Shearwater Perdix AI

[Mike1967]

Ok now I'm confused lol
I thought it was the SAC you needed to know so you could estimate the air you were going to use at a specific depth?
i.e. If your SAC is 2 bar/min then you'll use 4 bar/min at 10m and 6 bar/min at 20m etc

 

[Mike1967]

, now just multiply your bar/min x tank size (24.2 L) and get your RMV in L/min

Why do I need to know my RMV? I can't use RMV to track if my air consumption is getting better can I, I thought it had to be SAC because RMV varies so much with different dive profiles?

 

[Sevenrider860]

Ok now I'm confused lol
I thought it was the SAC you needed to know so you could estimate the air you were going to use at a specific depth?
i.e. If your SAC is 2 bar/min then you'll use 4 bar/min at 10m and 6 bar/min at 20m etc

SAC results in bar/min values and this changes based on cylinder size. A 7 liter and 11 liter cylinder will have different bar/min values because of service pressure and volume differences.

Once you know SAC (bar/min) for a specific cylinder size, take it to the next step and determine your RMV (Respiratory Minute Volume). This will be in liters/min.

 

[Sevenrider860]

Why do I need to know my RMV? I can't use RMV to track if my air consumption is getting better can I, I thought it had to be SAC because RMV varies so much with different dive profiles?

If you calculated your RMV and averaged 21 liters/min in January and after 50 dives you average 17 liters/min in April...isn’t that telling you that your air consumption is getting better?

SAC is pressure drop per minute. RMV is volume used per minute.

 

[stuartv]

So you're saying it doesn't need to know the tank size to get an accurate SAC?
The reason this has come up for me is I used twins for the first time on Saturday and the SAC was way low till I doubled my start and end pressures. I was using two 12.2lt tanks with an open manifold so as far as my perdix was concerned it could have well been one 24.4lt

You are comparing 1 12l tank to 2 12l tanks. Assume they all start at 230 bar. If you go from 230 bar to 0 on 1 x 12l and 230 bar to 0 on 2 x 12l, then haven't you used twice as much gas in the second case?

If using 230 bar in a twinset means you used twice as much as using 230 bar in a single tank, then doesn't that also mean using 1 bar in a twinset means you used twice as much as using 1 bar in a single tank?

Or, if you drop 1 bar in a single tank, you would drop 1/2 a bar in a twinset, right?

If you are measuring SAC in bar per minute, then doesn't it make sense that, if your breathing stays the same, your SAC would be half the number when you are using a twinset (of the same size cylinders)?

For people who only ever dive the same size tank, using SAC is fine. But, as you can see, once you start using tanks of different sizes, it is simpler/easier to calculate RMV and then compare that across different dives.

Unfortunately, as you see, the Perdix does not really make this especially easy, as it does not allow you to enter cylinder size, so it cannot calculate your RMV for you. You have to download your dive data and then let your laptop/tablet/phone calculate your RMV for you.

 

[KenGordon]

Ok now I'm confused lol
I thought it was the SAC you needed to know so you could estimate the air you were going to use at a specific depth?
i.e. If your SAC is 2 bar/min then you'll use 4 bar/min at 10m and 6 bar/min at 20m etc

Bar/min is a useful idea on the dive to figure out if you need to be a bit shallower or work less to make the end of the dive. For example on a typical Cozumel drift.

It isn’t so useful for planning though, in that case l/minute at the surface is the number you care about. It is a number personal to you and is independent of cylinder capacity. It multiplied by the absolute pressure (depth/10 + 1) gives the actual consumption rate in l/minute. Used in conjunction with the profile, say surfacing from Xm, gives you how much gas you will expect to need regardless of the cylinder size. Now you can make sure you have a big enough cylinder, in terms of free gas, for that profile. That might be 30 bar in a 10 or 100 in a 3.

Eventually you have to choose a cylinder for a dive. Personally I find that easier knowing how much gas I will use and comparing that with how much gas is in a cylinder than having some cylinder dependent constants inserted all over my calculations.

 

[Mike1967]

Ok thanks for all the feedback, I think it's sinking in. So the RMV is litres per minute used at the surface?

 

[stuartv]

Yes. The V in RMV stands for volume. Respiratory Minute Volume. Thus, liters (a measurement of volume vs bar which is a measurement of pressure) per minute.

SAC is ambiguous regarding whether it refers to volume or pressure. But, if you remember that RMV refers to volume (liter or cubic foot) per minute, then it's easier to remember to use SAC when talking about psi or bar per minute.

 

[ATJ]

From the Perdix AI manual:

Calculating RMV from SAC - Metric units


In the metric system, tank sizes are described using a single number, the tank’s physical size in liters [L].This is how much gas could be stored at a pressure of 1 Bar, so effectively the units of tank size are [L/Bar].

This makes converting SAC to RMV easy.When using metric units, simply multiply the SAC by tank size.

For example, a SAC of 2.1 Bar/min with a 10 L tank would be an RMV of (2.1 x 10) = 21 L/min.

Just to confuse things a bit, as I recently learned from the boys at Subsurface, the calculation above will only give an approximation for RMV. It's a good enough approximation for comparing dives but is not accurate.

First, 1 bar = 0.986923 atm, so that should be applied in the conversion.

Second, air does not act like an ideal gas under compression so the compression factor would need to be determine depending on the pressure of the air. The compression factor changes with nitrox, helium, etc.

As I said, ignoring compression factor and bar to atm conversion will still give a RMV which is useful for comparing one dive to another...

 

[Mike1967]

Yes

Cool, thanks mate.