Calculating the correct SAC rate for a dive sounds like a trivial and straightforward task, but if you want to do it right, you quickly get to some hurdles. SAC means “Surface Air Consumption” or for all German readers it’s the AMV = Atemminutenvolumen. The formula is indeed very simple:

*MetricSAC = (TankPressureDiff * TanksizeLiter) / (Divetime * (AvgDepthMeter / 10 + 1))*

*ImperialSAC = ((TankPressureDiff / WorkingPressure) * TanksizeCuFt) / (Divetime * (**AvgDepthMeter **/ 10 + 1))*

So where is the problem, you think? Well, you have to consider:

- Is there a dive profile? If so, I can calculate the average depth and dive time from the profile. If not, the average depth must be entered manually. Before calculating the SAC rate, I have to check if all required data is available
- Metric or Imperial calculation?
- Double tanks? If so, multiply tank size with 2
- Now it’s getting complicated: Does the dive have multiple tanks? If not, we can calculate right away and that’s how it worked in the past. But now since Diving Log supports multiple tanks it’s getting a lot more difficult:

If a dive has multiple tanks, the only way to calculate a correct SAC rate for the whole dive is by calculating the separate rates for each tank and then the average. So now Diving Log will calculate the dive time and average depth for each tank by running through the dive profile. When completed, it will calculate based on these values the separate SAC rates for each tank and then the average rate for the whole dive. Look at this dive for example (all metric values):

So SAC rate calculation for dives with multiple tanks works only if a dive profile is available and when each tank is assigned to a part of the profile. Each dive can have up to 10 tanks. In a logbook with 1000 dives, each dive 10 tanks, Diving Log will have to calculate the SAC rate for 10000 tanks when creating the SAC rate chart below. Some dive computers record the profile with a 1 second interval, so you could imagine the amount of required calculations to create this chart:

The improved SAC calculation will be available in the next beta.

Tags: Logbook, SAC, Statistics, Tanks & Gases, Technical Diving

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