SAC Rates

[Peter Guy]

I really would like to know if there is a reasonable need for knowing a SAC rate, where not knowing it would be less safe.

OK, real life situation -- There is a dive at our "local mudhole" called the boundary line dive. You descend in shallow water and follow a rope on the bottom which is the "boundary line" for a "no-diver zone" around a fishing/water taxi pier and go from "Cove 3 to Cove 2." This can be a very fun dive -- starts shallow, goes deepish (100' or so) and ends shallow.

An "average diver" should take about 10 minutes following the line to get to 100', stay at depth about 15 minutes and then take 10-15 minutes to come back up to the shallows.

While NOT an "overhead" environment, because of the "no-diver zone" (and the real possibility of getting hit by a boat or snagged by a fisherman), we all believe there is a "virtual overhead" especially in the deep area of the dive.

Which do you think would be a safer way to plan this dive? Have an idea of your average (normal) air consumption rate so you can estimate how much gas you need for your times at depth OR just wing it?

BTW, a couple of years ago an "average diver" and her buddy did this dive without any gas planning (I've heard the buddy didn't believe in it), she went OOA and went on his octo and started to the surface. At some point she evidently panicked and bolted, embolized and died. Now she didn't die because of not knowing her SAC rate -- BUT, in the chain of events, HAD she known her SAC rate, HAD she done a "quick and dirty" calculation of expected air consumption, she would have known she didn't have enough gas in her tank to do the dive. IF she had done the calculations and then concluded the dive wasn't feasible, my guess is she'd be diving today.

As I asked in a talk I gave a while ago:

HOW MUCH GAS DO YOU NEED?

 

[Teamcasa]

I got a few sharply worded PMs for starting this poll: http://www.scubaboard.com/forums/advanced-scuba-discussions/264147-how-small-your-sac.html

It was felt that the title implied that a big SAC meant a poor diver, and would encourage skip breathing. The sad truth was it was just a slightly lame attempt at adult humour, but the point was taken.

Sadly, whenever a scale can be applied to something, people will compete for a better spot on the list.

There is zero chance, even if I dive 3 times a day for the rest of my life, that my SAC will ever be as good as a 100lb girl with lungs no bigger than potatoes. OTOH, she will never be able to carry a 300lb man on her shoulders for 100 yards. So, I'm good with that.

I just want to see my SAC improve, or at least remain steady.

 

[Sinbad the Diver]

Interesting that I have not seen it mentioned, but Bob "NW Gratefull Diver" wrote what I consider an excellent article on gas management and it includes an outline of how to calculate SAC and how it is used in gas planning. I used it as the basis to build a spreadsheet to perform gas planning. The exercise of building the spreadsheet was in my mind a great learning exercise.

If you are interested, a quick search will pull it up (I have the link, but I prefer not to post other's work without their OK).

 

[Rhone Man]

There is zero chance, even if I dive 3 times a day for the rest of my life, that my SAC will ever be as good as a 100lb girl with lungs no bigger than potatoes.

I used to conduct careful examination of the chests of 100lb girls to try and find the secret of their lower SAC rates, but my wife believed that I had some other purpose in mind and so made me stop. Sadly, my research will never now be completed.

 

[Teamcasa]

...BTW, a couple of years ago an "average diver" and her buddy did this dive without any gas planning (I've heard the buddy didn't believe in it), she went OOA and went on his octo and started to the surface. At some point she evidently panicked and bolted, embolized and died. Now she didn't die because of not knowing her SAC rate -- BUT, in the chain of events, HAD she known her SAC rate, HAD she done a "quick and dirty" calculation of expected air consumption, she would have known she didn't have enough gas in her tank to do the dive. IF she had done the calculations and then concluded the dive wasn't feasible, my guess is she'd be diving today.

Sad and good advice Peter.

OTOH, I suspect that her not paying attention to her SPG had a little something to do with her going OOA as well.

 

[BobinNC]

Interesting that I have not seen it mentioned, but Bob "NW Gratefull Diver" wrote what I consider an excellent article on gas management and it includes an outline of how to calculate SAC and how it is used in gas planning. I used it as the basis to build a spreadsheet to perform gas planning. The exercise of building the spreadsheet was in my mind a great learning exercise.

If you are interested, a quick search will pull it up (I have the link, but I prefer not to post other's work without their OK).

I have broken down the spread sheet that Kraken sent and looked up other equations for the psi/min as well just so I have an understanding what they mean and not just plugging in numbers with no understanding of how to get to the SAC rate (with the help of Sam1's post in this thread as well). You can really get some good nuggets of information of this board sometimes.

 

[SailNaked]

OK, real life situation -- There is a dive at our "local mudhole" called the boundary line dive. You descend in shallow water and follow a rope on the bottom which is the "boundary line" for a "no-diver zone" around a fishing/water taxi pier and go from "Cove 3 to Cove 2." This can be a very fun dive -- starts shallow, goes deepish (100' or so) and ends shallow.

...
Which do you think would be a safer way to plan this dive? Have an idea of your average (normal) air consumption rate so you can estimate how much gas you need for your times at depth OR just wing it?

I would use the rule of thirds and after entering the "overhead" for 1/3 of my available gas would turn around and go back. guessing how much gas I would use, from a very fuzzy calculation of SAC would not be my preference on this or any dive. there is no excuse for OOA unless you get entangled or lost in an overhead or have an equipment failure(the last 1/3 of your buddies air).

as far as the usefulness of knowing how much air to bring, I think most divers would know if they need a single 80, double 80s or double 120s with 2 spares for a planned dive. if in doubt bring more air.

Now I think I have hijacked the thread, it is my opinion and it has nothing whatsoever to do with how to calculate a SAC rate, but I think that has been answered.

 

[mrlipis]

While I don't think there is a particular need to know, I have watched my Sac rate improve and that is encouraging , yet at the same time I have been able to see the effects of tasking on my sac rate also. When I am taking pictures I am not as efficient. With that said, I am using a VT3 computer and simply download the information on my laptop and WAY LAH, I have my sac rates. It would seem difficult to calculate a Sac rate without doing a square profile just as it is difficult to calculate a pressure group using only tables during a sawtooth dive profile.

 

[TSandM]

I would use the rule of thirds and after entering the "overhead" for 1/3 of my available gas would turn around and go back. guessing how much gas I would use, from a very fuzzy calculation of SAC would not be my preference on this or any dive. there is no excuse for OOA unless you get entangled or lost in an overhead or have an equipment failure(the last 1/3 of your buddies air).

You can certainly use that approach.

But if you are planning a traverse, it seems to me that it would be nice to know if you can complete it, or whether you just need to bring more gas. These folks could have turned around and gone back, if they had watched their gas, and would have been safe but disappointed. On the other hand, had they done a quick calculation and taken a larger tank, they would have been safe and happy at the end of the dive. Safe and happy wins, in my book.

 

[peterbj7]

your SAC is not a single value. -resting, easy swimming, hard swimming, warm water, cold water all make a big difference

An enormous difference. Working hard by itself can increase gas consumption by a factor of 7!