SAC vs RMV, revisited

[Storker]

there is a loss in resolution going from degF to degC and from psi to bar but that probably doesn't matter much.

It won't. A temperature difference of 1C is hardly noticeable. A temperature difference of 1F is roughly half of that. And when do you relate to psi numbers with more than two significant digits? Heck, even in bars I rarely relate to pressure numbers with more than two significant digits. And if you need more resolution, you always have the digits to the right of the decimal point.

I'd posit that that loss of resolution has only a psychological effect.

 

[Marie13]

Shearwater gives you SAC in PSI/min. Discovered that with downloading dives to MacDive, you get RMV in cft/min. Handy to have it readily accessible.

 

[caruso]

Those of us who use the excellent (and free) Subsurface logbook software may be aware, or should be aware that Subsurface shows an entry in the box labeled SAC, only after the tank size is entered, at which point it is really displaying RMV. Not clear if this is a bug or intentional by the developer.

 

[atdotde]

Subsurface is intentionally showing the only thing that makes sense: Volume of gas consumed per minute (and not tank pressure difference per minute which changes when you use a different cylinder size). People disagree if that should be called SAC or RMV.

 

[Manatee Diver]

cue my soapbox

Americans are stupid. Our insistence on using indirect measurements for tanks leads to this confusion. Why our government didn't man up and just change to the metric system long ago is beyond me. The rest of the civilized world has seemed to do it. There are 3 countries that don't use the metric system, 2 of which are largely irrelevant, and everything important in this country is done in the metric system anyway.
In all of the sophisticated parts of the world, you have the actual capacity of the tank, and the pressure. Super easy and we don't have to deal with this crap. In our idiotic imperial measurement system, we create needless complexity and confusion in an effort to make things sound better than they really are.
So.

It has nothing to do with imperial vs metric, but the fact due to compressability that the only accurate way to really measure a gas is by weight, everything else is a calculated estimate. But that really isn't workable in the real world, with the different working pressures and container sizes the standard cubic foot or liter of air is the workaround.

 

[tursiops]

Those of us who use the excellent (and free) Subsurface logbook software may be aware, or should be aware that Subsurface shows an entry in the box labeled SAC, only after the tank size is entered, at which point it is really displaying RMV. Not clear if this is a bug or intentional by the developer.

This illustrates the potential confusion. If one uses the terms SAC and RMV without giving the units, they are inviting confusion. There is NO agreement or authority to turn to for the "correct" usage. You've got to include the units, somehow. You can say 'pressure SAC" or "volume SAC" and that is probably enough. RMV is more awkward, because it is always "volume" but it may not be "at the surface!" so you need to say the equivalent of "RMV at depth" or "RMV at the surface." Anything else is inviting confusion.

We return to our previously scheduled confusion.

 

[jgttrey]

Caution.... Shearwater has confirmed there is an issue with the SAC rate code that causes it to display incorrect values. A fix will be available with the next update.

Teric SAC rate confusion

I agree it is extremely valuable in conserving air as you. I've gotten my average rmv down from .5 to about .4 just by glancing at my wrist every once in a while.

It's a great data to track, but be careful.

Not suggesting you are doing this, but I would caution others about spending too much time monitoring this on a dive. It can lead to bad breathing habits as folks chase a lower consumption rate by trying to breath shallow or skip breathe. Best practice is breathe normally on your dives and use the RMV calculation after the dive to track progress and gas plan future dives.

Once a diver has decent breathing habits, any improvements in RMV are going to be the result of other things - streamlining gear, improving trim or finning technique, getting buoyancy perfect. Trying to improve gas consumption by changing your breathing is usually a recipe for developing bad habits and retaining CO2.

 

[tursiops]

Shearwater gives you SAC in PSI/min. Discovered that with downloading dives to MacDive, you get RMV in cft/min. Handy to have it readily accessible.

Presumably MacDive asks you what tank you are using?

 

[Storker]

f one uses the terms SAC and RMV without giving the units, they are inviting confusion.

Bingo.

I couldn't give a flying **** what you call it, as long as the unit is SLM (surface liters per minute) I know what you're talking about.

And SLM makes the math damned easy. Let's say my gas consumption is 20 SLM. I'm at 30m (4 ATA) and my buddy is OOG. I reckon I'll need one minute at depth to sort things out, that's 1x20x4=80L. Then we need 3 minutes to surface, at an average depth of 15m (2.5 ATA), that's 3x20x2.5=100L. I'd like to throw in a safety stop, that's 3min at 5m (1.5 ATA), or 3x20x1.5=90L. Total 270L. For me. Then my buddy will need their gas, so we'll have to double that. Total 540L. But my gas consumption is while diving normally, without stress, so let's double that again. That's 1080L. In a 10L tank, that's 110 bar. In a 12L tank, it's 90 bar, and in a 15L tank it's 72 bar. Add some 10-20 bar to account for SPG mismeasurement and my 1st's ability to deliver gas, and I have my min gas pressure (for my 10L tanks, that'd be some 120-130 bar).

 

[Manatee Diver]

Presumably MacDive asks you what tank you are using?

Yes it does, and you can set a default if you use one tank more than others.