turn pressure calculation

[ItsBruce]

Metric? What a great idea. I've been using firkins, furlongs and fortnights for my calculations. It might be easier using metric.

 

[Rick Murchison]

With as much open ocean as there is, I'm not sure I'll ever need to go into a cave.

If my only overhead is deco then my gas plan is quite different than for a cave. "Turn" pressure is generally a convenient pressure to turn the dive to complete the navigation plan within the constraints of the overall gas plan. Depending on bottles carried, there may be planned and minimum switch pressures, min back gas, min total gas... etc. As "thirds" is generally more gas than any of these, it generally works well for caving, but it is generally unnecessarily conservative for open water.
Note the word "generally"... sometimes deco, current, use of scooters etc considerations dictate a turn at considerably less than thirds.
Rick

 

[OneBrightGator]

Hmmm... lessee now... if I were to round up to 2700 (the nearest hundred above 2640 divisible by 3) and figure thirds from there, I'd turn at 2700 - 900 or 1800... and the way I figure it 1800 is more than 2/3 of 2640. Ain't it? This method works for any pressure ... and is even easier to calculate and remember than dealing with the pressure below as well. I must admit, though, that I do go to the trouble of figuring things both ways, so for an actual pressure of 2500 I'd figure 1/3 at 800 - and then subtract from 2500 to get 1700, which I'd use in that case. So, from 2401-2500 I'd use 1700, from 2501-2700 I'd use 1800. There's just no sense in going up (in this case from 1800 to 1900) on turn pressure when you go down (from 2700 to 2640) on start pressure.

Looks good to me Rick, guess I was too busy with math to bother with common sense

 

[theskull]

Go grab a bottle of trimix. It sounds like you are narc'd at 0.79ppN2 and greater. :banana:


Of course, if you were doing the calculations using metric method of describing tanks by water volume, then everything would be much easier.

When I start diving with tanks full of water, I will start doing calculations by tank water volume. . . or maybe I'll just leave the tanks behind and breathe the water that's already surrounding me.



theskull