SAC rate calculations on non AI computer?

[Doctor Rig]

A friend has a non-AI Shearwater Peregrine and now wants to know his SAC rate. Is there a way to get (and record) an average dive SAC rate for a dive from the computer by manually inputting before and after tank pressures after the dive?

(buyers remorse, I’m thinking)

 

[grantmac]

Enter into Subsurface, input tank size plus beginning and end pressure.
Or just use average depth and do the math.

 

[Eric802]

I'm math challenged so I just use Subsurface. It's "garbage in, garbage out" though - the result will only be as accurate as the info you put in as far as starting and ending pressure. With an analog SPG, it's hard to be too precise.

 

[34109411]

A friend has a non-AI Shearwater Peregrine and now wants to know his SAC rate. Is there a way to get (and record) an average dive SAC rate for a dive from the computer by manually inputting before and after tank pressures after the dive?

(buyers remorse, I’m thinking)plent

A friend has a non-AI Shearwater Peregrine and now wants to know his SAC rate. Is there a way to get (and record) an average dive SAC rate for a dive from the computer by manually inputting before and after tank pressures after the dive?

(buyers remorse, I’m thinking)

plenty of formula out there

 

[ginti]

Or just use average depth and do the math.

This one. It isn't rocket science, and it's so helpful in your diving to understand the basic math behind.

SAC= {[P(start)-P(end)]×V_tank×(av_depth/10+1)}/time

 

[Sascha314]

This one. It isn't rocket science, and it's so helpful in your diving to understand the basic math behind.

SAC= {[P(start)-P(end)]×V_tank×(av_depth/10+1)}/time

Please then put in the right units for the formula. In this case Pressure (start) and Pressure (end) of your tank from the spg in bar
av_depth in metre from your dive computer
Time in minutes from the dive computer.

Then you get the volume (physical! Not gas volume) the lung has expanded during one minute on average on the dive. It basically sums up all volume increases by inhaling (=you take gas from your tank) and then "reset" it by exhaling the gas to the water.
With that you can calculate in average the used pressure per time unit for a given ambient pressure (or depth).
Its measured in litre per minute, if you use the above given units

 

[inquis]

Imperial version since the OP is in Michigan:

(Pstart - Pend) * (Vfull / Pfull) / (Davg_ft/33+1) / Time_minutes​

For an AL80, Vfull=77 cuft, Pfull=3000psi, Pstart & Pend use the same units as Pfull. Use 34 rather than 33 if you're in fresh water.

In practice, I use hundreds of psi and have memorized that tank ratio (Vfull/Pfull) is about 2.5 cuft per hundred psi (for an AL80). This makes it similar complexity to what the metric folks do (except they use the empty tank ratio, which is actually stamped on the side of the bottle if they forget an AL80 is 11.1 liters when empty or ~1 bar).

For example, using 2000 psi (20 hundreds) at an average depth of 66 ft for 35 min, the surface consumption in volumetric units (cuft/min) -- which is often called RMV -- is about 0.5 cuft/min.

 

[Manatee Diver]

I put together a spreadsheet where you input all the numbers and it spits out your SAC rate.