Nitrox training question

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czervos

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Location
Cincinnati, Ohio
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I plan to take the PADI Nitrox course soon, but bought the book ahead of time. I've run across a sample problem that I don't understand and hoped for some insight. Below is the question and the solution I arrived at:

A diver plans two dives. The first dive is planned to 87 feet using EANx36. Assuming the diver stays to the no decompression limit, followed by a 1 hour surface interval, what is the maximum allowable dive time for a repetitive dive to 48 feet using EANx40?

I found that the EAD of 87' is equal to 67' which the NDL is 40 min. This had a O2pp of 1.34 and 30% allowable oxygen exposure. This put the diver in pressure group T and after 60 min SI, entered pressure group G. Using EANx40 at 48', it would be equivalent to 30' using air and O2pp was 1.01. On the exposure table, I found that to use the remaining 70% allowable exposure, at O2pp of 1.1, the diver could stay 168 min.

The book says that the correct answer is 165 min. Any ideas?
 
I plan to take the PADI Nitrox course soon, but bought the book ahead of time. I've run across a sample problem that I don't understand and hoped for some insight. Below is the question and the solution I arrived at:

A diver plans two dives. The first dive is planned to 87 feet using EANx36. Assuming the diver stays to the no decompression limit, followed by a 1 hour surface interval, what is the maximum allowable dive time for a repetitive dive to 48 feet using EANx40?

I found that the EAD of 87' is equal to 67' which the NDL is 40 min. This had a O2pp of 1.34 and 30% allowable oxygen exposure. This put the diver in pressure group T and after 60 min SI, entered pressure group G. Using EANx40 at 48', it would be equivalent to 30' using air and O2pp was 1.01. On the exposure table, I found that to use the remaining 70% allowable exposure, at O2pp of 1.1, the diver could stay 168 min.

The book says that the correct answer is 165 min. Any ideas?

Group T cannot be right. Forget the oxygen exposure when you're working these out, as long as you're shallower than the allowed depth for the O2pp. My tables are old and may not be the same as yours, but a dive to 67' for 40 minutes should put you in Group H or so not T. One hour SI puts you in Group G. Find the residual nitrogen for a dive to 30 feet from Group G and subtract the RNT from the NDL at 30 feet to get your answer.

Adam
 
My Padi air table says Group G at 35 ft (shallowest it goes for EAD of 28.5 ft) says 40 minutes RTN and 165 minutes NDL. The dive is nitrogen limited.
 
I agree that you are pressure group T at the end of dive 1 and G at the beginning of dive 2.

I agree that EAD for 50ft dive on 40% is 30 ft. You have to round up to 35ft to use the RDP which tells you that the NDL for dive 2 at pressure group G is 165 min.

My O2 exposures are different than yours, but the tables may be a little different (though PADI). Still, the oxygen exposure limit is longer than 165 min, so I agree with awap. Nitrogen exposure is the limiting factor.

When figuring Nitrox problems, you have to take into account BOTH nitrogen and oxygen limits. The questions may focus on only one, but you have to remember that both are important.
 
czervos, you did everything right but just forgot to do the last step, which awap and wve explained above.

The allowable dive time is always the lesser of the O2 allowable time or the NDL. Some of the questions in the book ask specifically for the NDL, or the O2 allowable time. But this question, on page 80, makes you check both.

I'm doing my nitrox cert next weekend. Good luck on yours!
 

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