Question Definitions for SAC and RMV?

[tursiops]

Whatever........ Agree with it or not, the simple fact is that the SDI Solo Diver course that I just took teaches volume (not pressure) for gas consumption calcs.....

On the course chapter tests and final exam, answers based on volume are required if you want to correctly answer the questions related to gas calculations for this course.

Here's an example right from the SDI course materials.

SAC stands for surface air consumption and is a measurement in litres or cubic feet (never in units of pressure) that describes how much air a diver breathes every minute while on the surface at rest.

My point -- as clearly as I can make it -- is that SDI is inconsistent in its own use of those defintions, and nothing in the Solo standards disallows pressure units. However, having said that, if the exam question asks for your SAC (with no further clarification) then you'd be silly to use anything other than volume units, as per the Solo manual. The secondary point is you have a very strong leg to stand on if your instructor gets anal about what SAC means; you can easily use SDI's own publications (other than the Solo manual) to prove him wrong.

 

[lowwall]

So the executive summary is:

The definition and usage of units in both SAC and RMV are inconsistent. Precedent can be found for expressing either terms in both units of pressure (psi or bar) and units of volume (cubic feet or liters). To avoid confusion when discussing SAC/RMV, state your units. If using units of pressure also state the tank size.

 

[tursiops]

So the executive summary is:

The definition and usage of units in both SAC and RMV are inconsistent. Precedent can be found for expressing either terms in both units of pressure (psi or bar) and units of volume (cubic feet or liters). To avoid confusion when discussing SAC/RMV, state your units. If using units of pressure also state the tank size.

Not quite. RMV is always volume, as in its name.
It is SAC that could be anything...metric, imperial, volume-based, pressure-based.
Agreed, stating your units is essential; with units stated, you don't even need to use the ambiguous name!
"My gas consumption at the surface is 0.45 cuft/min."
What is really bad is if I say, "My SAC is 16." That could mean l/min, or psi/min; both numbers are quite common but mean VERY different things, unless you are using a 120cuft HP steel tank, in which case it turns out that 16 l/min is about the same "SAC" as 16 psi/min.

 

[ScubaWithTurk]

With RAID it is required to teach SAC and RMV calculations. When I teach the Advanced course and the Deep 40 course, we know our SAC (also called SGC) and we plan the dives based on that SAC..which is then converted to RMV (SAC*ATMs) to get our total gas used in liters (RMV*Minutes) for each segment of the dive.

SAC is not cylinder dependent. It could be any cylinder and I use it and teach it as "My SAC is 15 L/M at 35m my RMV is 67.5 l/m and we plan to spend 12 minutes of bottom time which means I will use 810 liters for that portion or phase of the dive"

I still am concerned that many instructors don't even know how to do this. Just month and a half ago I was a dive site and talking to my OW student about his SAC rate. I saw another instructor giving me a strange look, as was his student. He asked me what course I was teaching and I told him that it was the Open Water course and we just did dive 1 of the course. He then asked...and I am not making this up "What is SAC and why would a diver need to know this" After some conversation, I asked him what course he thought I was teaching and he said he honestly had no clue as he had never heard many of the words I said used when teaching scuba diving. How to plan deep dives if you have no idea what your gas consumption rates are??

 

[NW Dive Dawg]

After just taking the SDI Solo E-Course and then reading this entire thread......I think SAC may stand for Stupid Ass Cluster@%&* and can be calculated by taking your total SB post count, divided by your reaction score, then multiply by your number of dives, and finally...... divide by 1/3 of your estimated IQ while at rest watching TV.

PS... If you don't log dives, then just use whatever number you feel like making up.

 

[ScubaWithTurk]

After just taking the SDI Solo E-Course and then reading this entire thread......I think SAC may stand for Stupid Ass Cluster@%&* and can be calculated by taking your total SB post count, divided by your reaction score, then multiply by your number of dives, and finally...... divide by 1/3 of your estimated IQ while at rest watching TV.

PS... If you don't log dives, then just use whatever number you feel like making up.'

Ok, that was really funny!! But as far as not logging dives goes, I stated in my previous post, SAC comes into play when planning deeper dives. Are we gas limited or NDL limited? How much gas do are we going to actually use for ourselves and then how much is needed if things go South and we need to get us and a buddy to the surface? Do we have enough based on the planned bottom time and depth or should we shorten our bottom time or maybe look at going a bit shallower?

All part of being a thinking a diver as well as planning the dive and diving the plan.

 

[scubadada]

Hi @NW Dive Dawg @tursiops

After having solo dived for many years, I took the SDI Solo Diver course in 2013 to make it easier to dive with operators that did not know me. I already knew my RMV from several years of monitoring and used the same definitions for SAC and RMV that I use today, SAC is pressure/time and RMV is volume/time.

The SDI Solo Diving Manual I used, and still own, is dated 2007, 2010 and is edited by Steve Lewis. I don't know what is used today. I was very disappointed in the presentation of SAC and SRMV. It ended up not being important to me as I was adept at using my RMV to calculate required gas volumes. The test questions were a piece of cake.

The manual states SAC is only in volume/time, never in pressure/time. They want you to measure your SAC on the surface at rest and then convert to volume with the tank characteristics. You calculate your gas needs by multiplying the SAC by a dive factor between 1.5 and 3 to reflect overall dive difficulty (temperature, visibility, current, diver factors), If you use real dives to calculate SAC, that is SRMV (Surface RMV) and is not good because it already includes dive related factors and overestimates your SAC

So, is there any doubt why there is confusion with nomenclature?

It has been stated before, I quote my friend @lowwall "To avoid confusion when discussing SAC/RMV, state your units. If using units of pressure also state the tank size."

 

[NothingClever]

Setting aside inaccurate use of terms, I think the most proficient divers will just do the math for the profile and inform others how much gas they’ll need/use by stage without (and with) delays and their detailed numbers if required.

 

[Eric802]

I find this entire topic very confusing. My only knowledge of what my numbers may be comes from the number that Subsurface spits out after I download dives from my Peregrine and put in my starting and ending tank pressures, along with tank size. For example, for a 48 minute dive with an average depth of 49 feet, using an AL80, Subsurface tells me that my SAC is “0.54cuft”. I understood this to represent cubic feet per minute of gas use at the surface (so, at 66 feet, it would be approximately 1.6 ft.³ per minute). To me, it would seem that this number is independent of the size of the tank one is using and would be useful for dive planning.

From what I’m reading in this thread, it seems I’m on the right track as some people use the numbers and terms and completely wrong as others understand it.

 

[scubadada]

Hi @Eric802

You are perfectly on track. Subsurface chose to use the term, SAC, to describe gas consumption in volume/time/atm.

Nearly 2/3 of respondents who voted on the definition of SAC in the thread poll use the same definition.