question on RMV

[DEEPLOU]

SAC is Surface Air Consumption, not 20' Air Consumption

SAC remains constant

You adjust SAC by a factor of pressure(ATA) and exertion.

Calculate your resting SAC by breathing off a tank at sea level for several minutes and averaging the cf/min.

Multiply that by ATA's to get a depth compensated rate.

Then adjust for workload, light current, moderate current/tasking, heavy, etc...

I use the following EF multipliers:

Single tank recreational rig:
No exertion: 1
Light: 1.5
Moderate: 2.0
Heavy: 2.5

Double tanks tech gear:
No exertion: 1
Light: 2.0
Moderate: 3.0
Heavy: 4.0


My standing SAC is .23
Let's plan a 99' dive with doubles in a moderate current or work load

.23sac x 4ata x 3ef = 2.76rmv at depth

20 minutes at that depth

20m x 2.76rmv = 55.2 cf required for bottom time + decent/ascent and reserve

I record my actuals and the conditions to verify the EF values. It's dead nuts on for me right now.

That's how I was originally taught. However, another instructor I know wants you to calc your SAC by checking pressures upon hitting bottom and upon leaving (and of course marking the timing) and do the same for deco.
Then back calc to come up with the SAC.
This way you have SAC for actual dive effort (not including "more effort or stress" dives), and "rest" (on deco hang).

Problem I have found is that breathing at rest on surface, you are thinking about breathing and can artifically (accidently) throw the figures off.
On the other hand, the "other instructor" method I noted I find that when I hit the bottom I forget to record the pressure and time, or at the end of the dive do the same thing. I guess for that you have to waste a dive just consentrating on go down check time and pressure, swim around a little, and check time and pressure.

 

[mempilot]

DEEPLOU,

Agreed that the surface breathing can be manipulated. I will usually do it while watching tv for 5 to 10 minutes with a timer so I'm not looking at the gauges. My EF's were tuned over about 100 dives at various depths in 2002. I recorded actuals and conditions, and then plotted them in Excel. My spreadsheet works for me, but probably would be off for someone else. I'm usually right within a few cf either side of plan. I typically don't even need to round up to cushion it.

I actually enjoy the planning side and then checking the actuals back against it. If my actuals vary too much from plan, I know it's time to hit the gym! ...and recalc the SAC.