MacDive Forums

Good day to all of you!

I am wondering if there is a way to set the units for individual pieces of equipment. I am in Canada and the units here can be a bit of a mess. I am working on converting myself over to pure metric rather than the blend often used here in Canada. Therefore, I might select Imperial or Canadian units when entering a tank that I have rented from a shop (Aluminum 80s usually with a starting pressure around 3,000 psi). But if I switch the units back to metric the tank size remains as 80 which I think throws off the Surface Air Consumption rates. If it is not possible to select units for individual pieces of equipment, can someone tell me the equivalent metric size of an Aluminum 80 tank or should I simply keep working in Canadian units for now?

Thanks for any assistance available!

Shadallark

There's no way to mix and match units.

I think the equivalent is probably a 10 Litre tank? (so 10 for tank size, working pressure is ignored).

Calculating the metric equivalent isn't difficult, once you understand the difference between the two systems. In the metric system, the tank volume specifies the water capacity. If you would fill a 10L tank with water, you would need 10L of water. In the imperial system, the tank volume specifies the gas capacity at its working pressure. A 80 cuft tank with a working pressure of 3000 psi will contain 80cuft of gas (at atmospheric pressure) when filled at 3000 psi.

If you apply Boyle's law, you get:

1 atm * V_imperial = P_work * V_metric

or

V_metric = 1 atm * V_imperial / P_work

And convert the imperial units to metric of course:

1 atm = 1.01325 bar (or just 1 if you prefer)
1 cuft = 28.316846592 liters
1 psi = 0.068948 bar

For the example of V_imperial = 80 cuft and P_work = 3000 psi, you get V_metric = 11.097L

An Al80 tank has a volume of a little more than 11 liters.
So rounding towards conservatism, I use 11 l when I do dive planning with them.

They actually call it a 11 ltr Cylinder = 80cuft

3000 psi is about 205 Bar. 14.7 psi = 1 ATM (BAR)