mikerault
Contributor
Actually,at least for me, SAC usually decreases slightly with depth...and for everyone I have gotten enough data to plot. It is not a constant.
Do you avoid people with bad sac rates?
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Granted, physical conditioning is a part of lowering your air consumption but it is only one of dozens of factors. Those who think it is just being relaxed and in good physical conditioning may be full of the stuff they processed after eating.
An example: My wife has MS (Multiple Sclerosis) and was dependent on a walker, crutches, wheel chair, scooters and any other mobility device to get her from point A to point B. Her physical conditioning was to say the least lousy. But at 53 and 130# she can max out the tables on TWO 100 to 120 dives on a single alum 80 and still have 500 to a 1000 pounds left. The woman does not breathe so to find some to match her SAC, its not me, would be tough. Many times Ill grab her octo and stay on it for a while just so she can stay down longer.
Gary D.
An example: My wife has MS (Multiple Sclerosis) and was dependent on a walker, crutches, wheel chair, scooters and any other mobility device to get her from point A to point B. Her physical conditioning was to say the least lousy. But at 53 and 130# she can max out the tables on TWO 100 to 120 dives on a single alum 80 and still have 500 to a 1000 pounds left. The woman does not breathe so to find some to match her SAC, its not me, would be tough. Many times Ill grab her octo and stay on it for a while just so she can stay down longer.
Gary D.
SAC and depth have an indirect relationship. SAC is affected by depth through experience. There is no direct relationship between sac and depth as per definition of SAC.
Did I really just read that "SAC rates increase with depth" ???!!?!?!
The amount of gas you breathe increases with depth, but all else being equal, your SAC doesn't!
Again (as I said in POST 38) the "S" stands for "SURFACE", so it can not increase.
I guess you could make a case for consumption at depth to increase due to WOB the denser gas (assuming non-HE mix).
Did I really just read that "SAC rates increase with depth" ???!!?!?!
The amount of gas you breathe increases with depth, but all else being equal, your SAC doesn't!
I can think of a few imperfections that might cause SAC to change with depth-
- higher breathing gas density will increase the work of breathing
- poor buoyancy control/trim
- mental pressure from the ambient pressure
- euphoric narcosis/paranoid narcosis
- temperature
I think most of these will only have small effects and not cause dramatic differences, although never underestimate the power of poor buoyancy and mental stress.
mikerault
Contributor
Ok...here are the data sheets and calculations. As you can see when the SAC's are arranged by depth for each diver the rates decrease with depth using a linear trend.
Mike
Mike
OP
alphonsis
Contributor
Just to clear up some apparent confusion. SAC (Surface Air Consumption) doesn't compensate for depth. DCR (Depth Consumption Rate) which is your SAC multiplied by the absolute pressure at depth determines how fast you suck down your cylinder.
For example:
I have a SAC of 0.8. If I do a dive to 66ft, which is 3 atm absolute (66/33 + 1), my DCR would be 2.4 cu ft per minute.
For example:
I have a SAC of 0.8. If I do a dive to 66ft, which is 3 atm absolute (66/33 + 1), my DCR would be 2.4 cu ft per minute.
The Kraken
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I have to back up Rick Inman's post.
ALL OTHER THINGS BEING EQUAL
A person breathes at a relatively constant rate.
The rate at which a person breathes is generally expressed in cubic feet.
So, if a person, while at the surface (1 ATA), requires a volume of 0.5 cu. ft. of per breath, his SAC rate is said to be 0.50 - SAC=0.50 cfm.
So, if this diver were sitting at the surface breathing from his tank, he would require 0.50 cubic feet of gas per breath.
Now if this same person is doing a dive at 100' and is perfectly relaxed as he would be at the surface, then his SAC (read "surface") rate would still be 0.50 cfm.
BUT the VOLUME of gas he would consume from the tank each breath would be 2.0 cu. ft. instead of .50 cubic feet because the ambient pressure upon the body is 4 ATA.
the K
ALL OTHER THINGS BEING EQUAL
A person breathes at a relatively constant rate.
The rate at which a person breathes is generally expressed in cubic feet.
So, if a person, while at the surface (1 ATA), requires a volume of 0.5 cu. ft. of per breath, his SAC rate is said to be 0.50 - SAC=0.50 cfm.
So, if this diver were sitting at the surface breathing from his tank, he would require 0.50 cubic feet of gas per breath.
Now if this same person is doing a dive at 100' and is perfectly relaxed as he would be at the surface, then his SAC (read "surface") rate would still be 0.50 cfm.
BUT the VOLUME of gas he would consume from the tank each breath would be 2.0 cu. ft. instead of .50 cubic feet because the ambient pressure upon the body is 4 ATA.
the K
AbyssalPlains
Contributor
I don't even know what those numbers mean. I select my buddies based on whether I think they are cool people and whether they come across as easy going and with a genuine interest in a mutually enjoyable dive. I also take on a buddy who has nobody else to dive with. IMHO, this whole "Who you should and shouldn't dive with" thing is a bit overrated. I mean, I'm not looking to marry those people. Then again, if the right buddy should come along, I'd definitely consider it. Anybody in the Tucson area need a buddy?
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