SAC Rates

[Parrothead_Diver]

I was just going back through my log and computing my SAC for each dive. While doing this I noticed something rather strange that I cannot explain. That is for all of my local dives (Southern California) my SAC has been in the range of 0.90 to 0.95 (but trending down), and for the dives in places like Cayman, Roatan & Cozumel my SAC was around 0.65. Can anybody help me explain what maybe the problem?

My initial thought was that it had to do with the fact that I was diving off of boats while traveling and diving from shore when local. But I looked at some of my local boat dives and those were around a rate of 0.88. Also, my equipment is probably not a factor as I have always used my own gear.

I realize that all of these values are rather high, but I have only been diving since November and have logged about 25 dives. I am just trying to figure out why there is such a large difference in my SAC rates.

 

[D_O_H]

I was just going back through my log and computing my SAC for each dive. While doing this I noticed something rather strange that I cannot explain. That is for all of my local dives (Southern California) my SAC has been in the range of 0.90 to 0.95 (but trending down), and for the dives in places like Cayman, Roatan & Cozumel my SAC was around 0.65. Can anybody help me explain what maybe the problem?

My initial thought was that it had to do with the fact that I was diving off of boats while traveling and diving from shore when local. But I looked at some of my local boat dives and those were around a rate of 0.88. Also, my equipment is probably not a factor as I have always used my own gear.

I realize that all of these values are rather high, but I have only been diving since November and have logged about 25 dives. I am just trying to figure out why there is such a large difference in my SAC rates.

My SAC is .1 to .15 higher here in Norcal than it is in tropical places. Cold is definitely a factor that increases air consumption. You could also be (consciously or not) a little less at ease in murkier water, causing you to hoover a bit more. For dives in places like Cozumel, which are almost all drift, you should notice an even bigger disparity, since the current does most of the work for you.

 

[donacheson]

I was just going back through my log and computing my SAC for each dive. While doing this I noticed something rather strange that I cannot explain. That is for all of my local dives (Southern California) my SAC has been in the range of 0.90 to 0.95 (but trending down), and for the dives in places like Cayman, Roatan & Cozumel my SAC was around 0.65. Can anybody help me explain what maybe the problem?

My initial thought was that it had to do with the fact that I was diving off of boats while traveling and diving from shore when local. But I looked at some of my local boat dives and those were around a rate of 0.88. Also, my equipment is probably not a factor as I have always used my own gear.

I realize that all of these values are rather high, but I have only been diving since November and have logged about 25 dives. I am just trying to figure out why there is such a large difference in my SAC rates.

Colder water and (probably) a much heavier exposure suit are important contributors. For reasons I don't understand, murky waters tend to produce larger SAC rates than clear waters for me.

Yes, you have a lot of room to improve - but with time and experience you should get down to a SAC rate of 0.5 cubic feet per minute or maybe less.

 

[jbd]

I was just going back through my log and computing my SAC for each dive. While doing this I noticed something rather strange that I cannot explain. That is for all of my local dives (Southern California) my SAC has been in the range of 0.90 to 0.95 (but trending down), and for the dives in places like Cayman, Roatan & Cozumel my SAC was around 0.65. Can anybody help me explain what maybe the problem?

My initial thought was that it had to do with the fact that I was diving off of boats while traveling and diving from shore when local. But I looked at some of my local boat dives and those were around a rate of 0.88. Also, my equipment is probably not a factor as I have always used my own gear.

I realize that all of these values are rather high, but I have only been diving since November and have logged about 25 dives. I am just trying to figure out why there is such a large difference in my SAC rates.

Just some things to think about

Are you wearing the exact same gear for all of your dives? Is the current the same? Are you working against the current at all in your local diving and drifting with the current while on vacation? Are water conditions identical? Regarding thermal protection, your exposure suit may be keeping you toasty warm in the tropical locations but may only be keeping you marginally warm in the local waters.

 

[JustinW]

OK, so I have done a search and I think i understand how to calculate my SAC rate. thread I think I did this right. I'll post the info for my last dive and tell me if I got the right answer

-diving a PST 102, 3500psi capacity
-air consumption during my dive was 1071 psi
-dive time 45 minutes
-average depth 33 feet
-31.2 cubic feet used

so I calculated 3500/102= 34.3 psi/cubic foot
1071/34.3= 31.2 cubic feet used
(31.2/45)/2=.346

so is my SAC rate .346 or is it .69 (not incorporating atmosheres)?

I"m not sure if the calculation should be (31.2/45) or if it should be (31.2/45)/2

 

[kidspot]

Hey Justin699 I cheated and use the calculator (I know... rotting my brain lol) at http://www.spearfishing.org/bruces_tips/java/sac.html and it gives the same value you arrived at .3468 cf/min -

Tim

 

[JustinW]

cool, I always thought of myself as being an airhog

 

[fmw625]

Just because the 102s have a 3500 psi capacity you can't use it for the starting pressure unless the tank was filled to capacity. Psi used is SPG starting pressure minus SPG ending pressure. If you started with 3000 psi (a typical fill pressure around here) you are about 14.43 cu ft short of a max fill. This being the case your SAC would be even lower.

 

[The Kraken]

Just for funsies . . . a SAC fromula that accounts for fill volume:

(((Pb-Pe) / Pb) x ((Pb / Pw) x V) / T) / ((D/33)+1)

Where:
Pb = Beginning Pressure (SPG)
Pe = Ending Pressure
Pw = Nominal Working Pressure of Tank
V = Nominal Volume of Tank at Nominal Working Pressure
T = Time of Dive in Minutes
D = Average Depth of Dive

Needless to say, the kicker is determining your average depth of dive.

Have fun !!!

 

[MXGratefulDiver]

Just a nit ... but what most people refer to as SAC (surface air consumption), is in reality RMV (respiratory minute volume).

To be accurate ... SAC is the amount you breathe ... in PSI (pressure differential) ... in one minute on the surface. It's specific to the cylinder you're breathing from.

RMV is the equivalent value, expressed in cubic feet or liters per minute (volume) ... in one minute on the surface. It's independent of the cylinder you're breathing from.

Both numbers are important ... SAC because you do read your cylinder pressure in PSI, and RMV because when you do your dive plan you generally know what size cylinder (in cubic feet or liters) you'll be using.

You can calculate your SAC by subtracting your ending pressure from your starting pressure, dividing that number by the minutes of the dive, and dividing that number by your average depth, in ATA. The resultant value is expressed in psi per minute.

You can convert SAC to RMV by calculating your cylinder's "baseline" ... volume divided by working pressure ... and multiplying SAC by baseline. The resultant value is expressed in cubic feet (or liters) per minute.

Getting back to the original question ... you will ALWAYS consume more gas in colder water than warmer. The reasons are several ...

- you are wearing heavier exposure equipment and more gear, so you work harder
- cold makes your body metabolize more oxygen for heat generation ... so you breathe harder
- dark water causes subconscious changes in your breathing pattern (human nature) which cause you to breathe more rapid and shallow (less efficient)

The last is also why if you track your consumption rate on all your dives, you'll notice that you have a higher SAC rate on deeper dives than on shallower ones. It's not for physical reasons ... it's because your psychology causes you to breathe differently.

... Bob (Grateful Diver)