Maybe the third time's the charm.

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Posted by Chuck Tribolet on March 07, 2005 at 20:06:29:

In Reply to: you can have the last word posted by northcoast diver on March 07, 2005 at 18:00:18:

Let's try this again putting back in the deleted blanks. Maybe third time's
charm.

"I'm giving you the last word" I doubt it.

Karl, you have claimed that I was incapable of calculating air consumption.
You've also asked if I was ever trained in how to calculate. I was, long ago,
in high school physics. It's not complicated. I can do it with pencil and
paper, but, being a software engineer by training and trade, I whipped up a
little program to calculate it. That took about 15 minutes.

Now let's apply it. Here's your dive, assuming an 80 cu. ft. 3000 PSI tank.
My calculated air consumption is within .1 CuFt of yours, which is within the
range of round off error. SAC is your 0.75 cu. ft. / minute throughout.


        ---------- Dive Segment----------   -----At Seg End ---
Seg.     Seg.  Start   End    # of   CuFt      Run   PSI   CuFt
Nbr.     Time  Depth Depth  Divers   Used     Time  Left   Left
   0     0.00      0     0       1    0.0     0.00  3000   80.0
   1     2.00      0   100       1    3.8     2.00  2859   76.2
   2    20.00    100   100       1   60.5    22.00   591   15.8
   3     3.33    100     0       1    6.3    25.33   356    9.5

356 PSI left at the end of the dive. That's more than the 300 PSI
which I think is what you as a NAUI instructor should be teaching.
So you could do the dive. EXCEPT that California practice is to be
on top with 500 PSI so you can swim out from under the kelp.

THAT PROFILE ON THAT TANK ISN'T SAFE SAFE IN CALIFORNIA.

Now let's some doses of reality.

Reality Dose 1: You're a NAUI instructor. The NAUI tables say to do a stop at 15'
for three minutes. That gives us:


        ---------- Dive Segment----------   -----At Seg End ---
Seg.     Seg.  Start   End    # of   CuFt      Run   PSI   CuFt
Nbr.     Time  Depth Depth  Divers   Used     Time  Left   Left
   0     0.00      0     0       1    0.0     0.00  3000   80.0
   1     2.00      0   100       1    3.8     2.00  2859   76.2
   2    20.00    100   100       1   60.5    22.00   591   15.8
   3     2.83    100    15       1    5.8    24.83   373    9.9
   4     3.00     15    15       1    3.3    27.83   250    6.7
   5     0.50     15     0       1    0.5    28.33   233    6.2

That puts you on top with less than 300 PSI, which violates the NAUI
protocols you are supposed to teach.

Reality Dose 2: Your buddy goes OOA right at the end of the dive:


        ---------- Dive Segment----------   -----At Seg End ---
Seg.     Seg.  Start   End    # of   CuFt      Run   PSI   CuFt
Nbr.     Time  Depth Depth  Divers   Used     Time  Left   Left
   0     0.00      0     0       1    0.0     0.00  3000   80.0
   1     2.00      0   100       1    3.8     2.00  2859   76.2
   2    20.00    100   100       1   60.5    22.00   591   15.8
   3     2.83    100    15       2   11.7    24.83   154    4.1
   4     3.00     15    15       2    6.5    27.83   -91   -2.4
   5     0.50     15     0       2    0.9    28.33  -126   -3.3

Oops, you ran out of air on the safety stop, or maybe even before
you got there (most regs start getting wonky at low tank pressures.

Reality Dose 3: Your buddy runs out of air at the end of the dive, and you
blow off the safety stop because you know you will run out of air
on the stop:


        ---------- Dive Segment----------   -----At Seg End ---
Seg.     Seg.  Start   End    # of   CuFt      Run   PSI   CuFt
Nbr.     Time  Depth Depth  Divers   Used     Time  Left   Left
   0     0.00      0     0       1    0.0     0.00  3000   80.0
   1     2.00      0   100       1    3.8     2.00  2859   76.2
   2    20.00    100   100       1   60.5    22.00   591   15.8
   3     3.33    100     0       2   12.6    25.33   120    3.2

You're on top with 120 PSI. MAYBE the reg is still working, maybe
not.

Reality Dose 4: Now let's take it in a different direction. AL80s don't hold 80
cu. ft. They hold 77.4 cu. ft. See
http://www.luxfercylinders.com/products/scuba/specifications/us_imperial.shtml

So, let's do the dive, no stuff goes wrong:


        ---------- Dive Segment----------   -----At Seg End ---
Seg.     Seg.  Start   End    # of   CuFt      Run   PSI   CuFt
Nbr.     Time  Depth Depth  Divers   Used     Time  Left   Left
   0     0.00      0     0       1    0.0     0.00  3000   77.4
   1     2.00      0   100       1    3.8     2.00  2854   73.6
   2    20.00    100   100       1   60.5    22.00   511   13.2
   3     3.33    100     0       1    6.3    25.33   267    6.9

You're on top with less than 300 PSI and nothing went wrong. Obviously
this is a bad thing. Note: Somebody pointed this out a while back
and you ignored it.

Reality Dose 5: I don't do just 3 minutes at 15 feet, and I come up slower
than two seconds per foot. My ascent from a square profile to 100'
would be to take two minutes to get to 50' (I used to say 1 minute, but
I've found it actually takes two minutes because the anchor line runs
parallel to the bottom down there. Three minutes at 50'. A minute to get
to 30'. Three minutes at 30 feet. A minute to get to 15'. Three
minutes at 15'. Ascend to 10' (this only takes a few seconds, and
we'll lump it with the next item). A minute at 10' swimming to the
tag line and hanging the camera, and getting out of my BC (it's easier
than on the surface, and good practice for the Farallones). A minute
from 10' to the surface. Now, I agree that this is probably overkill,
but I enjoy it because I see all kinds of cool stuff in the water column.

Here's the numbers, and nothing went wrong:

 
        ---------- Dive Segment----------   -----At Seg End ---
Seg.     Seg.  Start   End    # of   CuFt      Run   PSI   CuFt
Nbr.     Time  Depth Depth  Divers   Used     Time  Left   Left
   0     0.00      0     0       1    0.0     0.00  3000   80.0
   1     2.00      0   100       1    3.8     2.00  2859   76.2
   2    20.00    100   100       1   60.5    22.00   591   15.8
   3     2.00    100    50       1    4.9    24.00   407   10.9
   4     3.00     50    50       1    5.7    27.00   195    5.2
   5     1.00     50    30       1    1.7    28.00   133    3.5
   6     3.00     30    30       1    4.3    31.00   -28   -0.8
   7     1.00     30    15       1    1.3    32.00   -75   -2.0
   8     3.00     15    15       1    3.3    35.00  -198   -5.3
   9     1.00     10    10       1    1.0    36.00  -235   -6.3
  10     1.00     10     0       1    0.9    37.00  -267   -7.1

Nope, nothing went wrong except I was OOA before I left the 50' stop.

Now, in all of this I have not addressed what happens when the diver
has a really laid back dive, and their SAC goes way down. If they use
the tank to limit their dive, they get bent.

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