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How is SAC rate calculated for mutiple gases?
Posted: Sat Aug 13, 2016 8:02 pm
I am a very satisfied user of DiveLog DT on the Mac. I like that the software calculates the SAC rate for you. My question is how does it do the calculation for multiple gases? From today's dive where I breathed 50% O2 and 32% O2 mixes, it appears that the software simply calculates the SAC rate for each gas. I use a Shearwater Predix. The profile clearly shows the portion of the dive on 50% and the portion of the dive on 32%, I haven't hand checked the calculation but it seems that it has the smarts to work with the appropriate segments of the dive. In my case there were two segments on the dive on 32% and one on 50%. Can the folks from DLM provide some details on the SAC rate calculations on a multi gas scenario?
Re: How is SAC rate calculated for mutiple gases?
Posted: Sun Aug 14, 2016 8:37 am
Dive Log DT and Dive Log Manager (as well as Dive Log on iOS) calculate the SAC rate for each gas individually (i.e. the SAC rate during the portion of the dive that that gas was used) as well as an "overall SAC rate". In order to calculate these SAC rates we do need to have enough information to do it. If that information is not available, the the SAC rate for that portion of the dive or the overall Sac rate will not be calculated.
In order to calculate the SAC rate, you need the amount of gas used (i.e. a Start Pressure, End Pressure, Tank Size and, in the case of US/Imperial calculations, the Tank Working Pressure) as well as the average depth during the period of the dive in question as well as the duration of that portion of the dive. In order to get the average depth and duration we typically need to have a dive profile downloaded from a dive computer (in the case of dives without a profile it is possible to manually enter the average depth and dive time and we can then calculate the overall SAC rate) that contains gas switches. As you note, the Perdix will provide that gas switch information so as long as you have filled in all the tank information for each tank used on the dive you should see a SAC rate for each tank as well as the overall SAC rate.
Things are a bit more complicated in the case of CCR rebreathers as the dive computer typically only tracks the Diluent as a gas so we use some heuristics to assign the Oxygen tank's use to the appropriate section of the dive.
We try to err on the side of caution, so if we do not have sufficient information to calculate a specific SAC rate or if there is conflicting information (for example a tank is specified and has gas used but does not show up in the profile then it is likely that the SAC rate would be inaccurate as there is no way to account for this "extra" tank).
Finally, the "overall SAC rate" is subject to a bit of definition. There is really no formal definition in diving terms (remember that SAC rate was originally used to calculate the likely air consumption when planning a dive in the days when SPG's (submersible pressure gauges) were not yet available). What we do is calculate the overall average for the dive. That is we add up all the gas use and apply that to the dives overall duration and overall average depth. One could also calculate each SAC rate individually and then average those averages but this would mean that the overall SAC rate could not be calculated unless you had the complete information for each tank individually. The approach we take allows you to get an idea of the overall consumption rate even if your profile does not have the information on a per tank basis required to calculate the average depth and duration of each segment of the dive.
If you are not seeing one of the SAC rates, you should double check that you have entered all the relevant tank information for each tank. In the case of the Perdix, you will need to manually enter the tank size, start pressure and end pressure as well as the tank's working pressure if you are entering the tank size in CuFt (rather than liters). You should then see each individual SAC rate (in Dive Log DT and Dive Log Manager look at the Environment tab and then select each tank in turn) as well as the overall SAC rate in the lower right hand corner.
I hope that helps. Let us know if you having a specific difficulty and we'll be happy to try to sort it out.