Simple SAC Calculation

[boomx5]

Okay, so grab your tank and regulator, get on the treadmill on high, and watch a porn flick . . . will that work? :blinking:

That may take a set of doubles and a loooooonnngggg decompression!

 

[AbyssalPlains]

Guys, what does SAC stand for? SURFACE Air Consumption, exactly. OF COURSE this will be different from the air consumption rate at a given depth, under given circumstances.

 

[muddiver]

Sure, then double it 'cause your working, double it again for excitement, adjust it for depth ... that'll be close, but I wouldn't bet my life on it.

Thal, couldn't you say something constructive, like "back caculating your gas use at depth to the surface is RMV, not SAC".

The SDI manual is correct in stating that Surface Air Consumption is measured at rest at one atmosphere ambient pressure.

Reread the section and pay close attention to RMV. RMV includes volume of gas breathed due to stresses associated with the diving activity, including cold water, swim aginst current, whare am I?, etc. Thus RMV will vary, where SAC should not very and can be used as a baseline for planning purposes.

As stated, don't realy on SAC alone to estimate your gas supply. Pay attention to your history of gas use on dives and adjust accordingly as AfteDark stated.

 

[MXGratefulDiver]

Guys, what does SAC stand for? SURFACE Air Consumption, exactly. OF COURSE this will be different from the air consumption rate at a given depth, under given circumstances.

Hmmm ... OK, so you're calculating it at the surface. Here's an analogy for you to ponder.

Go sit on the couch, like the book recommends, for 10 minutes. Measure the amount of air you consume while sitting there.

Now strap on that tank and go for a walk for 10 minutes. Measure the amount of air you consume while you're walking.

Now hop on the stationary bike and pedal hard for 10 minutes. Measure the amount of air you consume while pedaling.

Do you think those amounts will be the same? Of course not. Gas consumption measurements are best done under the conditions where you will be using them ... because your actual consumption rate is going to vary with conditions. It's no different than what happens to your MPG when you drive your car under different conditions.

The key thing is that you should be doing these measurements under different conditions because your SAC rate isn't a number ... it's a range ... and your actual consumption under any given condition will fall somewhere in that range.

What I do with my students is have them take two sets of measurements while diving. One set is taken while kicking very hard ... to simulate working against a stiff current. A second set is taken while swimming slowly and looking around ... to simulate normal, currentless diving conditions. The first set is called "working SAC" and the second is called "resting SAC". Your actual consumption on any given dive will fall somewhere in between those two values ... just as actual miles per gallon in your car will fall somewhere between the familiar "City miles" and "Highway miles" numbers you see posted for your particular type of vehicle.

The real key to looking at your consumption rate is to remember that it's just one piece of information you need in order to figure out how much gas you will need for a given dive. It works exactly like the MPG ratings in the analogy above. You need more information than that. You need to know how much gas is actually in your "gas tank" and you need to have some idea what conditions to expect ... to be able to figure out where in the continuum between "City miles" and "Highway miles" your consumption rate will actually fall.

I am surprised that a class on solo diving would give you such advice ... it's worse than useless, it's misleading and could result in you relying on information that's not reliable.

My advice is to take before and after readings over the course of several real dives, and use the simple arithmetic needed to calculate your surface consumption for each of those dives. You will notice that there will be slight variations ... which will be due to differences in diving conditions and your own personal physical condition on each given dive. Ask yourself what you experienced on each dive that could have caused the rate to go up or down slightly, and look for trends. By doing so you will develop a much better idea of how to accurately estimate the amount of gas you need for any given dive.

For more information, see this article ...

... Bob (Grateful Diver)

 

[Rhone Man]

Or, you can get an air integrated computer that calculates your SAC with great precision on each and every dive and note it in your log, preferably with a note about anything that would have adversely affected your SAC on a particular dive.

