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MOD Memory Aid September 17, 2010

Posted by Chris Sullivan in Technical Diving, Training.
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Maximum Operating Depth and Contingency Operating Depths for Nitrox diving are defined (by most) as the depths you reach PP02 of 1.4 and 1.6 respectively, indicating the recommended safe limits for continuous Oxygen exposure during the working and deco portion of a dive.

When I was teaching a Tec 40 class we were discuss MODs and other depth and pressure related calculations and I was using the formulas to explain the principles behind the math, the instructor mentioned Daltons’ Diamond, which I had to confess not remembering. It looks like this:

      /\
     /  \
    /PPO2\
   /______\
  /    |   \
 /Depth| FO2\
/______|_____\

It says that Depth*FO2=PP02, PPO2/Depth=Fo2, and PP02/FO2=Depth. The trick is that depth is in Absolute Atmospheres, so you need to divide FSW (feet of sea water) by 33, FFW (feet of fresh water) by 34, MFW (metres of fresh water) by 10, etc, then add 1. For instance, 50% mix at 33 feet is FO2=.5, Depth=2, so PP02=1 (and safe).

The memory aid is simple for people who used computers, and useless for those who do not. The triangle is made up of the first initials PDF (PPO2, Depth and FO2), which is the file format used by Adobe Acrobat.  Once I noticed it I never forgot it, and neither do my computer-using students.

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