SAC Rate, What is Normal?

[eponym]

Charlie,
You wrote "Don't forget to factor in your average depth."
But isn't the SAC an index based upon one's consumption at the SURFACE?

If I may . . .

Yes, indeedy.

Dividing your psi/min or cu.ft./min breathing rate by the number of atmospheres you were breathing is the way we restate the rate as though you were breathing at the surface--it just accounts for the compression of the air.

So I if breath two cu.ft. per minute at 99 fsw, the formula predicts my surface rate to be 1/2 cu.ft. per minute.

-Bryan

 

[Charlie99]

Just expounding on Eponym's comments ........

The main reason for measuring gas consumption at all is to be able to plan future dives. We all know that a tank of air doesn't last as long down deep as it does shallow. The SAC calculations are a way to compensate for that variation in gas usage with depth. By normalizing or scaling all gas usage back to the equivalent of what it would be at the surface, we are able to predict gas usage for a dive at a different depth.

 

[matt_unique]

Just expounding on Eponym's comments ........

The main reason for measuring gas consumption at all is to be able to plan future dives. We all know that a tank of air doesn't last as long down deep as it does shallow. The SAC calculations are a way to compensate for that variation in gas usage with depth. By normalizing or scaling all gas usage back to the equivalent of what it would be at the surface, we are able to predict gas usage for a dive at a different depth.

SAC rate is expressed in CF/MIN/ATA but most people list their rate as CF/MIN for short or perhaps they assume others know what they mean.

As basically everyone has said it has to do with a million factors - temp, stress, tasks, current, mental state, etc., etc. My rates are .3 Cf/Min/Ata for deco, .5 for normal swim, and 1.0 hard (nearly over breathing my reg). If you are going for lobsters or if you are shooting pictures for example expect to breath more gas.

--Matt

 

[Zibri]

On average I breath 15 litres per minute of air at surface, as calculated by DM5 (suunto dive manager).. sometimes more..sometimes less... I don't know what is the sac rate in this page.

 

[Neilwood]

On average I breath 15 litres per minute of air at surface, as calculated by DM5 (suunto dive manager).. sometimes more..sometimes less... I don't know what is the sac rate in this page.

Would be nice to have the metric conversion. I know I am nearer 23-24l/m but not got a clue what that is in relation to the poll.

 

[gfaith]

@Neilwood

1cu/ft = 28.3168 liters

RMV (SAC) * 28.3168 =

Like .5 * 28.3168 = 14.1584 liter per minute

Your close 24/28.3168= .847

 

[Taath]

The topic is SAC rate, however, all the options and conversation has been referring to RMV.

SAC = PSI per minute, tank size dependent (20 PSI/minute)
RMV = Gas volume consumption per minute (.5 CF per minute)

Am I wrong here?

 

[dumpsterDiver]

Way too many people are trying to redefine terms. Surface air consumption rate in cuft per min is easy to understand and is understood to be the EQUIVALENT air use at the surface,. If the diver was at zero depth.

 

[scubadada]

I believe Taath's definitions are most widely used. My download software automatically calculates both SAC and RMV. I only follow my RMV, as it is cylinder independent:

Combined results from nearly identical 2009 and 2016 polls on SB regarding average RMV. Obviously, the 250 participants in the polls are not necessarily average divers:

 

[kelemvor]

Man I would so love to be part of that 0.4-0.49 group...