Tank factors are a handy quick and easy method of converting between volume (cuft) and pressure (psi). It's especially handy when teammates are using dissimilar tanks.
Simply put, tank factors are the volume of gas per 100 psi, for a given tank.
To calculate a tank’s tank factor, you simply divide the tank’s rated pressure by the tank’s rated volume, then multiply by 100;
Tank Factor = (rated volume / rated pressure) x 100
For an al80 (al80’s actually contain 77cuft, not 80), the tank factor would be:
(77 / 3000) x 100 = 2.66
This means on an al80, there are 2.66 cuft per 100psi.
To make life simple, we’ll just call it a tank factor of 2.5.
It’s important to remember that you must use the rated pressure and volume.
Using Tank Factors
Using the aluminum 80's tank factor allows us to start converting between pressure and volume. The diagram below is actually a formula in itself, known to many as the "circle T". If you want to know the volume, you’d simply multiply pressure by the tank factor. If you want to know the pressure (psi), you’d simply divide the volume by the tank factor. Simply cover the item you wish to solve for.
For example, using the example on our Rock Bottom page, the 2 divers need 40cuft of gas. However, we typically work with volume on land, but pressure underwater, since that is what our spg's read. So, we need to convert that 40cuft to psi.
By covering up the psi section of the above equation, you can see we must divide the volume (40cuft) by the tank factor (2.5);
40 / 2.5 = 16
Keep in mind that tank factors represent the volume of gas, in cubic feet, per 100 psi. So, we must multiply the 16 by 100;
16 x 100 = 1600
So the Rock Bottom pressure for the Rock Bottom example, in an al80, is 1600psi.
To convert a pressure to volume, you'd cover the "volume" section in the formula above. So, to convert that 1600psi back into volume (if it were not already known), you'd multiply the tank factor and the pressure;
2.5 x 16 = 40 (cuft)
Remember, we multiply by 16, and not 1600, because the tank factor is solved for 100's of psi.
Tank factors are often used for double tanks as well. Perhaps moreso than single tanks.
When calculating the tank factor for your doubles, you use the same formula as above. Keep in mind that the rated volume of gas has doubled, due to the second cylinder. However, the rated pressure remains the same.
So, for a set of double AL80's;
154 (77 cuft each cylinder x 2 cylinders) / 3000 (rated pressure) x 100 = 5.13, round to 5
Using the Rock Bottom example from above, our Rock Bottom pressure in a set of AL80's is;
40 (cuft) / 5 (TF) = 88 x 100 = 800psi
Common Tank Factors
|LP85 / HP100||3||6|
|LP95 / HP 119||3.5||7|
|LP104 / HP 130||4||8|
Want to learn more? Check out our Consumption page
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