blending nitrox

[bell47]

If cascading from an O2 tank (it says 2015 and is about five feet tall, I don't know the CF), about how many alum 80's can you get 30% blend after you top off with air?

Before everyone says I'm gonna blow myself up, I'm just running some numbers! I'm just curious, and have no intention of doing partial pressure blending without proper training!

 

[paddler3d]

This will not directly answer your question. Check out Vance Harlow's Oxygen Hacker. Vance Harlow's OXYGEN HACKER'S COMPANION From AIrspeed Press

 

[bell47]

got it, thanks

 

[paddler3d]

p. 7 has a section about economics. VH uses the example of a 250cf K or H tank. It will fill aprox 15-20 tanks of EAN 32%. He doesn't say what size tank, but that may get you close enough for what you are looking for.

 

[bell47]

yeah, but that is with a booster. I am curious about just a cascade. Thanks though!

 

[350xfire]

Assuming 440 cuft, you'll have .11 cu ft per psi.... Now, you can only use 1000 psi out of the 4,000 psi cylinder (if that's the fill psi for the big cylinder). So 1,000 times .11 is about 110 cu ft. So only about 1 full fill plus 30cu ft... This is for 100% fills.

For 30% fills you'll only need 9% of O2 needed to fill the tank to 100% since air already has 21% already. So I would guess about 10-11.

I don't know if my PSI and cuft for the large tank are correct though.

 

[coldsmoke]

The 02 tanks you are talking about are probably about 250 cuft. If you are pp blending with pure O2 and topping with air (21%) to achieve 30% EAN at 3000 psi then you will get about 17-18 fills before the supply cylinder gets too low. It's not likely that cascading your O2 will be of much benefit. You'll only leave a few hundred psi in the supply bottle and 02 is pretty cheap.

HTH,

Hunter

 

[coldsmoke]

Assuming 440 cuft, you'll have .11 cu ft per psi.... Now, you can only use 1000 psi out of the 4,000 psi cylinder (if that's the fill psi for the big cylinder). So 1,000 times .11 is about 110 cu ft. So only about 1 full fill plus 30cu ft... This is for 100% fills./QUOTE]

The only bottles I have ever see that hold 440 cuft are at 4500 psi. I have never seen O2 supply tanks at that high a pressure. I'd sure hate to be the guy that has to fill those cylinders.

For 30% fills you'll only need 9% of O2 needed to fill the tank to 100% since air already has 21% already. So I would guess about 10-11.

Your math isn't quite right because you're not allowing for the volume displacement from the O2.

Hunter

 

[DIWdiver]

Here's my take on it.

5ft tall, 2150 psi, it's probably an A bottle (see Standard Cylinder Sizes). Or a K bottle, or a 1A, or a B. The name depends on who made it. Any way you look at it, its around 44 liters, which at 2015 PSI makes it about 215 cubic feet of O2.

To fill the AL80 to 30%, you don't fill 9% with O2 and top with air, as would seem intuitive. That gets you only 28%. Here's the math:
0.09*100% + 0.91*21% = 28%.

Though it would be foolish to do it this way, think of it as filling 70% with nitrogen, then top with O2. Since your source of nitrogen is only 79% pure (contaminated with 21% oxygen), you need
70%/0.79 = 88.6% of the tank filled with air, the remainder (11.4%) with O2.

Doing the math forward (as the first equation), you end up with
0.114*100% + 0.886*21% = 30%.

So in practice you add 11.4% * 3000psi = 342psi of O2. You can do this from any starting pressure of air in the tank, so long as your O2 tank has sufficient pressure to do the job.

Now, how many times can the O2 tank do this? Lets assume that on the last fill you're going to get, you will bleed your AL80 to 100psi before starting. Thus at the end of the O2 portion of the fill, you have 442psi in both tanks. That means you waste 442/2015*215= 47 cubic feet of O2, because you can't get it out of the tank. What you can get out is 215-47=168 cu ft.

Each fill requires 11.4%*80 cu ft, or 9.12 cu ft, so the number of fills is 168/9.12=18.4.

In practice, you may get MORE than this, because after the first fill, you are starting with 30% in the tank, and if you don't suck it dry you don't require the full 9.12 cu ft per fill. Of course, knowing how much to put in a tank already partially filled with EAN requires a good understanding of the math behind it. That, or a good understanding of the fancy app on your iPOD that does the math for you. You also MUST test the mix both before and after filling.

Using a booster would allow you to recover part of the 47 cu ft you would otherwise waste (all boosters require some input pressure). Using a blending stick would allow you to use every last cubic foot, but that's another thread...

D

 

[bell47]

Well if I start with an empty Alum 80 and a full O2 tank ( yes these are the standard type/size and look like a welding bottle). The top off program on V-planner says I need like 400 psi then top off with air. I guess the better question would be how many times can I get 400psi in an alum 80 from the o2 tank before the pressure gets below 400psi in the big tank? I'm pretty sure they are 250cf at 2150 psi.