Do you Plan your dive or Dive your plan within NDLs?

[stuartv]

I gave the source.
Fine.

However, if someone uses RMV for whatever reason, it is one simple step to get back to more familiar consumption rates.

Speaking only for myself, RMV is my most familiar way to deal with consumption. I could be diving an AL80, HP100, HP120, double 100s or double 120s (as a normal thing for me). Working with SAC as a normal thing would mean keeping up with a menagerie of different SAC numbers - and new ones every time I tried a new sized tank. RMV is one number for me to keep up with and I can use it for all my planning.

 

[tbone1004]

exactly, so since RMV is used to avoid confusion between pressure and volume per unit time, why even both? You have a surface air consumption expressed in volume per unit time, in this case lpm or cfm. That gets converted to DAC where you use pressure per unit time, in this case barm or psim to tell you how fast the gauge needle is going to go. There is just no point in using three terms when the third is completely useless. Why create a new definition for RMV, instead of just using SAC?

 

[lowviz]

...//... Why create a new definition for RMV, instead of just using SAC?

Because RMV is referenced to the big blue tank that surrounds the earth and SAC has a lot to do with what is on my back.

 

[stuartv]

Again, the reason RMV was used is to avoid confusion, not because it is the right term. RMV has a specific formula per the medical textbooks, and the original definition is volume per respiration, times respirations per minute. We can't measure either of those on scuba.

I feel like you are jumbling two things together here. There is the definition of RMV and the way you calculate it.

The definition of RMV is the volume of gas you consume in 1 minute.

The way you calculate it in the medical field is (taking from your post) to measure the volume of gas you consume in 1 breath, measure the number of breaths you take in 1 minute, and then calculate the RMV by multiplying those numbers together.

The way you calculate it in scuba is measure the volume of air you breath in one minute. That volume is measured by measuring the drop in cylinder pressure over time of a known cylinder. We use the measured pressure drop, the cylinder specs, and the ambient pressure to calculate the volume of gas consumed. The cylinder specs tell us the cylinder's surface air volume, so we have to use the ambient pressure to convert that number to the actual volume of the cylinder at the ambient pressure at the time of the measurement. That is, of course, assuming you're using those "American" style of cylinder specs, instead of the type of specs I seem to see used when talking about Euro cylinders. But the same concept applies even if the cylinder specs are given as water volume and working pressure and expressed in metric units.

For increased accuracy in scuba, we don't measure it over 1 minute. We measure it over a longer period and divide the total volume consumed by the number of minutes.

In either case (medical or scuba), it seems to me that the definition of RMV is the same. But, the measurements taken that are used to calculate the RMV is what is different. And it seems to me that the scuba way of calculating is just as accurate (barring differences in the accuracy and precision of the instruments you're using to take the measurements).

 

[tbone1004]

so the point still stands, what does SAC expressed in pressure per unit time do for you? the answer is absolutely nothing, it is just used as a stepping stone to get from DAC to RMV by those that think it is helpful to break the math up.

 

[stuartv]

exactly, so since RMV is used to avoid confusion between pressure and volume per unit time, why even both? You have a surface air consumption expressed in volume per unit time, in this case lpm or cfm. That gets converted to DAC where you use pressure per unit time, in this case barm or psim to tell you how fast the gauge needle is going to go. There is just no point in using three terms when the third is completely useless. Why create a new definition for RMV, instead of just using SAC?

Just for the record, I am pretty confused about your real point. It seems that you are making a case that we should either ignore psi/min or we should ignore cu-ft/min (to express it simply). But I'm not sure which.

Also, you keep using DAC, which I have never seen before. I'm assuming it is something that means your consumption rate at depth. But is that psi/min or cu-ft/min?

Regardless, my background is math and science. To me, a scientific term (which is how I would classify SAC and RMV) has a definition which will inherently identify what type of units are used to express it. In other words, if a term is defined as pressure over time, then it could be expressed in psi/min or bar/min, but it would be incorrect to express it in cu-ft/min or lpm.

Thus, it seems entirely appropriate to have SAC and RMV as terms we use and where one is defined as pressure over time and the other is volume over time. Sometimes you want to know or talk about one and sometimes the other. Trying to declare RMV (where the V stands for volume) as something expressed in units of psi/minute can't help but be confusing.

If I'm looking at my pressure gauge and trying to figure out how long I have until I get to my TP, knowing my SAC would yield the easiest mental arithmetic. But, if I'm calculating how big a tank I need to do a certain dive, then RMV will yield the easiest arithmetic.

It seems to me that both terms have a unique definition and they both have a place in scuba.

 

[Patoux01]

"Unreal" is the only thing that comes to my mind when reading you guys... If you'd simply always add the units and relevant info, it'd be so much easier.
I use 1b/min in a 15l tank (on surface). Hey, I have a surface air consumption of 15l/min, that's the same! Wow, great! We totally needed 15 pages of fighting to get to this conclusion...

 

[Bennno]

So SAC is not the volume used in one minute.

SAC is completely dependent on the specific cylinder.

No, SAC rate is expressed in liters/cuft per minute. So SAC is a volume and has nothing to do with the size of the tank.

Edit: Sorry, but this is stuff that is supposed to be taught in an Open Water Diver Course.

 

[Kevrumbo]

Am I doing something wrong? Doesn't 1 bar/min on an AL80 translate to an RMV of just less than 0.4 cu-ft/min?

Who does their gas planning based on an RMV that low?

I generally base mine on 0.7, which is 1.8bar/min (on an AL80), if I did the arithmetic correctly. Suddenly the arithmetic is not any easier than just sticking with Imperial. Especially since I almost never use an AL80 anyway.

Moreover, what is the point of giving such a detailed explanation for how easy it is to calculate when the whole thing is based on such a very specific assumption of RMV and tank size? Especially when the assumed RMV is so much lower than what it seems like most recreational divers actually consume.

What particular tank (or twinset) would that be? 10L? 12L? 15L? D7? D8.5? D12? Your 1 bar/min could be anything from 10 to 24 SLM (i.e. from mouse to hoover), depending on the tank/twinset size. I use 10L tanks, and I've never recorded a SAC as low as 10 SLM. And I log my SAC on every dive since my PDC is AI.

For me in cold water SoCal, 1 bar/min per ATA pressure Surface Consumption Rate is with twin AL80's which is 22 liters/bar total tank rating; it becomes 2 bar/min per ATA using a single AL80 at 11 liters/bar tank rating. Make sense?

Therefore my normalized volume Surface Consumption Rate across all tanks is 22 liters per minute per ATA, which is about 0.8 cuft/min per ATA in US units.

 

[stuartv]

Christ, and using 2 instead of 1.8 is omygodsofarfromtruthiwilldie?

Get your **** together buddy...

As I said before, I rarely dive an 80. So, if I'm using a 100, it's around 1.4? Still not simple arithmetic, like the poster who keeps going on and about multiplying and dividing by 1 keeps saying. And when I'm on a 120, it's 1.2? Doubles? 0.7 or 0.6?

If you're just going to round everything to 1 or 2 or something like that, well, I can do the same thing with Imperial numbers. The point being that the yammering about how using metric lets you do all this multiplying and dividing by 1 is really kind of, well, just that useless yammering, and how easy that is doesn't really apply to most people (not even close) from what I can tell.