Trimix gas blend

[JeffG]

Why don't you explain why you don't like 25/25? AG's reasoning seems sound to me. :huh:

Nothing wrong with 25/25 (other than it doesn't really have a use in real life)

How many "fills" are you going to get with your 130?

I suspect Jeff was refering to a prior post.

Unfortunately, Almost the whole thread.

 

[cool_hardware52]

I didn't think about that when I bought double HP 72s. I am thinking about getting a HP 130 and getting it filled with EAN32 and then I can PP fill AL 80s with 25/25 using just He and EAN32.

You might want to run the numbers first.

Assuming you first decant 750 psi of He into an empty 80 (3000 x .25 = 750)

You will then need 60 cu ft of 32% to finish the fill

130-60 / 130 = .54 .54 * 3442 = 1858. How are you going to transfill an 80 to 3000 if your 130 is now at 1858?

You either need a booster, or a larger bank of 32 or a higher pressure bank of 32.

Even if you succeed how useful is a single 80 of 25/25?


Tobin

 

[amascuba]

I didn't think about that when I bought double HP 72s. I am thinking about getting a HP 130 and getting it filled with EAN32 and then I can PP fill AL 80s with 25/25 using just He and EAN32.

You won't get, but maybe one or two fills with a 130 cf tank unless you had a haskel booster. Yes 130 cf maybe bigger than 80 cf, but you won't get 130 cf worth of gas out of it. You will only get enough gas out of the 130 cf for the tanks to equalize in pressure.

If your LDS offered Modified Grade E air (oxygen compatible air) you would be better off contacting your local gas dealer and getting a 450 cf bottle of oxygen and/or helium, filling your tank with the required amount of oxygen/helium and then taking it to the LDS for a top off.

Around here I believe that a 450 cf bottle of O2 is running about $8/month for bottle rental + $.04 per cf of O2. Pretty darn cheap - around $18 for a full top off of O2.

I have no idea of what helium is going for.

Also doing this requires the tanks and any component coming in contact with high partial pressures of O2 to be O2 cleaned. I'm sure you already knew that, but I'm sure there will be people reading this thread that don't. So goes the generic warning: Don't do this at Home, without the supervision of a trained professional.

Even more so in the long run it would be more cost effective to just buy a compressor and do continuous blending, not to mention less risk of oxygen fires.

 

[rjack321]

Nothing wrong with 25/25 (other than it doesn't really have a use in real life)

If I were in New England, I'd probably use it. Alot of their wrecks seem to be just beyond 30/30 range and you never know where the boat's going until the day of.

 

[TheRedHead]

If your LDS offered Modified Grade E air (oxygen compatible air) you would be better off contacting your local gas dealer and getting a 450 cf bottle of oxygen and/or helium, filling your tank with the required amount of oxygen/helium and then taking it to the LDS for a top off.

That's the problem I wrote about earlier. The LDS will not top off tanks. They don't want anything to do with helium. And yes, it would take a lot of fills on the HP tank to a lower pressure tank to fill a tank. So there is not any easy solution to the problem aside from a compressor.

And finding an LDS that will top off tanks is not a useful solution in my area. They don't.

 

[JeffG]

If I were in New England, I'd probably use it. Alot of their wrecks seem to be just beyond 30/30 range and you never know where the boat's going until the day of.

I would go straight to 21/35, but thats just me.

 

[JeffG]

That's the problem I wrote about earlier. The LDS will not top off tanks.

They will fill with O2, but avoid helium? Tank Monkeyitis. You should show up to the LDS and hand out bananas.

 

[TheRedHead]

Even if you succeed how useful is a single 80 of 25/25?

Two would be perfect for my uses, Tobin.

 

[TheRedHead]

They will fill with O2, but avoid helium? Tank Monkeyitis. You should show up to the LDS and hand out bananas.

They sell premix and I don't know if they buy it that way, blend it, or use a membane system. The call it "Safe Air." As far as helium, they consider it technical and don't embrace it, although one instructor does teach Deco Procedures through TDI.

 

[amascuba]

That's the problem I wrote about earlier. The LDS will not top off tanks. They don't want anything to do with helium. And yes, it would take a lot of fills on the HP tank to a lower pressure tank to fill a tank. So there is not any easy solution to the problem aside from a compressor.

And finding an LDS that will top off tanks is not a useful solution in my area. They don't.

Define HP for the tanks you are thinking about. 3500?

For a 130 cf tank that is rated to 3500 psi has 26.92 psi per cf. If you took a 130 cf containing 32% and filled an AL80 cf tank, rated at 3000 psi, with 32% you would only be able to fill the tank to 1750 psi. That's 65 cf out of your HP 130 cf tank. That only leaves you with 65 cf left in your HP 130 cf tank and your AL80 is only has 46.67 cf in it. At that when the O2 cools down in the tank you can expect a 10% loss in pressure, which would leave you with about 42 cf of 32% in your AL80.

Not very many fills at all. Interested in the math of how I figured that out?

rated tank pressure/rated tank volume = psi per cf

HP130 = 3500psi/130cf = 26.92 psi/cf

AL80 = 3000psi/80cf = 37.5 psi/cf

Difference in pressure between HP130 and AL80?

3500 - 3000 = 500 psi

When will the tank pressure equalize in the tanks?

1500 psi = half of the pressure of an AL80

500 psi / 2 = 250 psi.

1500 psi + 250 psi = 1750 psi.

How much volume does that leave in each tank?

HP130 = 1750psi / 26.92psi/cf = 65 cf

AL80 = 1750psi / 37.5psi/cf = 46.67 cf

How did I figure the 10% loss in volume? To tell the truth it's something that I've always been told to expect when doing hot fills, so I always just calculate it in.

1750 / 10% = 175psi

1750 - 175 = 1575 psi

1575psi / 37.5psi/cf = 42 cf left in the AL80 after the 32% cools down.