Shearwater Perdix AI SAC calculation

[BlueTrin]

If you upload to subsurface it will display a SAC once you enter the cylinder size.

Subsurface seems to be able to handle disconnects when I check the records where I switch to a stage and input manually the start and end pressures.

 

[scubadada]

If you upload to subsurface it will display a SAC once you enter the cylinder size.

Subsurface seems to be able to handle disconnects when I check the records where I switch to a stage and input manually the start and end pressures.

The Shearwater Cloud gives you SAC. Are you talking about RMV, multiplying the SAC by the tank factor?

In Imperial: SAC psi/min/atm X tank factor cu ft/psi = RMV cu ft/min/atm

 

[inquis]

Subsurface displays SAC with units of <volume> per min (though I agree with your addition of per atm as well). <volume> = cuft or liters, your preference.

It's a shame that two terms (SAC & RMV) are used for what are effectively the same thing but with different units, but fortunately, there is no confusion when units are given. (Units already need to be given to resolve the imperial/metric issue, so they can just as easily resolve the <pressure> / <volume> issue.)

 

[scubadada]

Subsurface displays SAC with units of <volume> per min (though I agree with your addition of per atm as well). <volume> = cuft or liters, your preference.

It's a shame that two terms (SAC & RMV) are used for what are effectively the same thing but with different units, but fortunately, there is no confusion when units are given. (Units already need to be given to resolve the imperial/metric issue, so they can just as easily resolve the <pressure> / <volume> issue.)

I believe my definitions are reasonably standard. SAC in pressure units/min/atm (cylinder dependent) and RMV in volume units /min/atm (cylinder independent)

Shearwater Cloud reports just SAC, you must convert to RMV by multiplying by the tank factor. My old Oceanic Oceanlog gives both SAC and RMV because the cylinder is designated

 

[inquis]

I believe my definitions are reasonably standard. SAC in pressure units/min/atm (cylinder dependent) and RMV in volume units /min/atm (cylinder independent)

Oh, I certainly agree they're standard, but in my view it's an artificial distinction. They're both consumption rates, just with different units. (I wasn't trying to say you're wrong by any means, just relating my opinion that two terms is a barrier to understanding.)

Sure, conversion between pressure-based and volume-based units requires additional information, but I really don't regard that as a novel concept. Converting from imperial to metric already requires additional information (in fact, converting temperature requires two additional numbers), but that doesn't seem to bother anyone.

Also no argument that the pressure-based values cannot be directly compared unless it's known the tanks are the same. However, people easily talk of their car's "mileage" -- a single concept -- while still understanding that there's a difference between city and highway driving conditions and they shouldn't compare across those.

 

[BlueTrin]

The Shearwater Cloud gives you SAC. Are you talking about RMV, multiplying the SAC by the tank factor?

In Imperial: SAC psi/min/atm X tank factor cu ft/psi = RMV cu ft/min/atm

Yes you are right, it is a RMV.

Subsurface uses the word SAC for the RMV, it will back out the RMV from the gas pressure points and the tank size like you said.