SAC rate question

[Jafo123]

I was wondering just what SAC rate a person has to have not to be considered to be a Hoover. I am overweight and out of shape but I have a SAC rate of .46 at least on my last dive. Is this good or not?

jafo123

 

[whitedragon13]

It depends on what you were doing, I suppose. Were you swimming your guts out the whole time, with a surface swim? Or did you drop of the boat, and hang out next to the anchor line?

Thought you were the new diver from my work...callsign JAFO.

 

[Jafo123]

It was about a 50/50 type dive. Did some swimming and did some picture taking. It was a boat dive in cooler water of about 70 deg.

Sorry I live in Iowa.

jafo123

 

[whitedragon13]

In my circles, our "Hoover" can blast through an AL80 in about 40-45 minutes of moderate swimming at 30'. Myself and a couple other non-smoker buddies will surface with close to 1000' PSI. I'm too lazy to do the SAC rates-I'm sitting in my hotel eating Thai food in Bangkok, but the easy answer is that if you're the reason everyone had to end their dive, you're the hoover.

 

[fgray1]

I've been as low as .3 and as high as .8 depends on the dive and the tasks involved.
It's only and average. A way to see if you improve. I once was with a guy that went through a al80 in 17 minutes at 50' Talk about a short dive.

 

[Jafo123]

White Dragon; Just had to rub it in didn't ya!!

 

[DA Aquamaster]

In my circles, our "Hoover" can blast through an AL80 in about 40-45 minutes of moderate swimming at 30'. Myself and a couple other non-smoker buddies will surface with close to 1000' PSI. I'm too lazy to do the SAC rates-I'm sitting in my hotel eating Thai food in Bangkok, but the easy answer is that if you're the reason everyone had to end their dive, you're the hoover.

If you crunch the numbers asuming you use 2000 psi from an AL80 on a 42.5 minute dive and your buddy uses 2500 psi on the same 42.5 minute dive to 30 ft. You get SAC rates of 24.5 psi/min (.33 cu ft/min) and 31 psi/min (.42 cu ft/min) Both are quite good assuming you were swimming the whole dive, and were at a constant 30 ft. for the whole dive.

Interestingly, the 50' dive for 17 minutes with an AL 80 mentioned above also produces a SAC rate of .44 which is also not bad but this again carries the assumption that the diver instantly descends to 50' and stays there the whole time and then instantly ascends. Consequently, in practice numbers generated this way tend to be overly optimistic and produce much lower SAC rates because most people do not swim constantly nor do they maintain the max depth of the dive for the entire dive.

What divers should do to figure a sac rate is descend to a fixed depth between 30 and 70 feet, note their SPG reading and then swim at that constant depth at a moderate and constant speed for 10 minutes and then note their SPG reading again in order to determine the gas used at that specific depth over a 10 minute period of constant swimming. This will give a more conservative number that will be more useful for planning purposes. A good average SAC rate by this method is probably around .6 with hoovers scoring around .9 to 1.0 and above.

Many air integrated computers now compute the average depth and combine this with the air used and the dive time to produce a SAC rate for the entire dive. In one sense this is probably more accurate for a given diver, especailly when the SAC is averaged over several dives, but it is again only applicable to dives with similar swim rates and work loads and could cause you problems if you use it for planning a more strenous where you will be swimming constantly.

 

[donacheson]

If you crunch the numbers asuming you use 2000 psi from an AL80 on a 42.5 minute dive and your buddy uses 2500 psi on the same 42.5 minute dive to 30 ft. You get SAC rates of 24.5 psi/min (.33 cu ft/min) and 31 psi/min (.42 cu ft/min) Both are quite good assuming you were swimming the whole dive, and were at a constant 30 ft. for the whole dive. ...

24.5 psi/min and 31 psi/min are correct, but they're equivalent to about 0.65 cu ft/min and 0.83 cu ft/min. You've already included the factor of (30+33)/33 = 1.91 in calculating the SAC in psi/min. (24.5 psi/min)(80 cu ft)/(3000 psi) = 0.65 cu ft/min
(Yes, I know, an Al80 doesn't quite hold 80 cu ft of air.)

While I don't know how to define a "hoover", IMHO a good SAC is about 0.4 cu ft/min (about 11 liters/min) for moderate swimming in cool or warm water.

 

[DA Aquamaster]

Good catch. I should not attempt any sort of math before the first cup of coffee in the morning.

The .83 would place the diver in question in the hoover category when you consider that part of the dive was by definition shallower than the constant depth used to figure the SAC.

I still think that .6 is a good average with continuous swimming, at least in cold water with 7mm wetsuits or drysuits.

 

[teknitroxdiver]

I have a low average of about .34, and a high average of maybe .42. I'd say a hoover would be anyone who cut someone elses dive short by a significant amount.