SAC vs RMV, revisited

[Bob DBF]

RMV is defined and used in medicine.
RMV (Respiratory Minute Volume)
the minute volume of breathing; the product of tidal volume times the respiratory frequency.

SAC is a SCUBA term used to track air usage in a tank in psi increments.


Both terms have been used interchangeably by divers and instructors for as long as it's been important to me. As long as I can understand what a diver is trying to communicate to me, it's in the fins vrs flippers catagory.


Bob

 

[tbone1004]

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.

NOAA uses three terms in the dive manual
RMV-in it's medical definition
SAC-cfm or psi/min *which is the confusing part*
Cd-consumption at depth *probably because Cd is better for formulas than DAC

Historically, SAC has been discussed in cfm in the dive industry. We teach DAC in psim because it is what you measure, and we teach SAC in cfm only to keep confusion to a minimum with students. surface PSIm is only useful IMO if you are only ever using one tank. The computers use it because it's easier and less risky to have you convert to cfm or lpm after the dive than it is to trust that you told it exactly what tank you were diving

 

[tursiops]

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.
We teach DAC and SAC. You measure the DAC, you calculate the SAC. SAC is ALWAYS expressed in cfm, and only cfm. The pressure/min is only a stepping stone in the formula. It is used in both directions to make sure you have a consistent running log of your consumption in something that is tank agnostic, and then for dive planning you can convert it to DAC with the specific tank you are using, at your intended depth.

Total psi consumed divided by total dive time=psim=DAC
DAC*tank factor/avg depth=SAC

If you only ever use one tank size, then psim is certainly significantly easier to keep track of, but you are screwed if you have to use another tank size.

Why we try to complicate this subject is beyond me. The only one of these three acronyms that actually has a defined unit is RMV, and the official unit for that is liters per minute. To make it even better, the definition of RMV is tidal volume * respiratory rate. As we can measure neither of those while we are underwater, why do we want to make something sound fancier than it is to make ourselves feel better, and actually use a specific term?

You can use DAC while you're diving because the gauges measure pressure delta and you can roughly extrapolate time remaining and the computers do the same. The computers give you the adjusted SAC because they don't know what your tank size is and it is too risky to have you program that in.

Good heavens, you've just made it even more complicated! DAC? Really? Who is "we" that you speak of....your group of instructors in your location? Not compelling! You are hard-over on definitions that are NOT in common use.

 

[tbone1004]

Good heavens, you've just made it even more complicated! DAC? Really? Who is "we" that you speak of....your group of instructors in your location? Not compelling! You are hard-over on definitions that are NOT in common use.

I edited the post, but yes the group of instructors that I teach with. DAC has been used for a long time, often seen as DACr. It may be a NAUI thing though, unsure if PADI uses it

 

[Miyaru]

Splitfins vs jetfins.....SAC vs RMV.....
As long as you can calculate your total gas requirements for a dive I couldn't care less which terminology somebody uses.

... 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.
...

Well.....there's the metric system and the impaired system.....
People must have had really large feet back when they came up with this measurement.

 

[Storker]

It's not exactly rocket science.

Start pressure minus end pressure = pressure used
Pressure used times tank water volume = gas volume used.
Gas volume used divided by run time = gas volume used per minute.
Gas volume used per minute divided by average depth in ATA = surface gas consumption per minute.

Disregarding, of course, compressibility, which would give an unrealistically high gas consumption for short dives.

Get a couple hundred dives and average the surface gas consumption per minute, and you have a decent number to base your min gas calculations on. If you haven't that number, some 15 to 20 surface liters per minute is a decent starting point.

 

[Pavao]

I really don’t get using pressure/minute as consumption measurement, it is directly related to a specific tank size used, which by default is only of meaning to that particular scenario.
Volume/minute at 1 ATA makes a lot more sense to me, and honestly, I really don’t care what you call it, I don’t even use anything other than SAC actually, I can get everything else I need calculated around this value.

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.

For real, we call it AL 80cf3, if that vessel was really 80cf3 I’m not sure a diver would be able to move much, that’s bigger than your fridge, way bigger.
But an 11 liter tank is exactly that, take the valve off and pour 11 liters of water in it and it will fit. Imperial tank designation is in relation to its working pressure, metric tank designation is the actual tank size, independent of working pressure. In a for sale post on FB a guy was trying to explain some tank sizing and it clearly illustrates how this creates confusion, I just walked away.
I bought a blender for my house a while back and just the other day I noticed the measurement engraved on the side of the container, clearly manufactured for imperial users, or by them more likely
Check out the conversation chart lol

Imperial system just sucks, ok, rant over

 

[saxman242]

Psi per minute (or bar per minute) is valuable in that it is the actual data that is being recorded and having the original test data is always a good idea. To be useful beyond that, however, it needs to be normalized for the tank in use.

And agreed on the metric system being better.

 

[txgoose]

That blender picture is classic. If anyone elected to congress is reading, I also vote to go metric.

 

[EFX]

My introduction on air usage under PADI used the term SAC to mean air usage at the surface in psi/min. It was drilled into us that the consumption had to be calculated for the correct diving depth and was useful only for the same tank volume. I was taught (eventually) how to convert SAC to RMV as a surface measurement of air consumption based on volume rather than pressure and that this value was more useful as it could be applied for different tank sizes. It seems that rec divers are taught and use the SAC term to mean pressure/min and tec divers use the same term to mean volume/min. There was a thread I responded to that became confusing because the meaning of SAC was not immediately disclosed. I would like to suggest two terms that are more descriptive to divers: SACP and SACV = surface air consumption pressure and surface air consumption volume respectively.

While I'm at it let's dispense with the term NDL (no decompression limit) and replace it with NSL (no stop limit) as a more useful and accurate term. As an engineer I prefer metric having used it in my career. The change from imperial to metric would be painful at first but we would all get used to it sooner than we think. However, there is a loss in resolution going from degF to degC and from psi to bar but that probably doesn't matter much.