Subsurface
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Stuart is not wrong, he is just different. To no purpose that I can see.
Doctormike is being kind when he ways Stuart just has a different way of conceptualizing it.
Most people and books and training that I am aware of have converged on taking things to the surface for comparison and calculation, rather than changing the meanings of perfectly good concepts and moving things to depth. I'm sure Doctormike sees the two ways of thinking; I'm not sure Stuart does, because he is arguing that he is "right" and any other way of thinking about it is "wrong." If my air consumption rate is 0.5 cuft at the surface, need I really also say that the cuft I am talking about are also at the surface, therefore my 100 cu ft tank has 100 of them? In Stuart's language of software development, perhaps he is comfortable with the "relative addressing" he is asking for, with everything being in the context of whatever depth you are at; I prefer absolute addressing.....take it all to the surface.
I am NOT discussing RMVs and SACs....those are definitions, not concepts.
Doctormike is being kind when he ways Stuart just has a different way of conceptualizing it.
Most people and books and training that I am aware of have converged on taking things to the surface for comparison and calculation, rather than changing the meanings of perfectly good concepts and moving things to depth. I'm sure Doctormike sees the two ways of thinking; I'm not sure Stuart does, because he is arguing that he is "right" and any other way of thinking about it is "wrong." If my air consumption rate is 0.5 cuft at the surface, need I really also say that the cuft I am talking about are also at the surface, therefore my 100 cu ft tank has 100 of them? In Stuart's language of software development, perhaps he is comfortable with the "relative addressing" he is asking for, with everything being in the context of whatever depth you are at; I prefer absolute addressing.....take it all to the surface.
I am NOT discussing RMVs and SACs....those are definitions, not concepts.
That may be the first analogy I've seen on scubaboard that makes complete sense.Stuart is not wrong, he is just different. To no purpose that I can see.
Doctormike is being kind when he ways Stuart just has a different way of conceptualizing it.
Most people and books and training that I am aware of have converged on taking things to the surface for comparison and calculation, rather than changing the meanings of perfectly good concepts and moving things to depth. I'm sure Doctormike sees the two ways of thinking; I'm not sure Stuart does, because he is arguing that he is "right" and any other way of thinking about it is "wrong." If my air consumption rate is 0.5 cuft at the surface, need I really also say that the cuft I am talking about are also at the surface, therefore my 100 cu ft tank has 100 of them? In Stuart's language of software development, perhaps he is comfortable with the "relative addressing" he is asking for, with everything being in the context of whatever depth you are at; I prefer absolute addressing.....take it all to the surface.
Think of your tank as having a certain number of CUF of gas in it. If you have an AL100, and it's filled to service pressure (3000 PSI) you have 100 CUF of gas. That 100 CUF is compressed into a volume much smaller than 100 actual cubic feet, but we are calling it 100 CUF because if you let it out at sea level, it would expand to fill a space that big. That's what you have to "spend" for the dive.
As you breathe from the tank, the "CUFs" come out, and the deeper you go, the faster they come out. On the surface (1 ATA) they come out at your sac rate - let's say 0.5 CFM (cubic feet per minute). At 100 feet (4 ATA), they come out four times faster - 2.0 CFM.
So the amount of gas you breathe from any tank of any size per minute (CFM) is the product of your SAC rate and your depth in ATA. As Stuart pointed out, the volume that you breathe doesn't change, but the number of gas molecules does. We are just using CUF (at surface) as an algebraic term for the number of gas molecules.
Then you multiply the CFM by the number of minutes to find out how many CUF you use over that time (i.e. for that leg of the dive).
If you want to solve for how long a tank will last, you just do what you said, and rearrange the equation to solve for time.
CUF used = SAC rate x Depth (in ATA) x Time (in minutes).
CUF Used / (SAC rate x Depth) = Time
For your example, 100 CUF / (0.5 x 4) = 50 minutes
However, you really wouldn't do that calculation quite like that - we don't want to know how long a tank will last, we want to know how much gas we have considering that we don't want to surface with an empty tank, and that in many cases (rule of thirds) we want to surface with 1/3 of our gas volume left. Also, this simple equation doesn't account for ascent time either.
But yes, generally when formally planning dives, most people calculate CUF for each leg of the dive, which only has physical meaning when considering the gas volume that that amount of gas would fill at the surface.
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I recall being told that my aluminum 80 held about the same volume as a phone booth. This shows how long ago I was told that, since today's yoots may never have seen a phone booth. Anyway, I was told to visualize having a phone booth's volume of air to consume for the dive, and the deeper I am, the more of it I consume per minute. That's hardly any sort of rigorous definition, but it worked well for us OW students. It made sense to me. Stuart suggested that the inclusion of "surface" in the name for the rate is potentially confusing to students. I think the way I--and apparently most of us--were taught is not confusing at all. Maybe using "RMV" in its strict definition is useful for physiologists, but I think using a rate having a surface-adjusted definition is more useful for divers.
If they have cable TV (also unlikely among today's youth) they have access to channels such as hallmark that play old/classic movies which might show a phone booth.
I saw something a year or two ago interviewing college kids about "old" technology. Most had never even seen an audio CD. I don't think my kids have (18, 17). I know they've never seen a real phone booth.
OP
GlennL
Contributor
i was told that I breathe more at depth. Not that I have less air to breathe. Whichever the case is technically true doesn't bother me. I ain't a rocket surgeon and I understand the basis.
Have seen/used pay phones. But not a booth.
I grew up on a farm and didn't get cable until I was a teenager. And yes I can still hear the tone in my head from the old dial up modem when logging into aol.
