Shearwater Perdix AI Sidemount SAC/RMV Calculation

[RayfromTX]

My situation is even harder. I typically dive three tanks, two in sidemount and one stage.
Today I did a four hour shore dive. The last 45 minutes was along the shore just looking at critters in five meters of water and cleaning the nitrogen out of my tissues down to GF40. To get my Teric to give me an RMV on the whole dive I get creative. I load it into Macdive and then edit the start and end pressures of the one tank it measures and check the box for double tank. I assume there is no adjustment in the software for the compression of gasses because if there is it would blow the accuracy.
My left tank started at 199 bar and ended at 48.
My right tank started at 184 bar and ended at 19.
The stage was breathed from 205 bar down to 15.
The calculation goes like this.
199-48=151
184-19=165
205-15=190
Total gas used is 506 bar. Divide that by two for the double tank setting and the result is 253.
Then I edit the gas in macdive to show a starting pressure of 253 and an ending pressure of 0.
Macdive does the calculation for me with a result of 14.05 liters per minute.
This method is only a tiny bit easier than doing the whole calculation myself using the average depth but it logs it into macdive in case I want to track RMV over time or in various configurations and conditions.

 

[napDiver]

@RayfromTX That's a good point. I use subsurface as my log and basically do the same thing for a multi tank dive. I never checked if it recalculates sac, but I just looked right now and it does! It'll do it as soon as I add the extra tank sizes with corresponding pressures.

@GLightcap
I'd just do what Ray suggests, but if you wanted to use your formula, it should work. I'm concerned with your formula to calculate ATM since I'm not sure if shearwaters S1 will be what we expect. I would prefer to calc it via average depth or use shearwater's "avgATM" instead. If you do both methods for a few dives and they're the same, then there shouldn't be any problems.

cheers

 

[J-Vo]

Am I correct that you only have a TX on one tank? To get the accuracy you are looking for on every dive, your going to need a second transmitter.

 

[kelemvor]

Download your logs into Subsurface and let the software do the math for you. Then you can look up your SAC at any point in the dive rather than relying on an average over the course of the dive.

 

[GLightcap]

Download your logs into Subsurface and let the software do the math for you. Then you can look up your SAC at any point in the dive rather than relying on an average over the course of the dive.

Thank you for the info. I will check Subsurface out.

 

[GLightcap]

Am I correct that you only have a TX on one tank? To get the accuracy you are looking for on every dive, your going to need a second transmitter.

I have a transmitter on each tank but the Perdix gives the SAC rate only for one of the tanks "T1."

 

[Mike1967]

I have a transmitter on each tank but the Perdix gives the SAC rate only for one of the tanks "T1."

Yeah when you download it to a decent app it'll give you your exact SAC. I'm very happy with this one....
‎Dive Log

 

[Wibble]

It’s amazing how much your breathing rate goes up under stress.

My SAC has always been reasonable and well under 10 at deco. Did a particularly deep dive and was overweighted. This caused no end of stress and was astounded at the speed the bottom gas was consumed. Left the bottom around min gas and almost consumed the reserve margin. Thankfully lots of deco gas available and no problems finishing the dive.

Massive lesson learned, particularly complacency about SAC and overweighting could easily kill.

If you're not correctly weighted, get out and sort it out.

 

[napDiver]

I have a transmitter on each tank but the Perdix gives the SAC rate only for one of the tanks "T1."

... that would of saved me a lot of time...

Another advantage of subsurface, it a line per tank showing gas pressure over time. The line is also colored from green to red for relatively low to high consumption based on the dive. Super cool to see what situations causes you to much higher than average and how much your sac is at that time

 

[J-Vo]

I have a transmitter on each tank but the Perdix gives the SAC rate only for one of the tanks "T1."

In that case you just have to average the used volume between the two tanks and use the average depth for the time frame you are analyzing. Calculation is the same as a single tank.

If you use a third tank without a transmitter the average SAC in subsurface uses the entre time and screws up the calculations. It also always assumes the second TX is on my first deco gas and updating it does not up date all the data in the dive to correlate.