Question Determining SAC Rate

[gbirch]

No, and I apologize for not being as clear.
I quoted your post as it makes a distinction from SAC and RMV, but I was commenting about it in more general sense, see examples of threads posted discussing definitions of SAC vs RMV.

And we have seen here on the board and elsewhere where people talk about SAC being psi/min at the surface and RMV being volume/min at depth.

Sometime somewhere someone has/is making it seem important that we have a measurement distinction/definition of a surface consumption and consumption at depth. To make matters worse, there's this psi/min vs volume/min.

PSI/BAR needs to be used at some point in these calculations, because that's how we measure the contents of our cylinders, but because it is cylinder size dependent, we convert it to volume, which is a more convenient straight forward way of planning our dives.

Having this psi/min way of measuring consumption at any ATA just adds unnecessarily confusion.

Thanks for the clarification. You may not hear this often on Scubaboard... but I think that we agree

Except, perhaps for "...how stupid I think the discussion is about using pressure/min". Since psi drop is the only thing that you can easily measure during a dive, knowing your anticipated SAC for a particular dive does, theoretically, allow you to match it against what your computer is telling you your SAC rate is at any point in the dive (Assuming your computer has this function - Shearwater has). I said this "theoretically" because, do I actually do this - Nah! I've done enough dives (and recorded my SAC after every dive) so that I pretty much know when my air consumption is where I expect it to be.

A word of advice to the Original Poster, and something that I tell ALL my students to do, is to "gameify" your air consumption during every dive, by which I mean take a guess at what your gauge is reading before you check it and see how close you are. Do this every time you check your gauges, and on every dive, and you soon become very aware of your gas consumption in a way more practical way that an artificial 10 min swim.

 

[scubadada]

LOL. You and I are obsessive...we actually download our computers!
Only (as far as I know) only rather recent computers display it on the computer.

Shearwater only gives you SAC, you must multiply by the tank factor to give you RMV.

 

[inquis]

Nearly all modern computers have average depth

Lol, my Suunto Mosquito showed it more than 20 years ago. I consider it ancient.

 

[tursiops]

Shearwater only gives you SAC, you must multiply by the tank factor to give you RMV.

ALL computers only measure pressure/min. It is ALWAYS necessary to put in a tank factor if you want volume/minute.

 

[Pavao]

Maybe you should take into account that many (most?) divers are using AL80s all the time, everywhere they go around the world....psi/min or bars/min is perfectly acceptable. I'm sorry you find pressure/min som confusing. And it is A:LWAYS surface consumption...no matter the names or units.

I also take into account that the divers you refer to largely have no clue what this discussion is about, thus making it a moot point as far as those types of divers are concerned
I’m sorry you misunderstood I’m confused by the use of psi/min, what I said is that brings confusion to others, look at the poll results on SAC vs RMV for proof of that.
Yes, it is ALWAYS surface consumption, you and I agree, but not everyone though, that was my point.

Btw, all those divers around the world using “AL80s” you mentioned, AL80s what? Cf3? Sounds like a volume measurement to me

 

[Pavao]

Thanks for the clarification. You may not hear this often on Scubaboard... but I think that we agree

Except, perhaps for "...how stupid I think the discussion is about using pressure/min". Since psi drop is the only thing that you can easily measure during a dive, knowing your anticipated SAC for a particular dive does, theoretically, allow you to match it against what your computer is telling you your SAC rate is at any point in the dive (Assuming your computer has this function - Shearwater has). I said this "theoretically" because, do I actually do this - Nah! I've done enough dives (and recorded my SAC after every dive) so that I pretty much know when my air consumption is where I expect it to be.

A word of advice to the Original Poster, and something that I tell ALL my students to do, is to "gameify" your air consumption during every dive, by which I mean take a guess at what your gauge is reading before you check it and see how close you are. Do this every time you check your gauges, and on every dive, and you soon become very aware of your gas consumption in a way more practical way that an artificial 10 min swim.

I know, it’s not seeing often. I found myself wrong in another thread and once I realized I was wrong I fully mentioned that on my next post, you could see some sort or celebration by some, in the sense they found some sort of victory, it’s sad really. Reinforces how many just narrow it to who can win the argument, whether the argument in itself is correct or not.

Well as I have mentioned earlier, our instruments measure in pressure, we can’t escape from that, but it doesn’t limit me to think in terms of pressure, I try to use the tank factor as my measurement, it just makes it easier to convert my pressure gauge readings into volume. I don’t use AI so my computers don’t display like yours, and I don’t need to look at my pressure gauge as often as I do my computer, we are in agreement here, we look at it to validate what we expect to see, not to be surprised by it. And the information shown in the computer will be sufficient to give me a pretty good idea what my SPG should show.

Well, I don’t think in terms of what “my anticipate sac will be” for a particular dive, my sac is my sac and it has been derived from numerous dives at various conditions, rec, tech, cave, dry suit, wetsuit… that measurement I use not to compare it to my current dive’s sac, but to plan it, I need to know how much gas will be required for the dive I plan to do, then later I will calculate my turn pressure, turn pressure which would be a number I arrived at from calculating the volume needed and converting back to pressure to know what that volume is represented by my SPG.
When we get to technical diving and where you could be using at least 3 different cylinder sizes in a single dive, trying to keep track of sac rate consumption by your computer AI will be tricky I guess, I find it more useful to use my sac rate for the planning phase.

 

[BoundForElsewhere]

My Suunto Eon Core has an SAC rate display. Are these things accurate or is more like a skinny mirror at Macy's?

 

[inquis]

My Suunto Eon Core has an SAC rate display. Are these things accurate or is more like a skinny mirror at Macy's?

Pretty accurate for what it is: an estimate based on the last few minutes -- as such, it will vary with effort. However if it's higher than normal but effort is NOT higher than normal, it could be an indication that you're agitated or task loaded. Possibly take a moment to mentally reset.

The gas time remaining may be useful if you have that feature, again under the limits of what it is: time at the current depth until the specified reserve is reached (again, using that recent estimate of consumption). Implementations vary, so it may or may not assume an ascent in there. I have no idea what the Eon does, but Shearwater does assume an ascent.

 

[BoundForElsewhere]

Not sure about the ascent but it's a handy little metric to help control my breathing.

 

[sea_ledford]

What a convoluted, and long winded set of calculations. Provided you know your average depth gas consumption can easily be calculated after a series of dives. For me it’s 16Lt/min in U.K. waters and 11Lt/min in the tropics.

Yes, you can certainly do it that way, and it is probably how most people calculate RMV. However, it is very useful to know RMV under different situations: Being calm and still for deco gas planning, normal movement for 95% of dives, and under work/duress for high energy environments or bailout gas planning.

You might go through all three types on a single dive if you have some current to swim against at some point, and then do a nice 5 min safety stop at the end. The differences will get lost using average depth and total gas used. That may not matter at all (or might even be better) for no deco dives, but may be important for gas planning. As a newer diver, it's probably useful into know how much of a difference there is between working and resting.