I am taking my nitrox course now and have a question for everyone. In the maximum and contingency depth formula you are dividing two different numbers by your O2 percentage. Those numbers are 46.2 for Max (1.4 ATA) and 52.8 for Contingency (1.6 ATA) depths. What do those numbers represent? It would make it a lot easier for me to remember those numbers if I knew what they meant. Thanks

I have no idea what those numbers mean, or where they come from Sounds like your after your MOD (maximum operating depth) for the ppO[SUB]2[/SUB] of 1.4 & 1.6 ata. Algebraically the formula looks like this. ppO[SUB]2[/SUB]/ FO[SUB]2[/SUB]=ambient pressure in ata. Then convert pressure to depth for MOD For eg. for a ppO[SUB]2[/SUB] of 1.4ata & 1.6ata for an FO[SUB]2[/SUB] of 32% we would have 1.4ata/0.32=4.38 ata or approx. 34mts 1.6ata/0.32=5 ata or 40mts

Possibly 46.2 and 52.8 are depth in meters. It sounds like an example where the fraction of O2 is 25%. Which is kind of an odd ball percentage. But then you are right it would be easier if they did not throw numbers out there without any associated units. BTW metric depths to pressures are really easy. 10 meters of salt water (msw)=1 atmosphere absolute (ATA). Of course do not forget to add the 1 ATA for the ambient pressure at the surface. So 46.2 msw=5.62 ATA (46.2 meters/10 m/ATA +1 ATA); and 52.8 m=6.28 ATA). 1.4 ATA/5.62 ATA=.249; and 1.6 ATA/6.28 ATA=.254. Is the half percent difference significant? Probably not, but you would not want to be much farther off than that.

The numbers by themselves don't pass the sniff test if used as max and contingent. 46.2/1 (100% O2) is far deeper than any suggested depth for oxygen. The PO2 is actually 2.4 ATA. 52.8/1 gives a PO2 of 2.6. Unless part of the formula calls for subtracting 1 from the result, then we'd end up with 1.4 and 1.6 respectively. The "real" formula for partial pressure is (depth+33)/33 x O2%, i.e. (20ft+33)/33 x .50 = 0.8ATA. .

That sounds good. However it means you're not looking for your MOD, but rather the best mix to use at a given depth. It also means that you're not dividing any numbers by your FO[SUB]2[/SUB]%, but rather your ppO[SUB]2[/SUB] in ata by your ambient pressure in ata. It may also mean that you may not have a grasp of the difference between ppO[SUB]2[/SUB] & FO[SUB]2[/SUB].

Umm, I thought this stuff was simple & straight forward. Then I got lost in some imperial hell. Please disregard everything I'v said to date.

I really pity the unfortunate "not the rest of the world" who have still not awaken to the joys of metric measurements.

Well . . .using the numbers given: 46.2% & 52.8% FO2 values of the Oxygen mixes; and the NOAA guideline PO2 limit levels of 1.4 ATA and 1.6 ATA --Max & Contingency . . .and solving for total pressure P in ATA by applying Dalton's Law of Partial Pressure: 1.4ATA/46.2% equals 3ATA or same as 20meters or 66feet. 1.6ATA/52.8% equals 3ATA or same as 20meters or 66feet. Conclusion: for a dive to 20 meters or 66 feet, a 46.2% Nitrox mix will yield a Max PO2 value of 1.4ATA; while a 52.8% mix will result in a Contingency value of 1.6ATA (and better refer to your NOAA Table of Maximum Time of Exposure as well). I'm not exactly sure if this is the answer you're looking for to your exercise --this is a basic Nitrox Class, right??? If so, you should not be using these high FO2 mixes in actual practice. . . Comment: I had a "hot" deco mix of 53% once, so I had to perform the six minute deco stop slightly shallower at around 19 to 20 meters, instead of the usual MOD of 21 meters for EANx 50 . . .which is why the above given value of "52.8" kind of reminded me of that experience.

I think Mr. Richard gave me the answer I was looking for. It is an ATA to depth conversion. Now why in the world could PADI not have just put that in thier book for over analyzing brains like me? Thank you everyone for you responses.

The only reason I knew what those 'magic numbers' represented is because, like you, I wanted to really understand the equations. All of the other equations were quite simple but when PADI decided to use 'magic' constants, I couldn't sit still until I knew where they came from. It's an engineer thing, I guess. Richard

They could... at an additional fee of $25. In fact, you might want to check if they don't have a speciality course on it.