Anyone not measure their SAC?

[northernone]

I know a good several dozen divers with decades of diving who haven't calculated their SAC rate and simply prefer a less technical approach to air consumption. I like knowing my SAC.

Regards.
Cameron

 

[Chilly_Diver]

My planning is usually based on my wife's air as she has a higher sac rate and a smaller tank.

Smaller tanks will return higher SAC rates as SAC is a measure of PSI per minute usage rather than volume. If the two of you are using different size or pressure rated tanks, SAC is not a good measure for comparison between the two of you.

 

[RayfromTX]

Smaller tanks will return higher SAC rates as SAC is a measure of PSI per minute usage rather than volume. If the two of you are using different size or pressure rated tanks, SAC is not a good measure for comparison between the two of you.

sac rate is measured in volume/time/ata. It has nothing to do with psi until you calculate for that.

A hp100 will be about 34.4 psi per cuft. A hp80 will be about 43 psi per cuft.

To do a gas calculation, I figure my time in minutes x my average depth in ATA x my estimated sac rate.

So an example would be: 10 minutes x 4ATA(99 fsw) x .6 sac = 24 cu ft.
My hp100 will expend 826 psi to deliver that 24 cu ft of gas.
My wifes hp80 will expend 1,032 psi to deliver that 24 cu ft of gas

 

[MargaritaMike]

I'm confused. I thought SAC is measured in psi/min and RMV is measured in CuFt/min in the Imperial system. I'm guessing that it correlate to bar/min and liters/min for those other guys. They are mathematically related but aren't the same. My RMV is about .46 whereas my SAC is about 10 or 11. Am I wrong --- again?

Cheers - M²

 

[RayfromTX]

No you're right and I'm wrong. I get them confused because my instructor used them interchangeably when we learned gas planning. My computer tells it to me in sac at somewhere around 19 and I still confuse the two terms.

 

[RyanT]

SAC = Surface Air Consumption. RMV = Respiratory Minute Volume. Both are volumetric calculations of gas consumption. Imperial versus metric, the units don't matter, so long as you are calculating them correctly. RMV consumption will directly reflect your depth as it increases with increasing depth (e.g. gas density). As the name implies, SAC is your consumption at the surface and allows you to calculate your gas consumption at any depth, just multiply the atm of pressure by your surface consumption.

 

[Kevrumbo]

SAC = Surface Air Consumption. RMV = Respiratory Minute Volume. Both are volumetric calculations of gas consumption. Imperial versus metric, the units don't matter, so long as you are calculating them correctly. RMV consumption will directly reflect your depth as it increases with increasing depth (e.g. gas density). As the name implies, SAC is your consumption at the surface and allows you to calculate your gas consumption at any depth, just multiply the atm of pressure by your surface consumption.

SAC can be either pressure or volume: if you want to be more definitive, you could prefix "pressure SAC" for values in psi/min per ATA or bar/min per ATA, but the total cylinder rating in use must be stated as well; or "volume SAC" for cuft/min per ATA or liters/min per ATA, which is normalized across all cylinder ratings.

RMV is of course always volumetric units per time normalized across all cylinder ratings, either referenced as a Surface Consumption Rate (SCR), or given as a Depth Consumption Rate (DCR) at a stated depth in ATA, meters or feet.

 

[lowviz]

It may now be useful to revisit HOW you get these numbers and the PURPOSE of each. Each scenario below has a purpose, and all gas usage (BC inflation, drysuit, inflating bags, purging) impacts your consumption rate. Anyone know of a good summary for those just starting out on this confusing subject?

Lying on the couch with a mask on and a reg in your mouth draining a bottle.

Average depth of dive, length of dive in minutes, and total gas used.

Doing laps at a constant depth, time, and total gas used.

Air integrated real-time plots. (Of which I know nothing, I don't do AI)

 

[lowviz]

Just found this: NWGratefulDiver.com


@NWGratefulDiver Thanks!

 

[Kevrumbo]

It may now be useful to revisit HOW you get these numbers and the PURPOSE of each. Each scenario below has a purpose, and all gas usage (BC inflation, drysuit, inflating bags, purging) impacts your consumption rate. Anyone know of a good summary for those just starting out on this confusing subject?
. . .

For starters, all that really matters for now and here in Basic Scuba Discussions is an Emergency/Stressed SAC rate and how to use it. Along with time of ascent and average depth, these are the critical factors used to figure out how much reserve gas is needed to bring two divers to the required Safety Stop(s) and the surface, in an emergency gas sharing contingency.

This video gives a great explanation (skip to 10:15 minute mark for an initial summary):