SAC Rate

[mikerault]

Ok, use average depth or max depth? Also I see a decease in SAC according to depth. For example the linear trend line in a XL plot of my SAC based on maximum depth varies from .72 at around 20 feet to .3 at 116 feet while basing it on average yields .89 at around 20 feet to .58 at 116 feet. So using the maximum depth I get an average SAC of .55 based on maximum depths and .75 based on average depths. This is based on nearly 100 dive profiles diving aluminum 80s.

Does this make sense?

Mike

 

[grazie42]

SAC= Surface Air Consumtion so in short, No.

Your gasconsumtion will vary with depth but not your SAC...
To get your SAC you simply convert your consumtion at depth to surface pressure...looking at the numbers you posted, I think you´re doing something wrong in your calcs...

A quick example:
You use 80ltrs/min @ 30mtrs / ~100ft = SAC of 20ltrs/min
You use 60ltrs/min @ 20mtrs /~70ft = SAC of 20ltrs/min
You use 40ltrs/min @ 10mtrs / ~30ft = SAC of 20ltrs/min

So while consumtion varies, SAC, typically does not...(except if your stressed, working hard, cold etc. on a particular dive)

Makes sense?

 

[TheHobster]

I have built a spreadsheet that has SAC calcs and you can us tank sizes for time at depth - I use it with AOW students to discuss the deep dive - we do the test in the PPB dive we do as dive 1; then create SAC and determine how long at 72 ft for the deep dive - kinda fun, but I would love other input

email me privatly if you would like that hphobbie@ameritech.net

 

[ClayJar]

My SAC is about 0.45 plus or minus 0.10, with the low end being an ISOSAD club dive (ScubUNO rocks!) and the high end being against currents and such.

My buddy on the last ISOSAD dive had an SAC of 0.25, and that included goofing off between rounds. (She got 2:47 out of an AL80 at 18' and came up with something like 600psi remaining. I did the dive in an AL100 and had maybe 300psi left.)

 

[mikerault]

Ok, I plotted your calculations for my dives data then ordered them according to depth and it clearly shows a decreasing SAC with depth.

 

[shoupart]

Interesting.... I looked at several of my dives at various depths to look for a trend.

DEPTH SAC
106 .51
88 .57
82 .41
63 .48
50 .48
48 .45
40 .50
29 .46
26 .55
15 .50

There did seem to higher rates the shallower I got, and all dives were from the past year, so my physical fitness shouldn't have changed that much.

 

[grazie42]

Bouyancy is harder to control shallow, just a thought...

 

[Soggy]

Bouyancy is harder to control shallow, just a thought...

Yup, those dives where you are at 10-20ft the entire dive kill my SAC.

 

[CompuDude]

You may use more gas at depth, but since compensating for depth is part of the calculation that gives you your SURFACE air consumption rate, no, there is no real change in SAC at depth due to the deeper depth. That said, it can be colder at depth, you may be more stressed, etc., so your actual rate of consumption may change while you are at various depths, but that doesn't mean there is some linear difference in SAC rates that is tied to depth, it all has to do with other factors.

 

[mikerault]

Air consumption will be driven by the amount of O2 your body believes it needs and the CO2 in the lungs, etc. At greater depth the amount of O2 in the air breathed will increase since the pressure is greater, however, the volume of the lungs will not change or the surface area of the aveole that does the gas transfer. This would seem to indicate at greater depths the amount of O2 molecules per square foot of lung tissue will increase which may cause SAC to actually decrease with depth. Just a thought.