SAC vs RMV - What is standard?

[Cave Diver]

I may be off here, but how can SAC be dependent on the tank size? If I breathe .53 c.f. per minute on the surface, I will burn 1.06 cubic feet per minute at 33 fsw, 2.12 cubic feet per minute at 66 fsw, etc. regardless of what size tank I am breathing from. Tank size has zero to do with my rate of consumption, or in figuring what that rate is. Am I wrong?

This is the distinction issue that I got into with several people in the thread I referenced above. .53 cfpm is not a SAC rate, it's actually RMV, Respiratory Minute Volume.

Most seem to disagree with me on the differences between SAC and RMV.

 

[TSandM]

I'm sort of surprised that Steve hasn't weighed in here. I can't remember his "official" username as an SDI/TDI rep, but his personal username here is Doppler. You might want to shoot him a PM and let him know about the thread; if I know Steve, he's off diving somewhere, and by the time he gets back to SB, he may miss it.

 

[Jax]

 

[Cave Diver]

I'm sort of surprised that Steve hasn't weighed in here. I can't remember his "official" username as an SDI/TDI rep, but his personal username here is Doppler. You might want to shoot him a PM and let him know about the thread; if I know Steve, he's off diving somewhere, and by the time he gets back to SB, he may miss it.

He was online and responding to threads in this forum earlier today.

 

[Jax]

He was online and responding to threads in this forum earlier today.

Given what little I 'know' about him, he'll be researching it, first. He was very quick to answer a question I had from the Solo Diver Manual.

 

[Steve SDI/TDI HQ]

I posted this in the basic thread, but am finding that definitions and useages are widely differing. So, my question to you, given this:



Q1. What, then, do you expect your diver to use to calculate gas needs?
Q2. Why the difference between SDI and TDI?
Q3. Are there plans to 'normalize' the two?

Thanks!

A1. I would expect a diver to use a constant multiplied by factors that reflect actual conditions on the day of the dive.

The constant should be just that: a fixed volume of gas per minute... let's pick 12 litres. This then can be adjusted to compensate for depth... a dive to 35 metres (ambient pressure of 4.5 bar) which means that 12 litres now becomes 12 x 4.5 = 54 litres per minute. Finally this has to be multiplied by a dive factor, which will be a function of work load, current, thermal stress, and so on. As a default guestimate on a simple dive a dive factor of about 1.5 works ok. But let's say for bloody mindedness that this particular dive is in cold, tannin-stained water with a stiff current approaching 1/2 knot. I'd use a dive factor of 2.25 for those conditions (3.0 being about top of the DF scale). This means that our 54 litres now becomes 54 X 2.25 = 121.5 litres per minute.

In stuff that I write, I use the term SAC as the constant, and RMV as a variable (literally the adjusted consumption when depth and DF have been accounted for).

A2. The difference between various materials published by us is that we have various authors and try as I might, few of them listen to me!

:cool2:

OK, let me rephrase that. There are many terms used in diving that are inconsistent or plain silly (take negative buoyancy for example). And although we have a very comprehensive editorial style sheet, things slip by and old materials have not been updated.

A3. Yes. But it is a slow and frustrating endeavor.

 

[Jax]

In stuff that I write, I use the term SAC as the constant, and RMV as the adjusted consumption when depth and DF have been accounted for.

Thanks, Steve!

Especially for that succinct and clear answer above.