Question Definitions for SAC and RMV?

[Belzelbub]

30 votes for definitions of SAC, 50% more than for RMV. I assume more respondents are familiar with the term SAC than RMV. Sixty percent voted that SAC was volume/min/atm, only 40% that it is pressure/time/atm.

That surprised me at first, and I was wondering why there were more votes for SAC vs RMV. As I thought back to where I heard the terms, it started to make sense to me.

In training classes that I've taken, I've only heard instructors use SAC. My OW courses were a long time ago and I can't remember if they taught PSI/Min or Cu ft/Min. I think my first exposure to RMV was with the Oceanic DiverLog+, then I started using Subsurface which uses SAC for the measurement that Oceanic called RMV. In my use, the only constant is that RMV is always volume. SAC can be both pressure or volume. Regardless of what I'm calling it, I don't drop the units, so it should be clear to anyone.

 

[inquis]

I'm curious as to what value do people think that having both SAC and RMV refer to volume unit per time unit?

Im pretty sure many of those who use volume-based SAC do not use or probably learned of RMV much later.

How do you feel about soda & pop having the same meaning?

 

[BoltSnap]

soda & pop

I Know it as "soda pop" in NY.

 

[Cheizz]

I Know it as "soda pop" in NY.

I though it was 'soft drink'?

 

[tursiops]

The oldest reference book I have available is Owen Lee's "The Complete Illustrated Guide to Snorkel and Deep Diving," published by Doubleday in 1963. Owen was one of Cousteau's divers (the first American, joined in 1960), and then went to work as an oceanographer at the US Navy Electronics Lab in San Diego, where I met him about 1968. The book's foreword was written by Cousteau.

The book talks about air consumption, which he says just to assume is about 1 cuft/min, but never gives it a name. He also mentions US Navy work on oxygen consumption, and refers explicitly there to RMV, which he says varies from 8 l/min (at rest, in bed) t0 85 l/min (uphill running). He suggests the ratio of O2 consumption to RMV is about 1/22, and that the O2 needs for a diver are about 1.5 l/min if swimming at about 0.5 to 0.8 knots (this suggests a RMV of about 33 l/min, or about 1.2 cuft/min). We appear to swim more efficiently today!

 

[wetb4igetinthewater]

Im pretty sure many of those who use volume-based SAC do not use or probably learned of RMV much later.

That could very well be, as training has been inconsistent and you have folks inventing new terms as opposed to establishing an agreed upon terms. Convergence is likely impossible. I just ask people to look at it from the point of view, if you were to teach someone gas planning and have them compare predicted versus actual remaining cylinder pressure, how would you do it? I do believe this necessitates some term to address pressure changes over time, and that is cylinder specific.

I honestly don't care what terms are used as long as:
a) previously established medical terms are not redefined
b) there is a common set of terms that everyone agrees with, kind of like indicating numbers by tech divers (oh how I wish that would filter to recreational divers universally)

Again, this isn't rocket science. We should be able to have a consistent method to teach a fairly simple concept to open water divers.

And I am open to any and all methods for teaching this to divers, as some students may need to hear it in a different way than what I present.

 

[scubadada]

So, the poll turned out to be quite interesting.

21/33 (64%) of votes for SAC were for volume/time/atm. The remaining minority voted for pressure/time/atm. There were 43% more total votes for SAC than for RMV, probably reflecting a greater familiarity with the term SAC than RMV.

22/23 (96%) of votes for RMV were for volume/time/atm.

There is a lack of uniformity in the definitions for terms describing gas consumption, particularly SAC. It would be best to include the units of RMV and SAC to ensure accurate communication.

The term to describe gas consumption in the thread and poll posted in July 2015, Average Gas Consumption, is RMV.

 

[Cheizz]

 

[stepfen]

Just to give you my 2c: I know and use only SAC (in lt/min) and with it I can do all the necessary gas planning calculations for my dives, if needed (not very often to be honest). On top of that, it is very convenient that Subsurface uses the exact same definition hence I can use its' gas planning feature out of the box. That's where my needs end and I am happy with it.

Now on top of that, I am aware that there is another term (RMV) widely used by the diving community (I have the feeling it is something mainly used in imperial/PSI unit countries, see the USA for example). I also know that some people call RMV what I (and Subsurface) call SAC and vise versa, but at the end of the day who cares? If there were divers who'd be comfortable enough to do their own gas planning correctly using spaghetti monster units/terms and it was working for them, why would I mind?

If I ever get to the point (highly improbable) that I have to plan a dive together with a diver who uses different units or method (RMV, psi, cuft etc), I will spend the 5 minutes (it doesn't take more) to understand the "other" way and to do the necessary conversions. Maybe add another 5 minutes to double check the results and we are good to dive. Until then, let it be.

 

[Wibble]

SAC -- SURFACE Air Consumption -- is the amount of gas you're breathing at any depth adjusted for depth, i.e. converted to 1 ATA/Bar.

Example: at 30m/100ft the pressure is 4 ATA/Bar. If my SAC was 20 litres/min, then I would be breathing 4 x 20 = 80 litres per minute at depth.

MEASURING your SAC needs you to observe the drop in pressure (in Bar) over a fixed time at a fixed depth and then multiplied by the cylinder's wet capacity.
Example: 30m for 5 mins on a single 12 litre cylinder. The start pressure is 133 bar, after 5 mins the pressure is 100 Bar; pressure drop is 33 Bar.
Gas consumed is 33 (Bar/ATA) x 12 (litres) = 396 (call it 400).
Adjust for depth: at 30m/100ft it's 4 Bar/ATA, so 400 ÷ 4 = 100 litres.
Adjust for time: 5 mins: 100 (litres) ÷ 5 (mins) = 20 litres per min SAC