BDSC
Contributor
I was off the coast this weekend as well and 30% is the standard mix for the group I dive with.
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I wonder if it has more to do with the cost to profit ratio?Is it common for shops to bank only 30%?
Remember when it comes to staying within NDL's, you generally get more bottom time by doing your deeper dives first.
Give me an example and use the same gas for all repetitive dives as the initial dive.
I cannot say if it is 'common', but it is certainly not unheard of on the East Coast, particularly as you move north into colder water areas. In fact, I think the trend is growing. I started a thread related to the topic at the beginning of the year (http://www.scubaboard.com/forums/ad...ox-mixes-you-use-best-mix-what-available.html) because I was curious about the issue - how many shops bank something other than 32% (or 36%) and what mixes do they bank. Based on the comments, it appears that quite a few bank mixes lower than 32%.Is it common for shops to bank only 30%? Particularly, is it common enough that I should be asking about this when planning a dive trip? The shops around the area where I live either do partial pressure blending or bank 36% from a membrane system.
From our perspective (we bank 30%), convenience / cost and the possibility of error. If we have to re-blend every time we fill a cylinder with nitrox, that is an extra step that takes time (which adds to cost) and opens the door to error. We would rather charge a modest amount for a nitrox fill, and would have trouble doing that if we had to partial pressure blend each time we filled a nitrox request.Also, I'm curious - why would a shop choose to bank a mix like that instead of blending or banking a higher mix they could thin out if necessary?
Okay...Dive 1, 30 minutes, 80 feet, air.
1 hour surface interval.
Dive 2, 60 feet, air. 36 minutes of residual nitrogen time apply, total time allowed within NDL is 14 minutes
....80' @ 30minutes makes you a "R" diver on a RDP..stay out for 1 hour now you are a "F" pressure group.now want to make 2nd dive to 60' your ANDL time is 36 minutes .. not 14..
60' for 30 minutes makes you a pressure group "L"..take a 1 hour SI now you are pressure group "C"..dive 2 for 80' as a pressure group "C" diver gives you 20 minutes of ANDL time..Contrast to:
Dive 1, 30 minutes, 60 feet, air.
1 hour surface interval.
Dive 2, 80 feet, air. 23 minutes of residual nitrogen time apply, total time allowed within NDL is 7 minutes
My answers came from a PADI RDP..if you used a different table,what was it? If it was US Navy table your answers are wrong there too.
US Navy table; 1st dive 80' 30 minutes makes you a "G" diver,,1 hr SI now you are at "F".dive #2 at 60'does give you RNT of 36 minutes with a ANDL of 24 minutes, not 14 minutes.
At first glance your dive profiles did not look correct. So I had to dig up a PADI RDP and an old USN table here at home;
....80' @ 30minutes makes you a "R" diver on a RDP..stay out for 1 hour now you are a "F" pressure group.now want to make 2nd dive to 60' your ANDL time is 36 minutes .. not 14..
60' for 30 minutes makes you a pressure group "L"..take a 1 hour SI now you are pressure group "C"..dive 2 for 80' as a pressure group "C" diver gives you 20 minutes of ANDL time..
My answers came from a PADI RDP..if you used a different table,what was it? If it was US Navy table your answers are wrong there too.
US Navy table; 1st dive 80' 30 minutes makes you a "G" diver,,1 hr SI now you are at "F".dive #2 at 60'does give you RNT of 36 minutes with a ANDL of 24 minutes, not 14 minutes.
Whether people plan on tables or a computer, the theory comes from the same place. Generally, I plan my dives using a computer, and if I have one mix leaner than the other, I very definitely look at which one to dive first. I don't see why it's bad advice, you have to pick one or the other, what's the problem with using some sort of reasoning to arrive at the answer?The reason I mention this, is because worrying about tables makes people give bad advice like "dive the leanest mix first" because "tables", which no one is actually using.