Teric SAC rate confusion

[hammet]

See the attached .csv file from a download from a dive I did. If you look at the SAC rates in column W and take the average, you get 20.66 psi/sec.

This gybes with the values I see during the course of my dive.

However, when I look at the SAC rate in shearwater cloud for the same dive, I see a SAC rate of 17.51 (See screenshot 1).

Screenshot 1
Start pressure: 3214 psi
End pressure: 664 psi
SAC 17.51 psi/sec

Screenshot 2
Average Depth: 52.0 ft
Dive length: 56 min


If calculate the SAC rate based on the above values in the screenshots, I get:
(3214-664)psi/(2.58atm * 56min)= 17.7psi/sec which is about what the end of dive result I see in Shearwater Desktop.

Yet this value is significantly lower than the average "instantaneous" values I see on my wrist while I dive and also with the data shown in the spreadsheet.

Why the discrepancy?

 

[scubadada]

That's pretty interesting. The app calculation and your calculation are nearly the same, perhaps the app uses the exact values whereas you use a rounded number. Using 3 sig figures, I get 17.45 psi/min, (3214-664)/(2.576 * 56.717).

I can't imagine why all the instantaneous SAC values are so much different than the average SAC, hope someone does. Have you already asked Shearwater? I use a nearly 9 year old Oceanic VT3. It does not calculate an instantaneous SAC while diving. I do not see my gas use and average depth until I download my dive to OceanLog. I have checked the SAC and RMV calculations several times and they have always come out the same as in the app.

In this case SAC is (2966-482)/(2.97 * 60) = 13.94 psi/min and RMV is 13.94 * (77.4/3000) = 0.36 cf/min

 

[uiowa96]

See the attached .csv file from a download from a dive I did. If you look at the SAC rates in column W and take the average, you get 20.66 psi/sec.

This gybes with the values I see during the course of my dive.

However, when I look at the SAC rate in shearwater cloud for the same dive, I see a SAC rate of 17.51 (See screenshot 1).

Screenshot 1
Start pressure: 3214 psi
End pressure: 664 psi
SAC 17.51 psi/sec

Screenshot 2
Average Depth: 52.0 ft
Dive length: 56 min


If calculate the SAC rate based on the above values in the screenshots, I get:
(3214-664)psi/(2.58atm * 56min)= 17.7psi/sec which is about what the end of dive result I see in Shearwater Desktop.

Yet this value is significantly lower than the average "instantaneous" values I see on my wrist while I dive and also with the data shown in the spreadsheet.

Why the discrepancy?
View attachment 509002 View attachment 509003

Can you get the Shearwater Cloud to display the SAC on the chart? I recently returned from my first trip wth the Teric and have been unable to get the SAC to graph. I think if the graph would display, the difference in calculation might be revealed.

 

[hammet]

That's pretty interesting. The app calculation and your calculation are nearly the same, perhaps the app uses the exact values whereas you use a rounded number. Using 3 sig figures, I get 17.45 psi/min, (3214-664)/(2.576 * 56.717).

I can't imagine why all the instantaneous SAC values are so much different than the average SAC, hope someone does. Have you already asked Shearwater? I use a nearly 9 year old Oceanic VT3. It does not calculate an instantaneous SAC while diving. I do not see my gas use and average depth until I download my dive to OceanLog. I have checked the SAC and RMV calculations several times and they have always come out the same as in the app.
View attachment 509006View attachment 509007

In this case SAC is (2966-482)/(2.97 * 60) = 13.94 psi/min and RMV is 13.94 * (77.4/3000) = 0.36 cf/min

I didn't bother with sig figs because in one instance on SW cloud, the time is only in even minutes and in another its minutes and second. I wasn't sure which one they used for their calculations and I think it's possible they don't account for the first couple of seconds or minutes of the dive until the SAC can be measured over an appropriate interval which is why is takes a little while before it is displayed in real time.

Either way, the numbers are far off from the measurements I see during the dive and in the spreadsheet.

I did ask shearwater. They asked to see the .csv file. I sent it to them a few months ago, but never received a reply. I was thinking about it again today and decided to recheck my figures.

 

[hammet]

Can you get the Shearwater Cloud to display the SAC on the chart? I recently returned from my first trip wth the Teric and have been unable to get the SAC to graph. I think if the graph would display, the difference in calculation might be revealed.

The SAC on the chart matches the numbers in the spreadsheet.

 

[Sol518]

I think I recall reading at one point that Sheerwater ignores the first couple minutes of the dive so the SAC isn’t affected by inflating the BC. I don’t remember where I heard that but I’ll see if I can find it later. That could explain why the average is different depending on where you look.

 

[hammet]

I think I recall reading at one point that Sheerwater ignores the first couple minutes of the dive so the SAC isn’t affected by inflating the BC. I don’t remember where I heard that but I’ll see if I can find it later. That could explain why the average is different depending on where you look.

I believe you are correct. However, that cannot explain the discrepancy I am experiencing.

 

[scubadada]

I believe you are correct. However, that cannot explain the discrepancy I am experiencing.

The difference between 17.51 and 20.66 psi/min is relatively large. Assuming an AL80 or a HP100, these would give you an RMV of 0.45/0.53 or 0.51/0.60 cf/min respectively.

Perhaps @Shearwater can assist here

 

[hammet]

bump @Shearwater

 

[scubadada]

Seems unlike @Shearwater to not answer