you, the NOAA diving manual has PP blending formulas in it, but it basically involves breaking down how much fraction of each gas you currently have, how much fraction you want, and what that equates to in pressure, translates independent of tank size since it is all done in PSI.

 

[scoobydrew]

FYI - that kind of stuff is covered in a typical gas blender course

 

[Keith.M]

I'd also recommend the Oxygen Hacker's Companion book from Airspeed Press.

 

[boulderjohn]

There is no reason to know how to do this unless you plan to blend your own nitrox. If you do, you should get proper training. You do not want the whole process to blow up in your face--literally.

 

[Subcooled]

Amount of oxygen:

- Let us assume an ideal gas: Amount of normobaric gas = amount of high pressure gas times pressure in ATA This is not quite accurate, but a decent enough assumption.
- In the equations below the pressure P is given in ATA (atmospheres absolute).
- The internal volume is 3.14 x Radius x Radius x Height = how much water fits inside.

Eq1: P(air in tank) x V(tank) x F(02 before ) + P(increase when adding oxygen) x V(tank) x 1 = P(total after adding oxygen) x V(tank) x F(O2 after)

You can divide this by volume of tank V(tank) to get:

Eq2: P(air in tank) x F(02 before) + P(added oxygen) = P(total when full) x F(wanted)

Eq3: P(air in tank) x 0.21 + P(added oxygen) = P(full tank) x 0.32

This equation has two unknowns: pressure change due to added oxygen and the amount of needed air. It is also clear that you could calculate either in ATA or psi or pascal. Any fixed scaling factor can be divided away

In addition you have:

Eq4: P(air)+P(added)=P(full), which tells the relationship between P(air) and P(added)

Solve Eq3 and Eq4 for P(added oxygen). Then top with air.

As this calculation is based on ideal gas laws, and ignores compressibility and temperature rise/fall, the result is approximate. It gives an idea though. In reality pressure change is not linear. After a certain threshold pressure increases faster than the amount of added gas. Then you would need to calculate using gas weights and real densities (kg/m3 @Pressure). Software can be used to calculate more accurately.

 

[lowviz]

... It seems like Blending should have formulas or is that just something you don't even need to know - leave it to the LDS?

If you own your own cylinders/tanks then the answer is: Nope.

Enlightenment (wrt all those equations) comes when you realize that a transfill whip will save you money..

 

[Subcooled]

If you have oxygen clean cylinders and personal filters to protect those and a decanting hose with a needle valve and an oxygen storage tank, then you could do partial pressure fills of nitrox at home and just top with cheap air at the LDS. For economical reasons. If you just drop your cylinders at the LDS for nitrox refills, then you don't need to know a thing. Just make sure you own an oxygen analysator. A CO-analysator would not hurt either if you travel a lot.

 

[boulderjohn]

If you own your own cylinders/tanks then the answer is: Nope.

Enlightenment (wrt all those equations) comes when you realize that a transfill whip will save you money..

Maybe. It depends upon how much nitrox you are making and a whole lot of other factors.

In some places, including the location in Florida where I spend part of each winter, nitrox in a shop costs barely more than air. You would have to make a whole lot of nitrox to break even on the cost of a transfill whip, and that does not count the cost of the oxygen supply.

Where I live in Colorado, if I want nitrox, I can pay a fee to one of the very few local shops that offer it, or I can make my own. The most expensive nitrox I have ever used, by far, is the stuff I make for myself. A good transfill whip is quite an expense, and getting your own oxygen supply bottle(s) is by no means cheap. I only do it because when I need to make it, I am usually off where I have no ability to buy it from a shop and have to blend my own in the field. If I could get and pay for local nitrox, I certainly would.