10 replies to this topic

[Dive_Girl]

Do you know the difference between SAC & RMV?

SAC = Surface Air Consumption rate
RMV = Respiratory Minute Volume

If so, give us your best non-techie explanation as many divers confuse the meaning of these.

Also often confusing are the reasons why a diver wants to know these. So second part to this "pop qiuz," tell us WHY a diver concerns themselve with SAC or RMV. What is the applied use of the working knowledge of these terms/calculations?

[BubbleBoy]

I think the units of measurement are different.

RMV units are volume per time, e.g. cubic feet / minute. It is the number of cubic feet of air (gas) inhaled or exhaled through your lungs per minute. Also equal to your volume per breath times your respiratory rate.

SAC units are pressure per time, e.g., psi / minute. This one is dependent on, and inversely related to the volume of the container you are consuming from. Anyone will have a higher SAC with a small tank than with a large one.

There is a mathematical relationship between RMV and SAC which can also be derived from unit analysis. But it's 12:30 at night, I'm tired and I don't feel like doing it.

I think the main use for both values is estimating the amount of time you have at any depth in water before you run out of gas in your tank and drown. You definitely don't want to slip a decimal point on that calc.

[gcbryan]

SAC must be calculated in order to figure out RMV. SAC is tank dependent and is measure in PSI/Minute. This is actually valuable to know while underwater since your SPG measures in PSI. It's good to know for instance that your usual rate is 15 psi/minutes.

RMV is measured in cu ft/minute and is useful for planning gas requirements before the dive as in .5 cu ft/minute at the surface which then needs to be corrected for depth (ata's).

[BubbleBoy]

I see this thread really sparked a lot of attention.

So, Dive Girl, did we pass? How well did we score?

[Dive_Girl]

I think the lack of activity is because a lot of people have heard the terms, but don't really know: what they mean; when they are used; how to apply them to their diving, if even necessary, or they think they are soley technical terms (which actually is how I was introduced to RMV.)

I had heard recreational divers toss around SAC rate, but after I learned about RMV, I found out that a lot of divers use incorrect terms or calculations. It's kind of like when I a diver says, "you'd better check your tides before you go diving" when they likley need to check the currents!

The explanations already posted are great. Let's see how others will ring in.

[AliKat]

There are probably many on the board like me - waiting to see what the answer was because I'm clueless

I've heard of SAC, but all I really know about it is that my computer calculates it for me every dive. I've never even heard of RMV before.

If SAC = psi/minute does this mean that when you say 15psi/minute that is the same as a SAC of 15?

[Capn Jack]

SAC is the first thing to compute - breathe on your tank at a fixed depth for a given period of time, convert to surface pressure and you know the RATE in terms of PSI/minute.

Then, knowing the size of your tank, you can compute the volume of gas consumed.

Both have to be converted to planned depth to have significance whether you're planning a tec dive and figuring your volume requirements, or trying to figure whether you'll hit NDL or turn pressure first during a rec dive.

Most rec divers I've met casually have no idea what their SAC is - usually they'll have a rough idea of how long an AL-80 will last on the typical dive they make.

I think it's a good thing to test now and then. It also helps when you're pairing up with a buddy you don't know. If you know your SAC you at least have a figure of merit for comparison.

You may want to play with it, try measuring it with exertion. It also will change dramatically (worse) when you are stressed or cold.

Dive_Girl - are you going to post formulas and methods you like?

Edited by Capn Jack, 09 May 2006 - 05:32 PM.

[Dive_Girl]

yep - I'll be posting formulas soon unless one of you smarties beats me to it (which is really my hope so I don't have to pull out my awful formula sheet from class! )

[BubbleBoy]

Let's see how others will ring in.

O.K. then, I'll try to add to my answer for a better grade.


I think surface RMV is the root number to start with. From that you can calculate SAC as follows:

SAC = RMV * Full Tank Pressure / Full Tank Volume Rating

At depth (measured in feet) your air consumption rate is equal to:

AC = SAC * (1 + Depth/33).

Example:

If your surface RMV is 0.8 cu ft / minute, you have an 80 cu ft tank that is full at 3000 psi,
and you are diving at 50 ft :

SAC = 0.8 * 3000 / 80 = 30 psi per minute.

Consumption Rate at 50ft = 30 * (1+50/33) = 75 psi per minute (approx.)

Keep in mind, even though your surface RMV is nearly constant when idle, your actual surface RMV increases as your activity and respiratory rate increases. Both are a lot higher for most people after running the 440 than when watching the Sunday night movie of the week starring Marideth Baxter Bernie, i.e., falling asleep.

So if we add just one more factor, call it R, the ratio of your actual respiratory rate to your idle rate then:


Air Consumption at Depth = RMV * R * (Full Tank Pressure / Full Tank Volume) * (1 + Depth/33)


Which leaves just one more problem; how do you remember this when you are Narced at 120ft? If you try to cheat and write it on your palm, it just washes off.

[Capn Jack]

yep - I'll be posting formulas soon unless one of you smarties beats me to it (which is really my hope so I don't have to pull out my awful formula sheet from class! )

Here's NOAA's version...

Attached Files

•  AirRqmtFormulas.pdf   7.31KB   14 downloads

[Dive_Girl]