SAC vs RMV - What is standard?

[Jax]

I posted this in the basic thread, but am finding that definitions and useages are widely differing. So, my question to you, given this:

I’m trying to figure (internalize) the differences and usages in these two breathing consumption rates.

First, to baseline the discussion:[1]

SAC: Surface Air Consumption. Measure by taking a tank of known volume, wear a mask or nose plugs, note the starting pressure, breath for 10 to 20 minutes, then note the final pressure. Compute the cu ft (L) used. SAC – Cu ft / minutes.

Surface RMV: Respiratory Minute Volume. Take an average from past dives, rework the figures of consumption at depth to extrapolate what the consumption would be at 1 atm (bar). Figure the consumption per minute.[1][2]

The SRMV supposedly has other factors ‘against it’ such as work during dive, cold, darkness, and other things that affect volume consumption. The SRMV is variable per dive.

The SAC is supposed to be a more-sure baseline as it is generally not affected much over time (except to vary with physical fitness).

Here’s an interesting difference. The TDI Guide to Advanced Nitrox[2] has nothing about the SAC, but does all computations according to a diver’s RMV.


[1] SDI Solo Diving Manual 2007
[2] TDI Guide to Advanced Nitrox 2009

Q1. What, then, do you expect your diver to use to calculate gas needs?
Q2. Why the difference between SDI and TDI?
Q3. Are there plans to 'normalize' the two?

Thanks!

 

[AndyNZ]

Not sure of the "official line" on this. My view:

RMV and SAC (and SCR) are essentially the same thing.

I use RMV when using volumetric terms, e.g. litres per minute. I would get students to measure it in the same as a SAC, e.g. maintain a constant depth for a specified period of time and measure the pressure drop in the tanks and convert this into volume used.

I use SAC in terms of pressure units, e.g. bar per minutes.

RMV then has the advantage that it is independent of tank size as you always adjust for tank volume. So a diver with a nominal RMV of 20 litres per minute will never get surprised if they change from a 12 litre to a 10 litre tank.... the disadvantage is that the calculations require one extra step.

SAC has the advantage that you can easily do calculations on the fly, but the disadvantage is that it is dependent on tank volume. If you use 2 bar per minute from a 12 litre tank, you will use more from a 10 litre tank.

I refer to both RMV and SAC in surface terms, and indicate consumption rates at depth by the terms dSCR and dRMV.

Saying that, I also don't get hung up on the terminology.... the end result is the important thing, being able to calculate your required gas reserves! As long as a student can do that, I'm not too fussed how they go about it or what terminology they use.

 

[Jax]

Oooh, oooh, may I ask you to repost that (or may I quote it?) in my other thread http://www.scubaboard.com/forums/basic-scuba-discussions/347007-sac-vs-surface-rmv.html?

I'm collecting data there.

Now, I'm waiting on what SDI/TDI says . . . .

 

[Red Sea Shadow]

Steve Lewis has written, IMHO, the definitive guide on SAC - RMV for technical divers. It comes in two parts and is titled "Gas Planning 101". You can find it at Doppler's Tech Diving Blog

 

[Jax]

Good link! This one is for finding out SDI / TDI's take on their definitions, since they don't seem to jive.

 

[Scott]

Good link! This one is for finding out SDI / TDI's take on their definitions, since they don't seem to jive.

If I'm not mistaken the Steve Lewis who's blog was quoted is also the the same Steve Lewis, Marketing & Communications Director for SDI/TDI.

 

[Red Sea Shadow]

If I'm not mistaken the Steve Lewis who's blog was quoted is also the the same Steve Lewis, Marketing & Communications Director for SDI/TDI.

You're not. And BTW, he's the editor of the Solo Diving Manual.

 

[Red Sea Shadow]

This one is for finding out SDI / TDI's take on their definitions, since they don't seem to jive.

My personal opinion is that, although they're sister companies, SDI is a recreational agency while TDI is a technical one. That's why it's not unusual to have different definitions. Those are different classes taught by different agencies.

 

[Jax]

Which is why I would really like to hear from SDI / TDI itself.

 

[merxlin]

Not sure of the "official line" on this. My view:

RMV and SAC (and SCR) are essentially the same thing.

I use RMV when using volumetric terms, e.g. litres per minute. I would get students to measure it in the same as a SAC, e.g. maintain a constant depth for a specified period of time and measure the pressure drop in the tanks and convert this into volume used.

I use SAC in terms of pressure units, e.g. bar per minutes.

RMV then has the advantage that it is independent of tank size as you always adjust for tank volume. So a diver with a nominal RMV of 20 litres per minute will never get surprised if they change from a 12 litre to a 10 litre tank.... the disadvantage is that the calculations require one extra step.

SAC has the advantage that you can easily do calculations on the fly, but the disadvantage is that it is dependent on tank volume. If you use 2 bar per minute from a 12 litre tank, you will use more from a 10 litre tank.

I refer to both RMV and SAC in surface terms, and indicate consumption rates at depth by the terms dSCR and dRMV.

Saying that, I also don't get hung up on the terminology.... the end result is the important thing, being able to calculate your required gas reserves! As long as a student can do that, I'm not too fussed how they go about it or what terminology they use.

I may be off here, but how can SAC be dependent on the tank size? If I breathe .53 c.f. per minute on the surface, I will burn 1.06 cubic feet per minute at 33 fsw, 2.12 cubic feet per minute at 66 fsw, etc. regardless of what size tank I am breathing from. Tank size has zero to do with my rate of consumption, or in figuring what that rate is. Am I wrong?