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Enriched Air Nitrox Formulas February 28, 2011

Posted by Chris Sullivan in Technical Diving, Training.
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Formulas for Nitrox are used for several purposes:

  1. Calculating the Equivalent Air Depth (EAD) to allow the use of standard Air tables to calculated no decompression limits, repetitive groups, or decompression schedules.
  2. Calculating the best or optimum mix for a given depth
  3. Calculating the maximum or contingency depth for a given mix to minimize risk of CNS Oxygen Toxicity
  4. Calculating the oxygen exposure for managing Pulmonary Oxygen Toxicity
  5. There are also formulas for partial pressure filling of tanks, which determine how much Oxygen needs to be put in before topping up with air. A more difficult problem is how much Nitrox to drain from a tank before topping off with air in order to reduce the fraction of O2 in the mix.

In basic Nitrox training we’re given the formulas for 1-3, and a table to manage 4 in recreational diving situations. The PADI table uses the NOAA or “CNS” clock only, with no surface interval credit. Surface interval credit makes sense in technical diving when we’re managing pulmonary toxicity using the Repex method, but when using one table to manage both I’m not convinced that it would be wise to apply it.

Even in basic training, I usually like to point out why the formulas work, instead of using rote learning.  I first starting doing this when I took the course myself in the Turks and Caicos in 2005, and found that the understand helps me both remember the formulas (which I’ve never been good at) and to catch errors in my calculations.

Except in case 5, all of the formulas relate to the pressure of Nitrogen or Oxygen in Absolute Atmospheres. Everything that in Imperial divides, multiplies, adds or subtracts 33 (or 34 in fresh water) is converting between gauge and absolute atmospheres, and the formulas themselves are much easier to remember in ATA than in ATG.

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