SAC Rates

[emoreira]

My SSI book has the spot for SAC in psi/min. The equation given for cf/min includes volume so that rate should be the same in any given size tank, correct?

That's in the deep diving SSI book. The calculations are done in psi/min considering an AL80 tank.
In the Luxfer web site (Luxfer: SCUBA Cylinders: Specifications) it is mentioned that the Al80 tank has 77.4 cuft of capacity.
If you consider your SAC in cuft/min you will independize your calculation of the tank size. If you are using a tank of your own and will always use the same tank, you could go on using SAC in psi/min.

 

[mrlipis]

Not sure what is considered a really good SAC rate. I am happy to be under .5 but would imagine there are divers that are under .4, especially women.

 

[DA Aquamaster]

I have a real world actively swimming SAC around .6 that drops to .4 or below during deco stops and that is on the low normal side for an experienced real world dry suited cave diver, who does not engage in skip breathing.

You can reduce a SAC a lot more by skip breathing, but the reduced ventilation means you retain a lot more CO2 and elevated CO2 levels potentiate both nitrogen narcosis and oxygen toxicity. I have learned over the years that artifically reducing the respiration rate (beyond a pause of a few seconds duration at the top of the inhalation) to further reduce SAC is just not worth the downsides, particularly in deep or technical diving. If I need more gas to do a particular dive, I'll take bigger tanks or a stage.

People tend to get too hung up on the number itself. It is essential as a gas planning tool, but beyond that it means very little once you get past the raw newby diver phase.

 

[The Kraken]

Dear Readers:

Do not use artificial breathing practices just in order to lower your SAC rate.

It's dangerous.

It could hurt you.

It's stupid.

Keep in mind, a SAC rate is like feet. If ya got a reeeeeeeeeeeeeeeal big body, then you probably have reeeeeeeeeeeeeal big feet.

If ya go an itsy bitty body, then you probably have itsy bitty feet.

And a low SAC rate ain't that big of a deal anywhoozit!!! Just get a bigger tank.

Be safe.

the K

 

[SailNaked]

I do not see the value of calculating a SAC rate. I know what people say they would use them for, however do people really use it and why and how? (not starting snot). 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.

 

[Sinbad the Diver]

I do not see the value of calculating a SAC rate. I know what people say they would use them for, however do people really use it and why and how?

If you are a recreational diver and your dive plan consists of drop to a depth of X, go in direction Y until you get to a defined psi, turn around and go back to the exit point and surface, then SAC is not a critical part of your planning. Assuming you planned an safe turn around preasure and you follow the plan, the worst thing that happens is you reach your turn around point and go back after a very short dive.

If you are doing more advanced dives and need to determine how much gas you need or whether or not a dive is possible given the gas available to you, it's a critical calculation.

 

[Rhone Man]

What Sinbad said.

 

[SparticleBrane]

I do not see the value of calculating a SAC rate. I know what people say they would use them for, however do people really use it and why and how? (not starting snot). 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.

It allows us to plan our dives more precisely. It can also be a good indication of stress or nervousness on dives if we are breathing too much (or the reverse, if we are extra relaxed).

 

[Teamcasa]

...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.

If you are a recreational diver and your dive plan consists of drop to a depth of X, go in direction Y until you get to a defined psi, turn around and go back to the exit point and surface,

For the typical recreational diver, calculating your SAC is really more of a tool you can use to track your diving improvement than it is a dive planing tool.

For most new divers 25-45 minutes at 30'-70' is the typical dive anyway. Most new divers pay attention to their SPG's and tend to surface with plenty of air in reserve. It's only after their confidence improves that they tend to push the limits of air reserves.

My lovely bride's SAC rate is better than mine but that just means I need a bigger tank to insure equal dive times.

 

[Rhone Man]

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.