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

[In10se]

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.

Thanks TSandM. I've never heard of the tank factor before. My reasons are to know how I would be doing if I were diving a single, to keep mental track of what progress I'm making on my SAC, and to just keep a running total of my remaining air in actual volume instead of just psi. If for some reason my dive plan changes I would like to be able to calc out where I am at regarding remaining gas.

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texdiveguy thanks for that link. It has some very useful stuff.

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vintagediver thanks for the raw math. That is what we should all know from our basic open water classes.

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willgr1 thaks for the help

 

[In10se]

Hehe ... scubaboard's web addy should be in every divers basic open water class as a reference source.

I was about to ask how to establish the tank factor for a pair of lp98's. Thanks richerso!!

 

[TSandM]

One of the big points about TF is to round it off to something you can use in your head. It doesn't have to be absolutely precise. If you're down at 70 feet, and you want to figure out how much longer you can stay there on the gas you have, you need easy numbers and they don't have to be perfect.

 

[amascuba]

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?

You can figure out the tank factor for any tank by figuring out how the cubic foot per psi, then multiply that by 100.

For example and AL80 rated to 3000 psi.

80/3000 = .026 cubic foot per psi

.026 x 100 = 2.6.
TF = 2.6

Hope that helps.

As TS&M said, the numbers don't have to be precise. Just something that you can do quickly in your head.

 

[richerso]

Does anybody see a theme in these math problems?

They're all ratios! Yes. I know. We all loved ratios in math class! See! Math is useful!

A note about rounding TF numbers: A diver would always like to be conservative when making estimates of air remaining or air used.

With this in mind, a TF should be rounded down when calculating air remaining in the tank. This underestimates the air remaining in your tank.

A TF should be rounded up when calculating air consumed. This overestimates your air consumption.

What should you round a TF to? The nearest round number that still allows you to perform mental math at 70 ft depth. Or, don't round at all if you are mathematics savant.

 

[texdiveguy]

I HATE math.............. :mooner:

 

[ZzzKing]

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?

Your mother must be so proud!

(BTW, an Al80 has 77.4 cuft or somesuch volume, not 80.)

 

[richerso]

Your mother must be so proud!

(BTW, an Al80 has 77.4 cuft or somesuch volume, not 80.)

I am glad you told me that. I have always been suspicious of such a nice round number. Thanks Craig.

 

[TSandM]

My favorite dive buddy is math challenged, and he has gotten quite good at doing a variety of gas calculations in his head, using tank factors and "scuba math".

 

[Charlie99]

All of the above derivation of tank factor are correct, but I find it easier to work it out slightly differently

An AL80 holds 77.4 cu ft at 3000psi. See Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan for numbers on other tanks.

3000psi/77.4 means 38.75psi/cu ft. Round that off to 40psi/cu ft. So if my SAC is 0.5cfm, then it is will be 0.5 * 40 = 20psi per minute using an AL80. At 100' or 4ata it will be 4*20 = 80psi/minute.

Take 40 psi/cu ft and flip it upside down and you get 0.025 cu ft/psi or 2.5 cu ft/100psi, which is the other version of the tank factor.

Sometimes 40psi/cu ft works easier. Sometimes 2.5cu ft/100psi works easier. I use whichever version lets me multiply rather than divide when solving the partiular question at hand.

Charlie Allen