How does Perdix calculate SAC rate?

[ScubaSteve85]

Like many of you, I use a Shearwater Perdix AI computer. Today, I decided to compare my SAC rates on my two most recent dives. Both dives were to the same wreck, at the same depth, on the same day. On my first dive, my average SAC was 27.2. on my second dive, it was 19.15. After thinking about why such a wide disparity, I realized that my first dive was on a HP100, while my second was in an HP130. Therefore, I surmised that the perdix was making the calculation assuming a certain tank size, but what tank size is it assuming?

While I can enter in a tank size in the Shearwater app, it's a non-standardized entry (i.e. no dropdown menu) so it doesn't affect the SAC calculation. I haven't found any menu options in the computer app to set a tank size.

Does this mean that if I'm frequently using different sized tanks, the displayed SAC isn't ever going to be accurate? Is there an easy way to caculate accurately using tank size, starting and ending pressures?

 

[Macan]

Hi, my response here assumes that the AI SAC function of the Perdix is the same as the Teric. I have no experience with the Perdix AI but I have a Teric.

SAC and RMV are two different values. One accounts for pressure and the other for volume. Shearwater doesn't compute for RMV on the dive computer which requires the input of tank size.

See sample SAC calculation below. Sorry it's metric. I shifted to metric 20 years ago for Tec and never looked back .

Start Pressure: 199.81 bar
End Pressure: 55.02 bar
Dive Time: 53.42 min
Avg Depth: 11.50 meters

Using the SAC formula:
SAC Bar/Min = ((Start P - End P) / Time)/ATA
SAC rate should be: 1.26 bar/minute

To check if your SAC is correct, manually compute your SAC using the above formula and match it with your Perdix AI value. If correct, it should match. On my Teric, there is a discrepancy between the manually computed value and the computer's displayed value. The SAC value on the Teric doesn't seem right. I have already reported this to Shearwater.

Your other point on entering the Tank capacity on the Shearwater cloud app so it can compute RMV - Unfortunately the SW Cloud app doesn't do that . Other Apps like Dive Log does that. Let's hope that SW fixes and enhances the Shearwater Cloud to make it as good as the Dive Log app.

Hope this helps.

 

[azstinger11]

Like many of you, I use a Shearwater Perdix AI computer. Today, I decided to compare my SAC rates on my two most recent dives. Both dives were to the same wreck, at the same depth, on the same day. On my first dive, my average SAC was 27.2. on my second dive, it was 19.15. After thinking about why such a wide disparity, I realized that my first dive was on a HP100, while my second was in an HP130. Therefore, I surmised that the perdix was making the calculation assuming a certain tank size, but what tank size is it assuming?

While I can enter in a tank size in the Shearwater app, it's a non-standardized entry (i.e. no dropdown menu) so it doesn't affect the SAC calculation. I haven't found any menu options in the computer app to set a tank size.

Does this mean that if I'm frequently using different sized tanks, the displayed SAC isn't ever going to be accurate? Is there an easy way to caculate accurately using tank size, starting and ending pressures?

My SAC rate varies a lot on the same dive day as well. For pretty good run down check out this alert diver article on the subject.

 

[hammet]

see attached

 

[rongoodman]

I calculated about .8 ft3/min on the first dive, and about .72 ft3/min on the second, not that far apart.

 

[ScubaSteve85]

I knew math would come back to bite me in the butt one day!

Thanks for the useful info all! It seems like Shearwater could imrpove this feature by giving the user an option to use either SAC or RMV plus tank size setting. Or, a happy medium might be to leave SAC alone on the dive computer, but allow the Shearwater desktop app to display RMV after inputting tank size. Seems RMV is a much more useful number.

 

[SJT1961]

According to Perdix AI documentation, shearwater's decision to use SAC not RMV was based on the concern that there was too much possible user error (either omission or commission) in inputting tank size/volume.

 

[broncobowsher]

What units does the Shearwater show in? PSI/minute? That would answer a lot of it.

 

[rongoodman]

What units does the Shearwater show in? PSI/minute? That would answer a lot of it.

Either PSI or Bar per minute.

 

[seeker242]

Or, a happy medium might be to leave SAC alone on the dive computer, but allow the Shearwater desktop app to display RMV after inputting tank size.

Subsurface dive log software can do that. One of the reasons why I prefer subsurface over shearwater desktop.