Calculating air consumption SAC / SCR / RMV

[TigerShark]

bonzjivar once bubbled...
I have been cetified for 25 years but have always been pretty good at sucking a tank dry in record time. What can a guy do to improve SAC?

Pls pardon my ignorance....what's SAC??

?? Air Consumption??

 

[TigerShark]

I guess Surface Air Consumption???

 

[DiverBuoy]

TigerShark once bubbled...
I guess Surface Air Consumption???

Here is a quote from an earlier post of mine for details:

SAC Surface Air Consumption - your consumption rate of breathing gas measured in ft3 (that's cubic feet) while at rest (normal breathing) at sea level with only the weight of earth's atmosphere (1 ata) above you.

There is no good or bad SAC rate - it is personal to your physiology and other factors. However, it is true that most people lower (or improve) their SAC over time as they relax and grow in experience. Many external factors contribute - your fitness, anxiety, comfort, work load, water current, water temperature, water depth, regulator tuning, buoyancy and thus weighting. Women and children and smaller people generally have a lower SAC. This allows them to stay down the same amount of time as their larger buddies while diving on a significantly smaller and lighter tank.

Be aware that SAC calculated directly from a dive is effected by how many times you used air from the cylinder to inflate your Dry Suit or BC too.

SAC is a good number to understand early in your diving experience because it can represent changes in your abilities or even aid you in spotting problems, and it certainly is a good number to know when choosing a buddy. It can help you choose equipment especially your choice of cylinder. Also you and your buddy with a similiar SAC will be happier diving together. But alas it is only one factor when choosing a buddy.

Of course most high end computers calculate this for you, when you combine with PC software. But if you want to know how to do this manually (imperial system), it sounds complicated but it really isn't. Here is how:

1. In the water before decending note your starting cylinder pressure.
2. After the dive is over at the surface note 3 things - max depth (average depth is best if you have a dive computer), dive time, and your ending cylinder pressure.

For the first number in the formula subtract end pressure from start pressure. Take this number and divide it by the rated pressure of the cylinder (Cylinder Listings ).

Note this as part I of your formula. This gives you the cubic feet of gas you sucked down on your dive.

For the second number take your depth add 33 then divide the result by 33. Multiply that answer by your dive time. Note this as part II of your formula.

Final Part: Your SAC is part I divided by part II. For a new diver the number is most likely to be between .3 and 1.5 but don't worry at all if your number is higher like 2 or 3, most new divers have a rate that high, and a few divers will always. If your number is lower than .3 check behind your ears for gills (or recheck your calculations.

 

[KeyLargoBrent]

weight yourself properly (most divers are overweighted)...
keep yourself in good physical condition...
streamline yourself and your gear...
don't dive on a full bladder...
move in slow motion...

 

[Scubamedicjoe]

what is a high sac rate ?

 

[Uncle Pug]

working or resting?

 

[Scubamedicjoe]

sac rate after the dive so I guess the working rate

 

[Uncle Pug]

We figure our working SAC higher than our resting SAC.


Why do you ask?

 

[Scubamedicjoe]

well just down loaded my last four dives noticed my sac rate from the cobra computer to be higher than normal normally its 58 sac todays was 70 sac

 

[Uncle Pug]

medic13 once bubbled...
well just down loaded my last four dives noticed my sac rate from the cobra computer to be higher than normal normally its 58 sac todays was 70 sac

you must be figuring your SAC as psi/minute... and that is tank dependent... we figure ours as cf/minute.

What size tank are you using?