How do I calc' the per-tank psi on my doubles?

[In10se]

How do I know the remaining volume (edited ... thanks grf88) in each al80 on my dbl's rig? I'm told that the tanks are EQ'd through the manifold to show the combined pressure on my spg but I'd like to be able to sort out where I stand per tank at any given depth and psi. For example w/900psi @40ffw on my spg what volume do I have per tank? I know there's a simple formula I can run in my head to sort out what's left per tank if any one would share that with me ... thanks.

 

[grf88]

If your SPG reads 900psi on manifolded doubles with the isloator open each of your tanks has 900 psi. Is it the volume of air you are trying to work out because that would depend on the size of the tank and the percentage of it's rated pressure (including + rating where applicable) that you are seeing on the SPG..

 

[willgr1]

just like what grf88 said....as long as the isolator is open air is allowed to pass from both tanks to the valves equally, so when you take a breath half the air in that breath is from one tank, half from the other (in theory), so the tanks will always have equal pressure (and volume). Now, if you closed the isolator then you would see a difference because you would only be able to breath from one of the tanks, but open the isolator back up and the air will flow between the tanks and make the pressure between them equal again.

 

[In10se]

If your SPG reads 900psi on manifolded doubles with the isloator open each of your tanks has 900 psi. Is it the volume of air you are trying to work out because that would depend on the size of the tank and the percentage of it's rated pressure (including + rating where applicable) that you are seeing on the SPG..

Yes indeed you nailed what I'm after. What will my volume per tank be at 900psi? There's gotta be some math I can use to keep a running total in my head ...


Thanks

 

[willgr1]

PV=PV is the equation you want, (initial pressure)(initial volume) = (current pressure)(current volume)

 

[vintagediver]

How do I know the remaining volume (edited ... thanks grf88) in each al80 on my dbl's rig? I'm told that the tanks are EQ'd through the manifold to show the combined pressure on my spg but I'd like to be able to sort out where I stand per tank at any given depth and psi. For example w/900psi @40ffw on my spg what volume do I have per tank? I know there's a simple formula I can run in my head to sort out what's left per tank if any one would share that with me ... thanks.

3000 PSI / 77.4 CUFT= 38.75 PSI PER CUFT

900 PSI / 38.75 = 23.23 CUFT PER TANK

2x 23.23 CUFT= 46.45 CUFT

 

[texdiveguy]

Tank Volume

 

[TSandM]

There's a pretty easy way to do the math in your head. I was taught something called a "tank factor", which is the volume of gas in cubic feet that corresponds to 100 psi in your tank or tanks. For an Al80, the TF is 2.5; for doubles, it's 5. So, if you have 900 psi in double Al80s, it's 9 x 5 = 45 cu ft. If, for whatever reason, you want to know how much gas is in EACH tank, it's half of that.

 

[Wookie]

There's a pretty easy way to do the math in your head. I was taught something called a "tank factor", which is the volume of gas in cubic feet that corresponds to 100 psi in your tank or tanks. For an Al80, the TF is 2.5; for doubles, it's 5. So, if you have 900 psi in double Al80s, it's 9 x 5 = 45 cu ft. If, for whatever reason, you want to know how much gas is in EACH tank, it's half of that.

Cool beans, I love thumbrules, when I mix trimix (or other gasses) I usually use rules of thumb, and my mixes usually come out better than if I used a blending software (but not always). Do you have a list of "Tank Factors" for other cylinders, specifically steel 95's and Hp 100's?

 

[richerso]

Divewookie,
You can create your own Tank Factors (TF) for any tank or set of doubles that you own.

Let's say that I own two aluminum air tanks. Let's say each air tank is rated as 62 cu. ft. rated at 2900 psi. Yes. I know these are odd numbers. I want you to go through the math for your tanks.

If I am diving singles (standard recreational OW stuff), then I have 62 cu ft of air at the beginning of the dive. I am assuming that the tank was filled correctly to 2900 psi. If I am diving doubles then I have 124 cu ft of air. So far, so good? I will assume we are tech diving with a set of doubles (124 cu ft).

Now it's simple math to create your own tank factor. Let's develop a tank factor (TF) based on 100 psi pressure (diving doubles). Tank factor is:

100 / 2900 = .0344828

Let's multiply this by our "full" tank volume, that is 124 cu ft.

.0344828 * 124 = 4.276

That is to say, diving these fictitious tanks (total 124 cu ft full), you have 4.276 cu ft of gas per 100 psi.

Note that this method only works if you base all of your calculation numbers on a fully charged (to correct pressure) tank and use the published volume for a fully charged tank.

Coming back to a standard Al 80 tank, that is to say the standard numbers are 3000 psi and that makes this a volume of 80 cu. ft.

If you begin a dive with an Al-80 tank that is charged to 1400 psi (did a dive earlier today), then the math looks like this:

100 / 3000 = .033333
.033333 * 80 = 2.667
That is 2.667 cu ft / 100 psi.

Since this tank only has 1400 psi, 14 * 2.667 = 37.3 cu ft air remaining in this tank.
To be a safe diver, you should always reserve 500 psi in your tank. That is 5 * 2.667 = 13.33 cu ft of air held in reserve.

Does this help?