SAC rate too low?

[Diver0001]

I've been working on my trim and breathing techniques in the water to lower my SAC rate and I've got it down to .497 cu ft/ min, my question is, when is your SAC rate considered too low?

This is really a personal issue. Mine is normally at about 0.42 in cold(ish) water and wearing a twin 12 and a drysuit.... That's not incredibly low but in my case it's lower than many people I dive with and nearly exactly the same as my regular buddy.

and that's what's important.... knowing what it is so you can use that number for planning.

If you're doing anything "special" like skip breathing to bring down your SAC then you're doing something wrong. But with slow and even breathing then.... well ... it is what it is. It will change over time and from day to day but just take it as a given.

Having said that, if it's below about 0.28 and you're a normal sized adult then I would assume you're doing something wrong.

R..

 

[Mattleycrue76]

I'm surprised that noone has mentioned it yet but what everyone here is discussing is not SAC but RMV

SAC = PSI per minute (which makes it specific to a given tank)
RMV = cubic feet per minute

For more details on the difference see here:

NWGratefulDiver.com

 

[Kingpatzer]

How many digits in a calculated SAC and RMV rate are really significant?

To me, figuring out SAC to be .745 or whatever is kind of goofy. There are plenty of variables at work that will change each dive slightly? Wouldn't one digit be significant enough for most any application?

As a further aside, to me it's a road to ruin to make judgments of value when comparing one's gas usage to another. The utility of you knowing mine and me knowing yours is pretty much limited to when we are diving as buddies or as part of some shared endeavor where are relative times in the water matter to each other.

Lower rates are dependent in some way with comfort in the water and diver skill. But they also have a lot to do with lung volume, muscle mass, and other factors that aren't related to skill as a diver at all. More time in the water being a good thing means that we all want to improve our fitness and skill levels; but SAC rate is a simple physical fact, and shouldn't be seen as a measure of value. Just my humble opinion, but there it is.

 

[Bubbletrubble]

I'm surprised that noone has mentioned it yet but what everyone here is discussing is not SAC but RMV

SAC = PSI per minute (which makes it specific to a given tank)
RMV = cubic feet per minute

For more details on the difference see here:

NWGratefulDiver.com

@Mattleycrue76: This is truly splitting hairs. Lay people misuse scientific terminology all of the time. I would venture a guess that all of the participants in the thread understood what the OP was talking about.

If you really want to split hairs, RMV can be expressed in other units beside "cubic feet per minute" and SAC rate could be defined in terms of cubic feet per minute.

 

[mikemill]

I'm surprised that noone has mentioned it yet but what everyone here is discussing is not SAC but RMV

SAC = PSI per minute (which makes it specific to a given tank)
RMV = cubic feet per minute

For more details on the difference see here:

NWGratefulDiver.com

On who's authority?

SAC = Surface Air Consumption
RMV = Respiratory Minute Volume

There is no reason why SAC couldn't be calculated in lots of different measurements (cuft/min, psi/hour, etc). RMV is locked into a volume/minute but that's it.

 

[Thalassamania]

On who's authority?

SAC = Surface Air Consumption
RMV = Respiratory Minute Volume

There is no reason why SAC couldn't be calculated in lots of different measurements (cuft/min, psi/hour, etc). RMV is locked into a volume/minute but that's it.

I tend to like (mega parsec barns) per (sidereal year).

More to the point, yes your SAC rate can be too low. In my late teens and early twenties I went on a binge to stretch my tank ... guess what? Wicked headaches. I found out why and stopped that foolishness, your SAC rate is what it is and if you want to stay down longer that your SAC rate will permit, there is an easy solution: BRING MORE GAS.

 

[mikemill]

I tend to like (mega parsec barns) per (sidereal year).

Who doesn't?

 

[Kingpatzer]

I'm partial to hogshead per fortnight, just like Grandpa Simpson

 

[DA Aquamaster]

On who's authority?

SAC = Surface Air Consumption
RMV = Respiratory Minute Volume

There is no reason why SAC couldn't be calculated in lots of different measurements (cuft/min, psi/hour, etc). RMV is locked into a volume/minute but that's it.

Exactly. 4-5 years ago I had the same belief that SAC = psi/min and RMV = cu ft/min.

So I set out to do some research to prove my point, and I did - but only to a certain point in history. I have a few new old stock SAC rate calculators that I got in a box of stuff from a dive shop closing and they are intended to be used with psi, and that was the practice at the time they were sold, which was about the same time I started diving. But when I started looking at older sources, I started finding references to SAC in cu ft /min, not psi. So...over the long term, SAC has been used to reflect gas consumption in both psi and cu ft and it would be equally correct to associate it with either one.

Conversely, that also means that the effort to associate RMV with cu ft/min and SAC with psi exclusively is a much more recent convention.

-----

I had an aeronatical engineering professor / former CAG that liked to work in furlongs per fortnight for velocity - I am not sure what his preference would have been for flow rates, but I am sure it would have been equally interesting.

 

[Kern]

I'm surprised that noone has mentioned it yet but what everyone here is discussing is not SAC but RMV

SAC = PSI per minute (which makes it specific to a given tank)
RMV = cubic feet per minute

For more details on the difference see here:

NWGratefulDiver.com

From all but the US point of view, this is parochial & spurious. The US & Liberia are the only places left where it is even a consideration. Everyone else just uses litres/minute for both, & so SAC is not specific to any given tank size.