What's a good SAC

[RainPilot]

As I'm better looking.

Whatever you need to tell yourself to get through the day......

Once the stress and exertion got my "CO2 engine" revving, it took quite a while for my body to calm down. Just trying to zen-out wasn't enough to cool my jets.

You bring up a very good point. One of our few sites here for doing the AOW Deep dives easily is a wreck called Inchcape 1. There is usually a fair current at the surface and since it is a popular site, it is common for a few boats to be daisy chained together on the descent line.

What that ends up meaning is that once you splash, its an effort to get to the mermaid lines and then pull hand over hand till you get to the front of the queue and to the descent line. Add in a drysuit and twins etc and I am usually breathing a little hard by the time its descent time. In the past I used to just go down on reaching the buoy, after some advice I now hang loosely on the buoy for a few minutes till I am breathing normally (or close to) before I descend. The difference in my gas consumption is huge if I take advantage of the big gas tank in the sky to flush the CO2 rather than my scuba tanks.

 

[Diving Dubai]

The funny part is, I actually do track my consumption fairly obsessively because I started with such high consumption, and it amuses me to see my progress. I quickly learned, though, that is was a pointless fool's game to worry about how I compared to others. That doesn't change anything.

I agree. I used to track my consumption rates obsessively, as a measure of my improvement when I was a newer diver, now not so much. It is what it is. As long as I have enough gas to complete the dive I wish to to, keeping a reserve all is good.

I often tell other divers, that the easiest way to reduce your air consumption, is not to worry about it. Stressing that you'll be the first diver to signal that your at low pressure/ agreed minimum, naturally causes increased consumption.

@RainPilot post above is also a good point. get yourself relaxed before you descend, take a moment or two and don't allow yourself to be rushed. If you're stressed, agitated or unsettled at the start of the dive, this will have a direct impact.

Also a certain level of fitness is useful - but I don't believe that above a certain threshold it has much bearng. I would be breathless running a short distance, yet kicking hard into a current I'm okay - mainly because I mentally control my breathing - and won't allow my breathing to run away with itself. Some of that has to do with being comfortable with the situation. I'm used to currents, and thus my body doesn't get stressed or agitated.

Remember we all have the "fight or flight" reflex, where if triggered subconsciously, our diaphragms drop increasing our lung volume, and then adrenaline kicks in as the second part which increases heart rate, blood pressure and expands the air passages in your lungs. So being comfortable and relaxed in a situation plays a huge part.

 

[Diving Dubai]

Nothing wrong with that - why would anyone want to dive with more weight than needed? I've steadily decreased my weighting as I've gained more experience. The side benefit is less weight also helps reduce my RMV so I can get a little more dive time out of a tank.

I don't disagree, and I used to be obsessive about my weighting, and happily track the reductions each month, changing from a SS plate one season to Ali, then plateless.

Nowadays not so much. When teaching I'm always over, so I have enough to pass on if needed as well as keeping extra, so if a student loses buoyancy, I can simply dump gas and bring them down. Obviously I prefer not to be overweight, but I can't say it contributes significantly to my air consumption and I can maintain the same levels of trim and position in teh water column as I would if ideally weighted (it just takes a little more concentration) . My higher consumption whilst teaching is because like others, you're always on high alert and not chilled and relaxed with students.

Again, the fight or flight reflex comes into play, if people are worrying about or concentration on buoyancy they're not relaxed, similar effect can be found from a bit of task loading, whether it's launching a dsmb, or even a mask flood and clear. You're out of "the zone"

When I teach OW course, I deliberately carry out the skills through out the dives while the students are swimming, (which is more realistic anyway) rather than corral them to do the skills. This is because once they're swimming and looking at the pretty fishes they've forgotten that I've briefed a mask flood for instance and are more relaxed, if I were to coral them they know it's "skill time" and become tense, perhaps lose some buoyancy, get taks loaded and it all becomes a cluster.

Instead, they carry out the skills while relaxed and are generally pretty good. I rarely have a student lose buoyancy at 15' carrying out a mask remove and replace (I always leave mask skills to the end of the dive)

 

[MargaritaMike]

I Love SB! Ask an innocuous question and get some GREAT discussion! Some germane and some not so much, but it's ALL good!

Cheers -

p.s. I need 3 more "likes" to be at 75%. Not important, but I'm a "numbers guy". - M²

 

[Miyaru]

A good SAC is anything above zero.

A bad SAC is zero, regardless of how you express it, in cft/min or psi/min (for the 3 countries using the impaired system) or bar/min, l/min (for the rest of the world using the metric system), because in that case you're not breathing anymore.

