Rule of 1/3s and different tank sizes

[bax737]

hi all

Looking for a program that will allow me to punch in tank and pressure and get turn pressures for each

I know the mathmatics etc - but just want to able to have it as a quick calculation

Oh im in metric too

 

[stuartv]

You want a program to tell you what 2/3 of your tank pressure is?

For a cylinder with 3000 psi in it, it's 2000 psi.

For a cylinder with 3500 psi in it, it's 2400 psi.

For a cylinder with 2400 psi in it, it's 1600 psi.

Or, you can use a calculator program, punch in the current tank pressure and multiply by .67.

 

[Hoag]

A tank is typically rated to 3000 psi, which is about 206 BAR regardless of its size unless it is an HP tank. This would mean that in general, for most standard tanks, they will typically be filled to (roughly) the same pressure. For simplicity, if you round it from 206 BAR to 210 BAR it works out as:
Full is about 210 BAR
2/3 Full is about 135-140 BAR and
1/3 Full is about 65-70 BAR

I would tend to use the more conservative of the estimates, but I think that this should get you into the right ball park unless you need to know exact numbers.

 

[Trace Malinowski]

V-Planner has such a feature.

 

[teknitroxdiver]

Are you wanting turn pressures for a single tank size, or something that will do gas matching for dissimilar tank sizes?

 

[Lorenzoid]

If you're talking units of pressure, then what Stuart said. But maybe you are talking volume?

If you're in metric, that too seems awfully simple. A "12L" tank is a 12L tank, and you know the pressure. Now, if you were working in Imperial and couldn't be certain of capacity and rated working pressure, then I could see value in being able to just punch in a manufacturer's model name, such as "Worthington LP85." At least when dealing in Imperial units and tanks with names that don't exactly reflect their capacity at rated working pressure (like the aluminum 80 that holds 77 or 78 cubic feet at 3000 psi), that seems like it could be useful.

 

[AlexL]

I know the mathmatics etc - but just want to able to have it as a quick calculation

i think the min gas calcs are the 'harder' part for deriving usable gas and therefore turn pressures. having wet notes with lookup tables for depth -> volume min gas, and then cheater tables for min gas -> turn pressures in common tanks (imperial, al80s, al100s) makes it pretty straightforward.

 

[stuartv]

If you're using Rule of Thirds and calculating Turn Pressure (i.e. what the OP specifically said) then why do you need to do anything but multiply your actual tank pressure times 2/3?

What does tank capacity, metric vs Imperial, etc. have to do with it?

If it has 3000 psi in it, then the turn pressure is 2000 psi.

If it has 210 bar in it, then the turn pressure is 140 bar.

Or have I lost my mind?

 

[Doby45]

If you're using Rule of Thirds and calculating Turn Pressure (i.e. what the OP specifically said) then why do you need to do anything but multiply your actual tank pressure times 2/3?

What does tank capacity, metric vs Imperial, etc. have to do with it?

If it has 3000 psi in it, then the turn pressure is 2000 psi.

If it has 210 bar in it, then the turn pressure is 140 bar.

Or have I lost my mind?

You are exactly right @stuartv if you are talking about a single diver. If you are talking about gas planning for team of two and you have dissimilar tank volumes and pressures then things get a bit different. Such as, your friend has sidemount AL80s and you have dual LP 108s pumped up to 4000psi.

And all though the OP did not mention gas matching, more than likely that is the reason for the question.

 

[AlexL]

why do you need to do anything but multiply your actual tank pressure times 2/3?

minimum gas reserves and having to match by volume for dissimilar tanks as @Doby45 mentioned.

for an AL80 at 3000psi, with a shallow water min gas of 500psi, leaves 2500psi as usable, round down to 2400psi to make math easy, divide by 3, and your thirds are 800psi, turn pressure at 2200psi. start going deeper and your min gas goes up, for min gas of 900psi, usable is 2100psi, thirds are 700psi, turn at 2300psi.

unless you're talking about 'thirds' being 'halfs with (1/3-mininum gauge) reserved', then its just 1/3 the tank fill pressure.