How do I get 28% from 1000psi 32%

[Dexloy]

So I usually mix 32% EAN in 120cf hp tanks by CB, I f I want to mix 32 for the next dive no problem, but if I want a different mix then I have to either drain the tank and fill or guess what to top it off with. I know there must be a formula for what % to add to what % but I can’t figure it out. Anyone have the equation to do that? Say that I have a 120hp that has 1100 psi of 32% EAN but next dive I want 28%, what % would you add to the 32% to come out with 28% at 3500 psi.

 

[oly5050user]

need 157 psi of oxygen
and top off with 2243 or air.
it will give you a mix of 28%

 

[pescador775]

P1 X 32% + P2 X 21% = P3 X 28%

P1 = starting pressure
P2 = added pressure (P3-P1)
P3 = final pressure

If necessary, add or remove some 32% before top off with 21% per formula.

 

[pescador775]

Using the above method, just solve for P1, the starting pressure. Fill or drain the tank with 32% to that number. Add air to reach final pressure at 28%.

I wrote some other equations but the one above should be useful, especially for tanks which have to be drained by some amount before recharging. Also, try this second method which will be a little easier for the situation where the tank pressure is fairly low to begin with:

P1 + P2 = P3

Say that your tank has 1000 psi of 32%

1000(.32) + 2500(X) = 3500(.28)

320 + 2500X = 980

2500X = 980-320

2500X = 660

X = .264 = 26.4%

To get to 28%, top off the 1000 psi of 32% mix with 26% mix to final press of 3500 psi.

 

[Dexloy]

Just solve for P1, the starting pressure. Fill or drain the tank with 32% to that number. Add air to reach final pressure at 28%.

I wrote some other equations but the one above should be simpler to follow.

My algbra is a little rusty, I guess I don't understand..

P1 is known (but can be changed), P3 is known (what we want). P2 is what we want to solve (add 2500 psi of "what % EAN" to = 28 @ 3500)

so the equation is:

P1*.32+P2x=P3*.28
1000*.32+P2x=3500*.28
320+2500psi "x" = 980
2500x = 980-320
2500x = 660
x=3.78
3.78+21=24.78
so
add 2500 psi of 25%EAN to 1000 psi of 32%EAN to get 3500 psi @ 28%EAN

or am I just crazy and can't do math

I understand what you are saying Pesky, solve for P1 and fill with 32 to that PSI then and 21 to top it off but I am still not sure how to solve for P1:

P1*.32+P2*.21=P3*.28
P1*.32 + 2500*.21=3500*28
P1*.32 + 525 = 980
P1*.32=980-525
P1*.32=455
P1=1421
so I fill 1421 PSI @ 32%EAN then top with 21% to finish @ 3500 of 28%EAN

 

[dannobee]

Assuming you're continuous blending:

You have 1100 psi of 32%. That's 352 psi of O2 (1100 x 0.32).

You want 3500 psi of 28%. That's 980 psi total (3500 x 0.28).

Subtract 980 from 352. That's 628 psi of O2 total in the tank that you'll need to add.

The pressure difference is 2400 psi. (3500-1100).

Your 2400 psi will need to have 628 psi of O2. For that, divide 628/2400 = 26.17%.

Set the regulator on the stick until the oxygen sensor reads 26%.

 

[pescador775]

Example of method one. This is primarily for situations where a high O2 in a full tank needs a lower O2 but it will work either way. It is also useful if only a single mix is banked and a compressor is available. Original tank press 3500 psi. Reduce 32% mix to 28%.

P1 + P2 = P3

P1 X .32 + P2 X .21 = 3500 X .28

.32 P1 + .21 P2 = 980

P2 = 3500-P1

.32P1 + .21(3500-P1) = 980

.32P1 + 735-.21P1 = 980

.11P1 + 735 = 980

.11P1 = 245

P1 = 2227 psi

Drain or lift the tank to 2227 psi and top off with air.

I have offered two methods and provided two examples. This is the example for method one which is now being included per your request. I hope that these two economical methods work for you.

 

[Ed Jewell]

Scuba Diving - New Jersey & Long Island New York - Dive Gear & Training - Gas Calculators

Use the Nitrox Fill Calculator (The second calculator down from the top of the page)

Just an example using calculator:
Current Fill: 32% at 1100 psi
Target Fill: 28% at 3500 psi
Fill Mix: I plugged in 32%
Punched Calculate: Add 1127 psi of 32% & top off with 1273 psi of air

 

[Dexloy]

Assuming you're continuous blending:

You have 1100 psi of 32%. That's 352 psi of O2 (1100 x 0.32).

You want 3500 psi of 28%. That's 980 psi total (3500 x 0.28).

Subtract 980 from 352. That's 628 psi of O2 total in the tank that you'll need to add.

The pressure difference is 2400 psi. (3500-1100).

Your 2400 psi will need to have 628 psi of O2. For that, divide 628/2400 = 26.17%.

Set the regulator on the stick until the oxygen sensor reads 26%.

yep that one works and I understand it.

 

[Dexloy]

Scuba Diving - New Jersey & Long Island New York - Dive Gear & Training - Gas Calculators

Use the Nitrox Fill Calculator (The second calculator down from the top of the page)

Just an example using calculator:
Current Fill: 32% at 1100 psi
Target Fill: 28% at 3500 psi
Fill Mix: I plugged in 32%
Punched Calculate: Add 1127 psi of 32% & top off with 1273 psi of air

hehe this is fun, now I need to play with all of the info untill understand