SAC differences between AI computer and Manual Calculations

[rfwoodvt]

Just playing around with manually calculating my SAC and comparing it to my Dive Computer's numbers. I've found that the manual calcs invariably come out about 50% higher.

For example on my last dive the SAC reported by my computer was 0.53 but the manual calculations come out 0.72

What might account for such a wide difference?

 

[Lorenzoid]

Does the computer simply use the average depth over the entire dive, the same way we do when calculating manually?

 

[inquis]

An error on your part?

Just kidding, can you elaborate how you are doing it? You may also be using a different cylinder size than the dive computer/software.

 

[tursiops]

Some computers ignore the first part of the dive, which is when your tank is (usually) rapidly cooling off and thus appearing to lose pressure.
Is it actually your computer that is reporting SAC, or is a program into which you download your computer? What computer, what program, what tank?

 

[inquis]

SAC_pressure = (Starting PSI - Ending PSI) / (Avg Depth_ft / 33 + 1) / Time_mins
This is in PSI per minute per atm. If you want cubic ft per min per atm, convert based on your rated tank parameters.

SAC_vol = SAC_pressure / Rated PSI * Rated Vol

Be aware that the latter is more commonly known as RMV (Respiratory Minute Volume), and "SAC" is usually the pressure version. However, some software uses the term SAC with units of volume per min.

Since you mentioned 0.53 and 0.72, that's going to be the volume/time version (RMV) with units of cuft/min. (Everyone drops the "per atm" part.)

 

[rfwoodvt]

An error on your part?

Just kidding, can you elaborate how you are doing it? You may also be using a different cylinder size than the dive computer/software.

At first I thought is was an EOMP but I ran the numbers through several online calculators and they all came up with the same 0.72

Here's the data I used as logged in the computer.
Starting Pressure: 2308
Ending Pressure: 878
Volume of air in Tank: 80 Cuft

Length of Dive: 53 minutes
Avg Depth: 10

And I just noticed my Error. Since the tank was not full, it was not 80 cuft. At 2308 psi an AL80 only has 61 cubic feet. Plug that in and it comes out close to the computer values.

I suspect the slight difference between them is due to the computer calculating periodically as opposed to with the ending average as Lorenzoid was wondering.

Does the computer simply use the average depth over the entire dive, the same way we do when calculating manually?

 

[tursiops]

You appear to be using the wrong numbers, AND the wrong calculation.

First, your 80 cuft tank does not hold 80 cuft, but only 77.4, when filled to 3000 psi.
When filled to 2308 psi, it contains 2308/3000 * 77.4 = 59.5 cuft.
At 878 psi, it contains 878/3000 * 77.4 = 22.7 cuft.

So over the course of your 53 minutes dive, you used 59.5-22.7 = 36.8 cuft.
That give 36.8/53 = 0.69 cuft/minute, at an average depth of 10 ft.

But you want a surface-equivalent value....and 10 ft is not at the surface.
At ten ft in salt water, your ambient pressure is 1 + 10/33 = 1.33 ATA.
In fresh water, your ambient pressure is 1 + 10/34 = 1.34 ATA.

Assume salt water: your consumption rate at 10 ft is 0.69 cuft/min, but at the surface that would be only 0.69/1.33 = 0.52 cuft/min. THAT is your RMV.

 

[macado]

EDIT: I hit post without refreshing but @tursiops' math is correct and explained better than me.

In the above scenario you used 36.89cuft of gas. AL80 (77.4cuft)
2308/3000 x 77.4 = 59.54cuft available gas
878/3000 x 77.4 = 22.65cuft gas left in cylinder
59.54cuft - 22.65cuft = 36.89cuft gas used
36.89cuft / 53 minutes = .696cuft/minute
10ft / 33 + 1 = 1.303 ata
.696cuft / 1.303 = .53cuft/minute

 

[tursiops]

EDIT: I hit post without refreshing but @tursiops' math is correct and explained better than me.

In the above scenario you used 36.89cuft of gas. AL80 (77.4cuft)
2308/3000 x 77.4 = 59.54cuft available gas
878/3000 x 77.4 = 22.65cuft gas left in cylinder
59.54cuft - 22.65cuft = 36.89cuft gas used
36.89cuft / 53 minutes = .696cuft/minute
10ft / 33 + 1 = 1.303 ata
.696cuft / 1.303 = .53cuft/minute

Yeah, the BIG factor he was missing was the depth correction.

 

[inquis]

You can also do it in PSI and convert at the end.
SAC = (2308 - 878) / 53 / (10/33 + 1) = 20.7 psi/min at surface pressure
RMV = 20.7 / 3000 * 77.4 = 0.53 cuft/min

Sometimes SAC is more useful, other times RMV is more useful. It's good to know how to convert either direction.