Calculating SAC Rate and RMV

[miketsp]

I agree SAC is by definition expressed in PSI per minute
..snip..

?????
By who's definition?
Any definition that requires complementary information like the size of the tank should be thrown in the garbage. Otherwise the result is meaningless to a third person.

 

[DA Aquamaster]

How old are your text books? The vintage would tell a lot.

SAC as an expression of psi per minute goes back to 1978 that I am aware of and was associated with the SAC-Rate computer at that point and was a trade marked term. RMV went back even farther and RMV was used to measure "surface air consumption". At the time AL 80's and Steel 72's were the most common tanks encountered and were more or less standard in recrational diving so using volume was not all that essential if you always used the same type of tank. In that regard a SAC rate measured in PSI was preferrable once the SAC-Rate computer came out as you could do it in a few seconds with no math.

So it may be a case of semantics where RMV predates SAC as a way to measure surface air consumption but the specific term "SAC" was a much more specific reference that has since been adopted as a generic, as opposed to trademarked term, that has replaced RMV. Sort of what happened to the term "Aqualung" during that period of time when any reg was known as an "aqualung in the general lexicon even if it was made by Voit, Dacor or any other company.

You are entirely correct that a SAC rate required a reference to the specific type of tank as 30 psi per minute means something much different in a 6 cu ft tank as opposed to a 100 cu ft tank. Converting a SAC you did in an AL 80 was imporant if you went to doubles, a 50, etc. Instructions with the SAC rate computer included instruction how to convert your SAC rate to an RMV using a tank factor.

As for saying a SAC rate is garbage if it relies on a specific tank and pressure is not entirely true as tank factors and pressures are still used for planning purposes between divers in the cave community and the goal is still much the same - to be able to get a feel for how many psi are required to make up a given volume in cubic feet for easy reference on an SPG.

 

[nereas]

It's not universal; it depends entirely upon who is doing the defining.

I have a number of dive texts which refer to SAC Rate in CFM.

But really, it's a non-issue. As TSM noted, the bottom line is what matters.

Indeed, for dive planning and tank size selection, I have found CFM (cubic feet per minute) to be the most useful denomination of SAC-RMV. So I agree whole heartedly with Blackwood.

And I have found SAC and RMV to be virtually identical in conversation. Medically speaking, I believe RMV is denominated in mmHg, and that this is the origin of the term, and not scuba, but I will defer to TSM (a surgeon) for more on that. But RMV certainly predates scuba.

My thanks to DA for his excellent perspective on the history of these terms.

 

[coldsmoke]

?????
Any definition that requires complementary information like the size of the tank should be thrown in the garbage. Otherwise the result is meaningless to a third person.

SAC is not used by a third person. It's for the individual diver to monitor his/her consumption during the dive. RMV - actually let's get rid of that term since the consensus is that it's dated - So let's use cuft/min and psi/min. It's to your benefit to use and understand the value of both - not just cuft/min. Your consumption in terms of cuft/min in great for surface dive planning. ie - I want to do a dive to (x)Feet for (y)Time with a specific gas usage plan (ie halves, thirds, sixths, whatever). My cuft/min consumption rate is (z) so I need (xx)cuft of gas.

Now once you hit the water you don't really want to monitor your gas in cuft. You don't have a device that tells you that. Your spg reads psi. Sure you could convert but why?

Plus, by getting in the habit of knowing and monitoring your consumption during the dive in terms of psi/minute if either your time keeping device or your spg fail you can use the other as a backup. Not sure how much sense that made so let me explain it differently. If I have a left post failure and shut down that valve my spg will read zero. Obviously the dive gets called, but what if I'm in an overhead situation either real or due to a deco obligation, or maybe I just don't want to surface here because I know there's lots of boaters and the surface marker that I'd like to shoot will only attract them like flies to, well you know. I will always have a pretty good idea of how much gas I have regardless of even looking at my spg by how much time has elapsed. I know that I use 8 psi/min in my HP100's at one ATA. Say I'm at 100 feet. I should be using about 32psi every minute or about 160 psi every 5 minutes.

