RMV math…
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johndiver999
Contributor
For the OP. A good way to start is to assume your SAC is 1, pretty easy to remember. 1 cu/ft per minute at the surface. Then just figure what pressure you are at in atmospheres at any depth (don't forget to add 1) and multiply. Not too hard to remember to use the number 1 twice.
Don't make it overly complicated. When you get some decent data on your own use, then the sac will probably be around 0.6 or 0.7 for a lot of situations, but 1.0 is a good relatively safe ball park figure especially for doing the rithmetic in your head. going to 100 feet.. that is 99 feet that is 3 plus 1 = 4 atmospheres of pressure times 1 cu-ft/min means you use 4 cuft per minute at depth. If you have an 80 tank, that will be gone in 20 minutes. Very rough calcs, but they do serve as a reality check.
Don't make it overly complicated. When you get some decent data on your own use, then the sac will probably be around 0.6 or 0.7 for a lot of situations, but 1.0 is a good relatively safe ball park figure especially for doing the rithmetic in your head. going to 100 feet.. that is 99 feet that is 3 plus 1 = 4 atmospheres of pressure times 1 cu-ft/min means you use 4 cuft per minute at depth. If you have an 80 tank, that will be gone in 20 minutes. Very rough calcs, but they do serve as a reality check.
Since this is the Basic Scuba forum, I want to note that we have not always been using the terms RMV and SAC (surface air consumption) rate correctly in this thread. RMV varies with pressure. SAC is RMV at 1ata and is thus a fixed number.
It can get confusing because you tend to use RMV to figure out your SAC rate. And then when you are planning a dive, you use SAC rate to figure out RMV.
For example, let's say you completed a 40 minute dive at an average depth of 20m/66’ and used 50 cubic feet of gas.
Your RMV for the dive is 50/40 or 1.25 cubic feet per minute. To figure out the SAC you divide RMV by the average ata which is 3 in this example, so SAC is .417.
Now that you have the SAC rate, you can calculate RMV at any depth. It's simply SAC x ata.
You know, this might be easier to grasp if we used weight rather than volume. I'll save that thought for another post.
It can get confusing because you tend to use RMV to figure out your SAC rate. And then when you are planning a dive, you use SAC rate to figure out RMV.
For example, let's say you completed a 40 minute dive at an average depth of 20m/66’ and used 50 cubic feet of gas.
Your RMV for the dive is 50/40 or 1.25 cubic feet per minute. To figure out the SAC you divide RMV by the average ata which is 3 in this example, so SAC is .417.
Now that you have the SAC rate, you can calculate RMV at any depth. It's simply SAC x ata.
You know, this might be easier to grasp if we used weight rather than volume. I'll save that thought for another post.
OP
quietlife4me
Contributor
As a newb...this had me confused as hell for the last hour. The documentation I was using online (see above) was calculating SAC using PSI. So the SAC would be shown as 16.15 not 0.42 and I was racking my brain with how the heck can his SAC be a number lower than 1 when my calculations are always a number greater than 10.
I may be more special than others....but it took me that long to figure out people are not saying what unit they are using when they quote SAC
SAC in cuft/min or SAC in psi/min
Not as I understand it. Your lung volume is what it is, and thus RMV is not depth dependent. What varies is the number of gas molecules a given breath will pull in as depth varies.
In the big picture, Volume (and thus RMV) is applicable across cylinders. SAC will be specific to a particular cylinder (or cylinder). The ratio of gas capacity and cylinder pressure is the conversion between RMV and SAC and is true for both imperial and metric. (Capacity at 1 bar equals the water volume for metric folks. The "tank factor" I referred to above is one form of that ratio.)
If I'm concerned with the rate my cylinder pressure drops, I scale my SAC by the ambient pressure at whatever depth. Scaling my RMV by ambient pressure gives the rate of change of the volume that gas occupies at surface pressure.
Lol, I stopped fighting that battle long ago. Overwhelming usage is RMV for the cuft/min rate and SAC for psi/min (or the metric liter/min and bar/min rates).
Yes, they are both consumption rates, so Surface Air Consumption seems applicable, but that's definitely the minority view.
OP
quietlife4me
Contributor
I was working at NASA when two engineering teams decided to use different units of measure and didn't tell each other. So when the control unit sent "10" until touchdown to the landing unit...the control unit meant meters and the landing unit thought it was feet and all hell broke loose and destroyed the landing craft.
Bottom line is the fewest number of people will be confused (or try to correct you) if you give units and choose "SAC" vs "RMV" based on those units. It's an imperfect world... 
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