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36% and 30%
BenDavid - 10/13/2017 8:49 PM
Category: General
Replies: 5

I’m planning 2 dives to 65 feet tomorrow and have a tank of 30% and a tank of 36% nitrox.

Is there a benefit to using either one first or does it not matter?

I haven’t sat down and did all the math yet and I didn’t know if any gurus out there knew right of the top of their heads. I think last time had 2 tanks of 30% on this dive and ran out of gas before I ran out of time so it didn’t matter.

I was just pondering if there was a benefit if using the 36% first or last.
LatitudeAdjustment - 10/14/2017 6:26 AM
I was on a liveaboard once that was having compressor problems and was pumping 21%, that mattered :(

Unless you are diving 120’s and have a great SAC I’m guessing at 60’ you’ll run out of gas before bottom time and I would use the 36% first.
DiverfromBaskingRidge - 10/14/2017 3:34 PM
Interesting Latitude - I would have said use the 36% last.

Since we don’t know your SAC/RMV - my recommendation is to use the higher percentage nitrox last to get a longer NDL or at least a longer second dive. That longer NDL will in my opinion only benefit you on the second dive.
BenDavid - 10/15/2017 4:57 PM
Yes I run out of gas before I run out of time Latitude. Based on my dive log I get about 44 minutes out of my 100 Cu. Ft. steel and about 37 minutes out of my 80 ct. ft. aluminum on that dive. That’s if I run them totally out and not leave any reserve.

I don’t know if I did the tables right. Computers ruin you since if you don’t use it you lose it. But The way I calculated it using 36% and 32%, I come out 3 pressure groups better if I use the 32% first.

—> 65 feet for 35 min. using 36% puts me in a pressure group of "J" and after an hour interval I’m a pressure group "C". Another 35 min dive to 65 ft with 32% leaves me in a pressure group of S (with a safety stop required)

or if I switch the order of the gas

—> 65 feet for 35 min. using 32% puts me in a pressure group of "M" and after an hour interval I’m a pressure group "D". Another dive to 65 feet for 35 min. leaves me in a pressure group "P" (no safety stop required)

Again, that’s if I did the tables right, Haven’t actually manually done tables in awhile. It might also make a difference since I have to round 65 feet up to 70 on the 32% RDP since it doesn’t have a 65 foot column like the 36% dive planner does. I guess I would get a more accurate comparison if I just manually did both dives to 70 feet since both RDPs have a 70 ft column.
Curtis - 26 days 20 hours ago.
Using rough calculations, @ 65 FSW those mixes would bring you on a 50 ft table (AED), meaning your main issue is going to be gas, not NDLs, and you’re thinking way too hard about it. :-)