Hunter

 

[vladimir]

Maybe the next 30 years we should work on meters. But wait - what would that do to football? Crap, I guess we're doomed to imperial units forever.

Hunter

"2.7432 meters and a cloud of dust?" You're right, it just doesn't work in football; at least not at Ohio State.

 

[peterbj7]

I think the language has evolved. I know no-one who measures his gas consumption in pressure/time except as an intermediate stage in the computation, and I know no-one who uses the term RMV for the end result. I never use the term. It's not a question of what's "right" or "wrong", it's customary usage. The term "surface air consumption" is perfectly comprehensible and does not imply any units. The term "respiratory minute volume" is gobbledygook and certainly needs explanation.

I teach the concept of SAC (modern meaning) to both technical and recreational divers, the former because it's part of the syllabus, the latter because IMO it's a necessary part of their diving education. Just like nitrox calculations though I teach it using the metric system, as the maths is much simpler and the straightforward logic isn't clouded by lots of numbers. Then if people want to use the imperial system they can simply convert the end result. It's very rewarding when you see the light of comprehension in someone's eyes and know that they won't forget it because they UNDERSTAND it.

 

[Dwag766]

Now a new question on an old discussion for you very experienced divers.....I love using steel tanks and rent them whenever possible as I like the lack of fluctuation in bouyancy during the dive. I can make it easily through the dive with an 80cf but recently used a 100cf thinking I would have longer bottom time.

Upon dive completion I had used more than usual but did not have to surface early. Is the gas more compressed in the 80cf? I guess what I am trying to understand is if there is greater gas density in the 80cf.....I felt like breathing was more relaxed with the 80.......Is this just my imagination working overtime?

 

[sam1]

I love using steel tanks and rent them whenever possible as I like the lack of fluctuation in buoyancy during the dive.

The fluctuation in buoyancy is not a function of the tank but a function of the gas in the tank that is used during a dive. 80 CF of air weighs about 6 pounds so with a reasonable reserve, you use about five pounds of air during a dive regardless of the material the tank is made of. It is true, however, that steel tanks are usually negatively buoyant when empty. A particular AL80 is 4.4 pounds positive when empty, while a particular steel 80 is 3.3 pounds negative. This difference in tank buoyancy would allow you to use 7.7 pounds (let's call it 7) less lead. Also, with the two tanks in my example, the steel tank weighs 2.8 pounds less than the aluminum one. So you would be carrying about 10 pounds less weight on your back/body.

 

[TSandM]

Upon dive completion I had used more than usual but did not have to surface early. Is the gas more compressed in the 80cf? I guess what I am trying to understand is if there is greater gas density in the 80cf.....I felt like breathing was more relaxed with the 80.......Is this just my imagination working overtime?

The purpose of your regulator is to drop the pressure of gas from the couple thousand psi in the tank to ambient pressure. No matter what the tank pressure (2400 to 3500 or more), the gas comes out of the second stage at the same pressure, which is the ambient where you are. So it doesn't matter what kind of tank or how compressed the gas in it is, as far as your ease of breathing goes. What DOES matter is the cracking pressure set on the regulator. I don't know if you were using the same regs, but if you were, any difference was largely in your mind.

You also said, "I used more than usual." But was that in psi, or in cubic feet? This is where the pressure in the tank and its size DOES make a difference. For example, an HP100 has about 3 cubic feet of gas for each 100 psi in the tank. But my LP95s have about 3.7 cubic feet per 100 psi. So, if I use 37 cubic feet on a dive, I'll have used 1000 psi from my tank. If I used an HP100, I'd have used over 1200 psi for the same dive -- but it's the same amount of gas! So, if you want to compare gas usage between two different tanks, you have to know how much gas the tank holds at its rated pressure, and that will tell you the cu ft/psi for that tank. Then you can figure out the gas consumed in cubic feet, which is the only way you can compare dissimilar tanks.