Question Recorded vs Calculated SAC

[Divin'Papaw]

I’ve noticed that in the Gas section for a dive on the Shearwater Cloud app there is now a recorded and a calculated SAC. Is this new? Maybe it’s been that way for awhile and I’ve just not noticed? That is very possible because I don't review my dives in much detail. I just sync to cloud, update the location/site/buddy and then lookup key data for my official dive log which is a spreadsheet. But for my dives this past weekend, when looking up my SAC, I noticed both values for the first time.

Does anyone know the difference between these values? They seem to differ on every dive with recorded being a bit lower than calculated SAC. Which would be more accurate? Thoughts?

Here are two screenshots for reference.

 

[Alastor]

Is the "Calculated" one just referred to a single transmitter (e.g. tank) while the "recorded" is trying to account for a global (whole dive) one?
I'm wandering what would be shown in case you had 2 trasmitters and changed the gas during the dive.

 

[tursiops]

I believe the "calculated" is based on psi used during the dive and on average depth, whereas "recorded" is the average of the running 2-minute averages shown on the computer during the dive. The latter will be less because it does not include the first 2-3 mins of the dive when the tank pressure is (usually) dropping rapidly as the tank cools in the water.

ADDED: there is also the possibility that both the computer and the Cloud assume "European Standard EN13319" water (which is less dense) rather than salt water, so will drop the SAC slightly compared to using salt water.

 

[scubadada]

I believe the "calculated" is based on psi used during the dive and on average depth, whereas "recorded" is the average of the running 2-minute averages shown on the computer during the dive. The latter will be less because it does not include the first 2-3 mins of the dive when the tank pressure is (usually) dropping rapidly as the tank cools in the water.

I thought that also. I checked my last 20 dives and the recorded value for SAC was higher than the calculated value for SAC in 16 of the 20.

I sent an email to Shearwater over the weekend asking for an explanation.

 

[tursiops]

I thought that also. I checked my last 20 dives and the recorded value for SAC was higher than the calculated value for SAC in 16 of the 20.

I sent an email to Shearwater over the weekend asking for an explanation.

Interesting. For the same dive, using the same transmitter, my Perdix and Teric sometimes usually (not always) give opposite results on which is larger, and the sign of the difference seems random. The size of the differences is about 2% or less.

 

[scubadada]

...ADDED: there is also the possibility that both the computer and the Cloud assume "European Standard EN13319" water (which is less dense) rather than salt water, so will drop the SAC slightly compared to using salt water.

My Teric is set for salt water. The Cloud also shows salt water in the computer panel under salinity setting

In the 20 dives I checked the SAC difference between recorded and calculated averaged 0.14 psi/min with a range of 0.01 and 0.45 psi/min. This results in a minimal effect on the calculated RMV, <0.012 cu ft/min using the tank factor for an AL80.

I hope I hear back from Shearwater