SAC vs Surface RMV

[Blackwood]

I call it a SAC rate, but under the above definition I suppose "RMV" is the term used for that.

Eh, screw that RPV crap. Surface Air Consumption Rate. It perfectly describes it.

PSI, Liters, Bar, CF, whatever. It's all SAC.

Unless it's SEANxC or STxC or SHydrelioxC rate.

 

[PfcAJ]

So SDI takes your resting surface consumption, and multiplies it times two if you go below 60 feet? That puzzles me, because 60 feet is 3 ATA, so unless you are shore diving and bringing your average depth up closer to 30, a multiplier of 2 isn't even enough to compensate for depth, let alone activity.

It is not horrendously difficult to do a bunch of dives, figure out your average RMV, and then compare that to the average depth of the proposed dive for planning purposes. We learned to do this in Fundies, and we could all do it by the end of the three day class. We did use "scuba math", which involves some simplification and some rounding, but it's good enough for government work. I don't see any good purpose in sticking in a bunch of relatively arbitrary multipliers, when the real math isn't that hard.

My understanding was that the formula works out to .5(ata+1)x(multiplier for whatever)

So on a 80ft dive in cold, not clear water with current, your .5 sac rate becomes 2.5cuft/min... then you do the depth stuff. Either way, its silly, imo.

 

[Jax]

Wow. Did not figure such a basic question would have such a wide range of understandings.

I haven't seen any "consumption" rate in my training (NAUI, PADI, SDI, & TDI) expressed in psi/min (or bar/min), as that rate is extremely variable.

The one thing I am seeing is the factor used is normalized for the surface, and some dive computers take the temperature into consideration, as well.

Am I the only one amazed that such a basic calculation that is so necessary for gas planning is not standardized?

 

[HenrikBP]

I use Bob's nomenclature and methods for measuring/calculating both. In fact his article is standard reading for me prior to dive trips - just to keep it fresh

Henrik

 

[Jax]

First off, I'm not sure what SDI's version of SAC would be useful for ... a baseline is only any good if it applies to something. Taking measurements while sitting perfectly still on the surface have no real applicability to what goes on while you are diving.

My definitions of SAC and RMV are provided in my gas management article ... which is how I teach them. Essentially, they are two representations of the same value ... expressed in different ways ... and they do not represent a baseline, but rather a range of values that will be dependent on the conditions in which they are being measured.

When we breathe on the surface, we don't have a baseline ... our breathing rate will be dependent on what we are doing ... sitting on the sofa reading a book, walking, running, sleeping ... those will all give you different consumption rates. The same applies underwater. The only thing that truly matters is having some understanding of what the high and low end of your consumption range are, and where various activities will put you on that range.

SAC and RMV both represent that range. The difference is that SAC is expressed in terms of pressure per minute, and is dependent on the cylinder you are using ... while RMV is expressed in terms of volume per minute, and is independent of the cylinder.

RMV is used for dive planning ... because we know the volume of our cylinder.

SAC is used for dive execution ... because we know how much gas we have at any point in the dive by looking at our pressure gauge.

We don't develop an understanding of how different conditions affect our consumption rate by establishing a baseline ... we do so by tracking our consumption over the course of multiple dives, in different conditions, and considering why each is higher or lower than the others. In other words, you look for trends.

Gas consumption ... like so many other aspects of diving ... is not an exact science. There are a lot of different factors that will affect your gas consumption on any given day and any given dive. Understanding what those factors are, and how they affect you, will give you a much more accurate "picture" of how to plan your gas needs than sitting in your living room breathing off a scuba cylinder ...

... Bob (Grateful Diver)

Bob, your explanation above and on your site -- is that according to PADI, or is it your interpretation to make it more meaningful for students?

 

[Mike Boswell]

Wow. Did not figure such a basic question would have such a wide range of understandings.

I haven't seen any "consumption" rate in my training (NAUI, PADI, SDI, & TDI) expressed in psi/min (or bar/min), as that rate is extremely variable.

The one thing I am seeing is the factor used is normalized for the surface, and some dive computers take the temperature into consideration, as well.

Am I the only one amazed that such a basic calculation that is so necessary for gas planning is not standardized?[/QUOTE]

No you aren't the only one - I'm flabbergasted! I first learned about gas planning and SAC from Bob's excellent article (I love you, Bob!), and Suunto agrees with that as well, except they express SAC in terms of volume instead of pressure, and they correct for temperature which really is necessary for total accuracy.

 

[ZenDiver.3D]

Good luck with your question. After some of the ensuing discussion from that I decided it was easier to just do calcs my way and let everyone else do it theirs.

I agree with you there.
Here is some info from the TDI manual:

The manual says.... " Enter the SAC rate test, also known as the RMV."...... It states essential elements as
"1. measure cylinder pressure.
2. use a controlled dive environment.
3. set equipment as you typically use it.
Gas Usage
1. Determine cylinder capacity first
2. With good bottom timer or watch that shows seconds, enter the water and relax at constant depth. An excellent depth to pick is 9 meters.
3.Wait 5 minutes or so, for tank to cool down.
6. mark time and cylinder pressure
7. pass the time and wait at least 5 minutes. Then mark exact time and cylinder pressure.

8. Next pick a course to swim that will keep you at a constant depth begin light to moderate kicking for about 5 minutes.
9. Mark time and pressure.

10. Find something to allow you to hold on and push against and mark time.
11. Begin kicking hard.
12. after 5 minutes, mark time and pressure.

13. calculate each set of data to find the rest, moderate, and hard usage consumption rates."

It then goes on to note that these are benchmarks to like conditions. The benchmarks should be redone periodically. It also says they will vary form day to day, dive to dive, plan to plan, but they give a point of reference for planning purposes.
There. That's what TDI Advanced Nitrox text says- condensed and a bit paraphrased or it would take three pages......

 

[Jax]

I agree with you there.
Here is some info from the TDI manual:

<snipped>

It then goes on to note that these are benchmarks to like conditions. The benchmarks should be redone periodically. It also says they will vary form day to day, dive to dive, plan to plan, but they give a point of reference for planning purposes.
There. That's what TDI Advanced Nitrox text says- condensed and a bit paraphrased or it would take three pages......

Hey, thanks! I picked up the old TDI Advanced Nitrox text but haven't gone through it. The present Advanced Nitrox text says to do what I put in the OP.

 

[Scott L]

IANTD's Tek Lite manual does a very nice job of presenting the material in much the same manner as Bob...

 

[Jax]

IANTD's Tek Lite manual does a very nice job of presenting the material in much the same manner as Bob...

As a matter of fact, I've contacted them about purchasing the manual . . . :blinking:


I'm toying with the idea of doing a contrast analysis paper and submitting it to a dive magazine.