This is a common and absolutely silly statement made in many places. It is featured regularly in Dive Training Magazine, where Alex Bryskie is adamant about it. The theory is that you can either use nitrox to extend bottom times, or you can dive nitrox on an air table and this get a little level of increased safety. That's it. You have only two choices--nothing in between. That false dilemma assumes that once you enter the water, you have no choice but to stay for the full duration of NDLs. If the table says you can stay for 150 minutes at 50 feet, then, dammit, you have to stay for 150 minutes, no matter how much you want to surface earlier. Better bring a lot of gas.so you have to either dive on NITROX or cut bottom time. At least, this is my understanding of the article I found.
I have done comparisons of this kind a number of times. Here we go again.
Diver A is diving on air, using the PADI tables. He does the first dive to 80 feet for the maximum time of 30 minutes. He is in pressure group R. He has a one hour surface interval, putting him in pressure group F, and then he does a 50 foot dive to the maximum time of 56 minutes. This puts him in pressure group X.
Diver B is diving on EANx 36, using the PADI tables. He does the first dive to 80 feet for 40 minutes. He is in pressure group P. He has a one hour surface interval, putting him in pressure group E, and then he does a 50 foot dive for 70 minutes. This puts him in pressure group R.
Diver A had a total of 86 minutes of bottom time. He stayed to the end of his NDL's on each dive. He ended in pressure group X.
Diver B had a total of 110 minutes, He was 15 minutes short of NDLs on the first dive and a whopping 117 minutes short of NDLs on the second dive. He ended up in Pressure group R.
It sure looks to me like Diver B had more bottom time and less decompression stress.