Shearwater Perdix AI Sidemount SAC/RMV Calculation

[GLightcap]

Hello all. I a trying to calculate my RMV using the SAC rate calculated on my Shearwater Perdix AI. I'm hoping that someone who is better versed in this than I am can verify that either my reasoning and math is correct, or I am missing something. The problem is that I dive sidemount doubles but the Perdix only uses one cylinder's pressure along with the total dive time when calculating the SAC rate. So unless I use the same psi during a dive in each cylinder, the SAC rate given for one cylinder is not useful. I could easily calculate my RMV manually for the doubles using the average depth, but if I do that, I would be losing the benefit of the computer constantly calculating time, depth, and PSI drop. I'm trying to determine the ATM factor that the computer calculated to come up with the single cylinder SAC rate (PSI per minute) and apply that to the other cylinder. Below is what I came up with.

P1 = PSI used in cylinder 1
P2 = PSI used in cylinder 2
S1 = SAC rate for cylinder 1 (derived from computer calculation)
S2 = SAC rate for cylinder 2
T = Time in minutes
ATM = The atmosphere factor used by the computer to calculate S1
CF = Cubic feet per PSI for the particular cylinder you are using (example: for an aluminum 80 I use .026 (80cuft/3000psi) which is the volume of gas per PSI in that cylinder)

P1
T*S1 = ATM

Now we can use the ATM factor to calculate the SAC for the other cylinder.

P2/T/ATM = S2

Now that we have the SAC rate for the second cylinder using the same factor the computer used to calculate the SAC rate for the first cylinder, we can calculate the RMV.

(S1*CF)+(S2*CF) = RMV

 

[napDiver]

Why do you care about at the moment sac rate?

Here's some options
1) Easiest thing to do is drop to depth and swim at constant depth for 20 mins. You can write your starting and ending pressures and get your typical sac. If you want to be lazy just breathe the tank with the transmitter.

2) You can also do a single tank sm dive one day and just use that.

3) if you look at the shearwater documentation they describe how they calculate sac. I believe (please double check, I'm guessing off memory) they do it based on the last 2 minutes via average depth and change in tank pressure. So everytime your on the tank with the transmitter for more then 2 mins, the sac calculation should be accurate.

Once you have sac, you can convert to RMV as you'd like

 

[napDiver]

*note: I believe it disregards your first 30 or 60 seconds in the dive for the sac calculation

 

[GLightcap]

Why do you care about at the moment sac rate?

 

[GLightcap]

Why do you care about at the moment

I'm just looking to collect as accurate a RMV as possible to be aware of differences in rate for different types of dives, as well as identifying any trends or changes in rate. It's also useful for dive planning. Knowing how far I can go on a fixed volume of gas for a certain type of dive, or how much gas I will need to successfully complete a specific dive.

 

[GLightcap]

3) if you look at the shearwater documentation they describe how they calculate sac

Shearwater doesn't show how to calculate RMV (volume SAC) for sidemount doubles. Their method uses the pressure drop of one cylinder along with the time for the total dive (half of which you are not breathing from that cylinder). The result is a rate that is about half of what it actually was. This method works fine for a single cylinder dive or manifold doubles but not independent doubles.

 

[napDiver]

Per my number #3, read it again.

I'll also try to clarify, I'm saying per how shearwater calculates sac, it only calculates for a specific interval. If you are breathing from the tank with a transmitter for more than that internal it will be accurate.

For ex.

You have 2 tanks at 12L. You decend and use your left or the first 5 mins and use 30 bar. Your transmitter is on your left. Sac is accurate. You switch to your right tank and use it for the next 15 mins and use 60 bar. As soon as you switch to your right tank, your sac is inaccurate. You switch to your left tank and use 60 bar for 15 mins. The first 2 mins after you switch to your left it will be inaccurate. But at exactly two minutes and further sac will be accurate until you switch to your right tank.

If you want to know your at moment sac on the non transmitter tank. You would have to know the starting and ending pressure, time interval, and average depth during that interval for when you started and ended breathing from it. So itd look like... diving at 30m and staying at 30m average for 1 min, remember starting and ending pressure after 1 min and then calculating at moment sac.

Anyways... I believe shearwater doesnt take into account tank volume, but only psi change. You are correct, the rate of gas used would only be half since you have 2x independent tanks. But really, the way I'd look at it is the sac they give you while breathing on your transmitter tank for 2 mins or longer is correct. Calc whatever you want and then take into account volume of available gas is tank 1 + tank 2.

 

[GLightcap]

Per my number #3, read it again.

I'll also try to clarify, I'm saying per how shearwater calculates sac, it only calculates for a specific interval. If you are breathing from the tank with a transmitter for more than that internal it will be accurate.

For ex.

You have 2 tanks at 12L. You decend and use your left or the first 5 mins and use 30 bar. Your transmitter is on your left. Sac is accurate. You switch to your right tank and use it for the next 15 mins and use 60 bar. As soon as you switch to your right tank, your sac is inaccurate. You switch to your left tank and use 60 bar for 15 mins. The first 2 mins after you switch to your left it will be inaccurate. But at exactly two minutes and further sac will be accurate until you switch to your right tank.

If you want to know your at moment sac on the non transmitter tank. You would have to know the starting and ending pressure, time interval, and average depth during that interval for when you started and ended breathing from it. So itd look like... diving at 30m and staying at 30m average for 1 min, remember starting and ending pressure after 1 min and then calculating at moment sac.

Anyways... I believe shearwater doesnt take into account tank volume, but only psi change. You are correct, the rate of gas used would only be half since you have 2x independent tanks. But really, the way I'd look at it is the sac they give you while breathing on your transmitter tank for 2 mins or longer is correct. Calc whatever you want and then take into account volume of available gas is tank 1 + tank 2.

Thank you for the response. I understand what you are saying, and I am pretty sure I have a clear understanding of what and how the calculations are done with the Perdix. As far as the real time, last two minutes SAC rate, I'm not interested in that in this context. Shearwater shows your real time SAC rate during the dive but a few minutes after you surface, it calculates your SAC rate for the whole dive and displays that. That is the rate I am discussing here.

 

[napDiver]

Ahh ok, I misunderstood what you wanted.

You need average depth of dive
Start and end pressure for each tank
Tank size
And total dive time

Let's say 2x11L at 200 bar. Ending pressure 50 bar and 80 bar. 30 minute dive. 30m average depth.

Air volume breathe at depth = [(200 bar - 50 bar)*11L + (200 - 80 bar)*11L] / 30 min = X liters per min

To get sac,
X / (1 + average depth in meters/10)

I'll look at your eqs when I get back from diving today. Cheers

 

[sabbe]

This is the one point Shearwater seems to have made worse over the years instead of better. In 2017 I could select any portion of the dive and Shearwater Desktop would give me the instantaneous (every 2 min) SAC rate for each tank.

I've since switched to analyzing portions of the dive and manually doing the calculations myself since an overall dive SAC rate is meaningless and I'm not sure it does side mount correctly. I'll look at my at depth consumption, my deco / safety stop consumption separately so I can better plan future dives.

Sam