I'm surprised the topic started with SAC / RMV. If you express it in a pressure difference per minute, it's still useless. My rate can be 7psi/min on dive 1 and 14psi/min on dive 2, and I'm breathing at the same rate on both dives.
Besides, the term Surface Air Consumption isn't defined anywhere as an expression of pressure difference per minute. The pressure at the surface (sealevel) is always 1 bar (plus or minus the tiny weather influence), so you're consuming air at 1bar every minute. The only interesting factor is the amount of air you breathed during that minute.

So SAC and RMV are the same. Expressed in volume per minute. Written, both are three characters long, but in conversations it's just easier to use SAC. Those who come up with pressure-per-minute, are just trying to confuse you.

 

[markmud]

FWIW, I've been doing a lot more cardio since that happened a few years ago, and I believe my pulse and respiration rate return to normal after exertion much faster these days.

SAC vs. RMV

RMV vs. SAC: What’s the Difference? - Cave Diver Harry

Hi dberry,

Cardio is a huge cross training tool for diving.

I had to tow my wife into a current a couple of months ago. She has some heart issues that we need to manage. Better safe than sorry, so I help her when strong swimming is required. She gave me the signal, and the former tugboat skipper took over.

I had to force myself to breathe. My lungs were in such good shape, I did not need to breathe heavily, at all. I tried breathing more as I was worried about CO2 build-up. I guess my alveoli were working at a high level. The PSI remaining in my tank was no different than most other dives.​

You need to understand your consumption so you can plan your dive properly with the right size tank and realistic profile, but beyond that it is what it is and who really cares?

gcarter, yeah I agree with you.

The two articles listed in other posts do give different definitions and descriptions. I have found more articles that contradict the posted articles with an easy search. This article WHAT IS SAC AND RMV ? - Scuba Master has definitions that are contrary to what I was taught. The Cave Diver Harry article is more inline with what I was taught.

As my instructor said, after he rolled his eyes regarding this topic, this exercise is to determine your gas consumption while performing moderate work u/w (swimming moderately at a constant depth for a predetermined time) so we can form a baseline gas usage for you. A baseline used in planning dives.

A "SAC" that gives you PSI per minute on a dive where you photograph the same seahorse within 15 feet of the mooring line in benign conditions will not help you create a baseline for planning dives that end up requiring some work.

An RMV (or SAC, depending on whose definition you use) of X-PSI in some undetermined tank at a certain depth isn't helpful if you are planning a dive that is 99'/30m deeper.

A SAC/RMV of liters/cu ft per minute at the surface is a datum point that is useful. Use the principle of Boyles Law to convert your SAC/RMV to your planned depth, and now you can calculate gas requirements for the dive. For me, that number is .5 cf/14 liters per minute at the surface, using a controlled test to compile the raw data.
As some posters have opined, this can become self flagellation. Which is OK. This is an avocation.

I am a Shearwater fan-boy. I have dived with Shearwater people; they are my friends. Their "SAC" rate window, is based on PSI per minute used at whatever depth you are at. It is a fun data point, and it can be helpful while swimming into a current or while performing some other form of work, but it is not an indication of SAC/RMV, IMHO.

I am enjoying this thread,
markm

 

[Diving Dubai]

Even though I know my SAC - I prefer to deal with nice easy numbers.

Thus I plan at 15l/m (0.52) becuase I use 15L cylinders (generally) thus 15/m = 1 bar/min and so underwater maths is easy. If I'm using an AL80 then its' roughly 1.25 bar/m so I just add 25% to my calcs on gas needed.

Accuracy for me isn't significant as I continue to use the "P" for plenty in my gas considerations so gas volume for the required dive time isn't an issue but can re plan on the fly - say a Mola Mola pops up and I want to figure out how much deco I can take and still be within my personal reserve limits

On a typical guided vacation dive with an Al80 at 30 mins I need to have more than 110 bar to be sure to have a 50/60 min dive - I dive to that

 

[Bob DBF]

RMV is a medical term and has a precise definition.

SAC is a scuba term and is more dependant on popular usage in the scuba industry, rather than its definition in the industry, which is made to show air consumption by psi.

When faced with the different usage of SAC, I'm of the "whatever" school of thought, and pick up the use by context, and go on with the conversation, unless the diver is correcting me.


Bob

 

[markmud]

RMV is a medical term and has a precise definition.

Thanks Bob,

I did not know that.
respiratory minute volume

markm

 

[FreeFlyFreak]

Worst Ever 0.95 cuft/min
Best Ever 0.36 cuft/min

Average dive Im around
0.50-0.60 in 80F water
0.60-0.70 in 55F water
Depending on exertion.

Worst ever was cold water and it turned out I was sick, running a 101 fever when I got home.
Best ever was warm, just lying on the bottom watching the Manta's feed.

FFF