SAC vs Surface RMV

[Jax]

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.

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.

Q1: Why use a “variable” consumption rate?

Q2: What would be the general / average / expected difference in the SAC vs the SRMV?

Q3: What do you use to compute your gas use and why?

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

Lot of discussion on SAC here: http://www.scubaboard.com/forums/basic-scuba-discussions/344311-sac-rates.html but the definition of “SAC” doesn’t seem to be the same.

 

[Protist]

Very interesting topic.. Can't wait to see the responses.

 

[TSandM]

Hmm. Those aren't the definitions with which I am familiar. I am familiar with SAC defined as consumption in psi (which means you can't compare from tank to tank) and RMV as being measured in cfm, which means it's independent of tank.

Given the definitions you are using, the SAC would be more or less irrelevant to the diver, as it is a resting consumption rate, and diving is not done resting. RMV will vary within a range, but over time, one should have a sense for what it is when doing a reef dive where a lot of time is spent hovering and watching, and what it is on a dive where much more time is spent swimming.

I keep track of my RMV (and more often that not, CALL it my SAC rate) because it's the number which is relevant for dive planning purposes.

 

[Cave Diver]

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.

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.

Q1: Why use a “variable” consumption rate?

Q2: What would be the general / average / expected difference in the SAC vs the SRMV?

Q3: What do you use to compute your gas use and why?

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

Lot of discussion on SAC here: http://www.scubaboard.com/forums/basic-scuba-discussions/344311-sac-rates.html but the definition of “SAC” doesn’t seem to be the same.

Good luck with your question. I was taught that SAC and RMV are two completely different calculations with different uses. I've brought this up before and the general consensus from the forum at that time was that they are a different way of stating the same thing. After some of the ensuing discussion from that I decided it was easier to just do calcs my way and let everyone else do it theirs.

 

[Cave Diver]

I am familiar with SAC defined as consumption in psi (which means you can't compare from tank to tank) and RMV as being measured in cfm, which means it's independent of tank.

I keep track of my RMV (and more often that not, CALL it my SAC rate) because it's the number which is relevant for dive planning purposes.

Bingo.

 

[Jax]

Hmm. Those aren't the definitions with which I am familiar. I am familiar with SAC defined as consumption in psi (which means you can't compare from tank to tank) and RMV as being measured in cfm, which means it's independent of tank.

Given the definitions you are using, the SAC would be more or less irrelevant to the diver, as it is a resting consumption rate, and diving is not done resting. RMV will vary within a range, but over time, one should have a sense for what it is when doing a reef dive where a lot of time is spent hovering and watching, and what it is on a dive where much more time is spent swimming.

I keep track of my RMV (and more often that not, CALL it my SAC rate) because it's the number which is relevant for dive planning purposes.

Lynne, TDI/SDI use of the SAC as SDI describes it makes it a "constant", and the dive then gets a complexity multiplyer.

A warm, clear, still water dive is just your SAC (x 1)

If you are going deep (>60 fsw), the multiplyer is 2, if cold, add one, if against current, add one for each . . . .

So, a cold deep dive would make your consumption for planning purposes SAC x 3 for consumption purposes. Using the book rate of .5 cf / min, times 3, says to figure consumption at 1.5 cf/min.

My RMV ranges from .36 to .62, with an average of .44. Sure seems like the SAC times multiplier makes sense, but the "TDI" side of TDI/SDI says RMV . . .

 

[PfcAJ]

Those "multipliers" seem like a way to have to drag along a lot more tanks and gas than you need.

As an argument to that system, why not just call it what it actually is in those conditions?

 

[Jax]

Those "multipliers" seem like a way to have to drag along a lot more tanks and gas than you need.

As an argument to that system, why not just call it what it actually is in those conditions?

Sure. What is it, actually?

 

[PfcAJ]

Mine hovers around .6 for the majority of the dives I do, regardless of depth, dive length, visibility, flow, etc etc. It certainly doesn't double or triple just because the conditions are a little more sporty.

Is that what you were looking for?

 

[Jax]

Mine hovers around .6 for the majority of the dives I do, regardless of depth, dive length, visibility, flow, etc etc. It certainly doesn't double or triple just because the conditions are a little more sporty.

Is that what you were looking for?

So, I am to understand you use your RMV? A