How to convert Garmin PSi/min to Suunto CF/min SAC rate

[lairdb]

I made this and taped it into a page in my logbook:

After dives I can look up the psi/min in the appropriate tank column, do a rough interpolation as necessary, and record the cf/m with the dive. I also put a little dot to the right of the row, right on this table, which has become an interesting little histogram.

 

[lowwall]

They are both set for an 80 cu/ft cylinder with a pressure of 3000 psi. My Suunto says 0.43 cu/ft per minute but my Garmin gives me the reading in psi/minute.

3000 / (80/.43) = 16.125 psi/minute

You want psi per minute. You have the tank psi (3000) so all you need is to divide that by the number of minutes. You have an 80 cubic foot tank and you are using .43 cubic feet per minute. 80/.43 = 186.046511628 minutes.

You can rewrite the equation to make it easier to type into a calculator

3000 * .43 / 80 = 16.125

or more generally

tank pressure in psi * SAC rate in cubic feet/minute / size of tank in cubic feet

 

[lairdb]

You want psi per minute.

…except I’d suggest you want cfm as your final recorded value; it’s uniform regardless of cylinder, it’s the usual (non-metric) unit for RMV, it’s comparable with others…

 

[lowwall]

…except I’d suggest you want cfm as your final recorded value; it’s uniform regardless of cylinder, it’s the usual (non-metric) unit for RMV, it’s comparable with others…

I agree that cfm is a more useful number, but the OP had a cfm number and wanted to convert it to psi/minute.

If you have psi/minute and want to convert to cfm, the formula is:

Cfm = psi/m x cubic feet of gas in filled tank / tank fill pressure

For this example:

Cfm = 16.125 x 80 / 3000