 

[Jim Lapenta]

SAC rates vary depending on what your doing and at what depth your doing it at. If you're in still water, neutrally buoyed just cruisin' along relaxed, why would it be any different than sitting in a chair watching the news for example. I might use MORE air watching the news You need a baseline to work from on the surface in a chair is as good as any. Just don't use it to plan a deep dive in a strong current. You'll need to adjust for each dive. The more you dive and pay attention to your SAC the better your adjustments will become. I timed how long it takes for me to use 500psi of gas under different conditions. I use that base line to plan gas consumption for my dives. Even that needs adjusting if your fitness changes. My SAC at 55 years old is higher than when I started at 15.

That's weird. At nearly 50 ( 2 more months) mine is lower than it was at 15. Oh wait, wrong sac!

 

[muddiver]

The key thing is that you should be doing these measurements under different conditions because your SAC rate isn't a number ... it's a range ... and your actual consumption under any given condition will fall somewhere in that range.

What I do with my students is have them take two sets of measurements while diving. One set is taken while kicking very hard ... to simulate working against a stiff current. A second set is taken while swimming slowly and looking around ... to simulate normal, currentless diving conditions. The first set is called "working SAC" and the second is called "resting SAC". Your actual consumption on any given dive will fall somewhere in between those two values ... just as actual miles per gallon in your car will fall somewhere between the familiar "City miles" and "Highway miles" numbers you see posted for your particular type of vehicle.

The real key to looking at your consumption rate is to remember that it's just one piece of information you need in order to figure out how much gas you will need for a given dive.

My advice is to take before and after readings over the course of several real dives, and use the simple arithmetic needed to calculate your surface consumption for each of those dives. You will notice that there will be slight variations ... which will be due to differences in diving conditions and your own personal physical condition on each given dive. Ask yourself what you experienced on each dive that could have caused the rate to go up or down slightly, and look for trends. By doing so you will develop a much better idea of how to accurately estimate the amount of gas you need for any given dive.
... Bob (Grateful Diver)

Then one resting SAC and ones leasurely dive SAC and anything below a hard working SAC is usless because one must plan for the worst case scenario of being stressed out and paniced at the end of a dive. Otherwise one will always have less gas than necessary to make a "safe" dive, correct?

 

[MXGratefulDiver]

Then one resting SAC and ones leasurely dive SAC and anything below a hard working SAC is usless because one must plan for the worst case scenario of being stressed out and paniced at the end of a dive. Otherwise one will always have less gas than necessary to make a "safe" dive, correct?

Not necessarily. For open-water diving, you consider worst case to be your "rock-bottom", which is where you factor in not only your own working SAC, but that of your dive buddy ... which is why rock-bottom calcs always appear to be almost ridiculously high (and why it ain't a good idea to do a deep dive on an AL80). The nice thing about rock-bottom is that as you get close to it, all you gotta do to extend your dive is get shallower ... since your rock bottom assumes worst case of sharing air while stressed, coming up 15 or 20 feet will give you a new ... much lower one. If you stair-step your profile, you'll never reach it.

On the other hand, you calculate your turn pressure based on your anticipated conditions ... which may be your resting SAC if conditions are fairly benign, but you'd want a bit of a "fudge factor" if you're dealing with current, surge, or any other condition that's likely to cause your breathing rate to be higher than normal.

For newer divers, using resting SAC for calculating turn pressure is quite adequate ... because you're almost always going to be in conditions that aren't too deep, or aren't too challenging.

The key thing for most occasional divers isn't to bury yourself in math, but to understand the concepts sufficiently to make good decisions. You have a reserve for a reason. You have a buddy for a reason. Those right there give you quite a lot of "fudge factor". What gets people in trouble is just strapping on a tank and heading down on a dive without a thought or a clue how much gas they need, or how conditions will affect their air consumption ... and suddenly they find themselves at 90 feet with 300 psi left in their tank. Unfortunately, if you're that low, chances are your buddy hasn't got a lot in reserve to share either.

So it's worth thinking about all that before the dive ... asking yourself "what kind of dive do I want to do and do I have enough gas to do it" and to do that you not only have to know your SAC rate, but you have to understand enough about how to apply it to understand why you should ...

... Bob (Grateful Diver)