Rock Bottom Pressure and Turn-Around Pressure

[fjpatrum]

The OW diver is not taught to ask for air on a freeflow.

I was... I'm beginning to think my OW class was far better than I originally thought. Either that or I inferred an awful lot of information from what was taught that maybe I shouldn't have inferred.

 

[Dirty-Dog]

I was... I'm beginning to think my OW class was far better than I originally thought. Either that or I inferred an awful lot of information from what was taught that maybe I shouldn't have inferred.

We were too. We were also taught how to breath from a freeflow.

I think a fair bit of the "OW classes suck today" is unfounded.

 

[Kevrumbo]

. . .
Then why on earth are we talking about Rock Bottom calculations?. ..
. . .
Lets do BAR, I hate PSI.

Lets do the standard brief on the boat. "Be back with 50 BAR" (which is a lot more than 500psi)

So we have 150 bar to play with. Lets give, 20 Bar to a safety stop. so 130.

Divide by 2 gets us 65. So we have a turn point of 135 (assuming a 200 BAR Fill)

So we descend to depth, follow along until 135 BAR. Now we turn. Note that time has included the descent and part of the bottom time.

We now return to for the safety stop. Should take us 65 BAR. In reality it will take us less.
Why?

Firstly, as the tank hits the water it will cool, pressure will drop slightly.
Secondly, We always breathe a little faster in the first point of the dive, until we find our rhythm.

Working on those principles we now arrive back at 5m ready for our safety stop with 70 BAR +.

With the math. 20 BAR in a standard 80 (11.3 litre) tank. Gives us 226 litres of gas. Divide by 3 to get per min, then by atmosphere to get the worst case scenario SAC. (226/3/1.5) = 50.2 SAC.

I really don't try to injure my students, whatever people on the board may seem to think. I give them simple plans they can use straight after the course.

Easier method here, with due consideration for your buddy which is the primary reason behind Open Water NDL Rock Bottom Gas Planning (i.e. how much minimum gas in reserve should you plan for to get you AND your buddy to the surface in an emergency):

A Quick Rock Bottom Calculation and Gas Plan Estimate for Open Water. . .

For double 11litre tanks (double AL80's) for a total of 22 litres/bar metric tank rating and
a volume Surface Consumption Rate (SCR) of 22 litres/min -same as a pressure SCR of 1 bar/min*ATA-
using an example NDL air dive to 30m (4ATA) depth in Open Water.

Rock Bottom with one minute stops every 3 meters to the surface,
-->Just "tally the ATA's":
4.0
3.7
3.4
3.1
2.8
2.5
2.2
1.9
1.6
1.3

Sum Total: 26.5 --this also happens to be the pressure in bar needed for one person
to reach the surface.

For two divers: 26.5 multiplied by 2 equals 53 bar Rock Bottom absolute reading remaining on your SPG.

For two experienced divers stressed: 53 bar plus 30% of 53 bar equals 69 bar Rock Bottom SPG reading.

For two novice divers stressed: 53 bar plus 100% of 53 bar equals 106 bar Rock Bottom SPG reading.

If only a single 11L tank instead of double 11L tanks is to be shared, then multiply the above again by two:
For two divers: 53 bar times 2 equals 106 bar;
For two experienced divers stressed: 69 times 2 equals 138 bar;
For two novice divers stressed: 106 bar times 2 equals 212 bar (!!!)
--->obviously then, two novice divers on single 11L tanks should not be diving to 30m for any significant length of time. . .

 

[stevensamler]

Was taught sac rates NASDS 1973 ow.