Have seen/used pay phones. But not a booth.
I grew up on a farm and didn't get cable until I was a teenager. And yes I can still hear the tone in my head from the old dial up modem when logging into aol.
First, thank (most) all of you for engaging in an esoteric discussion and keeping it friendly.
Second, my last post or two was, as @tursiops said, no longer talking about definitions - they were about concepts.
To take a more practical view of it, ambient pressure (which is another way of expressing depth) is simply one term in any of the equations you use for figuring out things like "how much gas do I need" or "how long do I have at depth nnn?" So, whether you put that term on one side of the equals sign and it multiplies times the RMV or whether you put it on the other side of the equals sign and it divides the tank surface air volume doesn't matter. You get the same answer. So, which way you conceptualize it doesn't really change the mechanics of how you answer your practical dive questions. In other words, practically speaking, it doesn't change anything whether you choose to think of RMV as surface-corrected or not.
Strictly as a concept, in terms of explaining it to people who haven't already been trained and already have years of thinking of it in one specific way, it seems to me that it would make more sense (to a totally new diver) to say that your lung volume is your lung volume. It doesn't change, ever. That is easy to visualize. Explaining that the "amount" of gas (i.e. the number of molecules, if you will) that will fit in your lungs does change, based on your depth, is also easy. OW student books show the balloon at the surface and the half-sized balloon at 33'. It is, again, easy to visualize that more "amount" of gas fits in your lungs at 33' because the pressure is double which means the volume is halved. So, that big balloon that you could never inhale all of at the surface is suddenly so much smaller that yes, now you could inhale all of it at 33'.
Looking at it with a non-American-centric view, it seems like I've seen a non-trivial number of posts on SB that suggest that people sometimes struggle with how it all works in the Imperial system. And it seems like people who "grew up" on the metric system don't have as much trouble. I wonder if that isn't because the American way of doing it, as it seems to be normally taught, is that "surface correction" for tank volume is inherent to Imperial tank measurements. And, while RMV is not inherently surface-corrected, we nevertheless wrap up surface correction in RMV, instead of keeping that concept separate from RMV and keeping it strictly tied to Imperial tank volume. Meanwhile, the metric way of doing things doesn't really "internalize" the surface correction into the tank measurements. A 12l tank at 200 bar, is 2400l at 1 bar. Or 1200l at 2 bar. When that measurement is expressed that way, RMV is clearly not a surface-corrected concept and it is easy to visualize how the tank contents, the ambient pressure, and the RMV all "go together".
Or so it seems to me.
Anyway, it is clear that the horse was dead a while ago. Not to mention in the totally wrong corral. So, I'll shut up now.
Second, my last post or two was, as @tursiops said, no longer talking about definitions - they were about concepts.
To take a more practical view of it, ambient pressure (which is another way of expressing depth) is simply one term in any of the equations you use for figuring out things like "how much gas do I need" or "how long do I have at depth nnn?" So, whether you put that term on one side of the equals sign and it multiplies times the RMV or whether you put it on the other side of the equals sign and it divides the tank surface air volume doesn't matter. You get the same answer. So, which way you conceptualize it doesn't really change the mechanics of how you answer your practical dive questions. In other words, practically speaking, it doesn't change anything whether you choose to think of RMV as surface-corrected or not.
Strictly as a concept, in terms of explaining it to people who haven't already been trained and already have years of thinking of it in one specific way, it seems to me that it would make more sense (to a totally new diver) to say that your lung volume is your lung volume. It doesn't change, ever. That is easy to visualize. Explaining that the "amount" of gas (i.e. the number of molecules, if you will) that will fit in your lungs does change, based on your depth, is also easy. OW student books show the balloon at the surface and the half-sized balloon at 33'. It is, again, easy to visualize that more "amount" of gas fits in your lungs at 33' because the pressure is double which means the volume is halved. So, that big balloon that you could never inhale all of at the surface is suddenly so much smaller that yes, now you could inhale all of it at 33'.
Looking at it with a non-American-centric view, it seems like I've seen a non-trivial number of posts on SB that suggest that people sometimes struggle with how it all works in the Imperial system. And it seems like people who "grew up" on the metric system don't have as much trouble. I wonder if that isn't because the American way of doing it, as it seems to be normally taught, is that "surface correction" for tank volume is inherent to Imperial tank measurements. And, while RMV is not inherently surface-corrected, we nevertheless wrap up surface correction in RMV, instead of keeping that concept separate from RMV and keeping it strictly tied to Imperial tank volume. Meanwhile, the metric way of doing things doesn't really "internalize" the surface correction into the tank measurements. A 12l tank at 200 bar, is 2400l at 1 bar. Or 1200l at 2 bar. When that measurement is expressed that way, RMV is clearly not a surface-corrected concept and it is easy to visualize how the tank contents, the ambient pressure, and the RMV all "go together".
Or so it seems to me.
Anyway, it is clear that the horse was dead a while ago. Not to mention in the totally wrong corral. So, I'll shut up now.
You still sound like a young whippersnapper, to me.
We lived on the edge of a swamp (north FL) until we moved closer to town to live on a farm. We didn't have a color TV until I was in high school. Cable TV was a fancy city thing. When we lived down on the river, picking up the phone was a gamble on a dial tone or hearing the neighbors because it was a party line. 
OP
GlennL
Contributor
You still sound like a young whippersnapper, to me.
Im definitely not old. lol
But it does amuse me to talk to teenagers nowadays and compare what they think is outdated technology
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