Wisdom of trusting one's dive computer?

And these are just the ones with GF settings.
Many apply conservatism by adjusting the altitude setting to something higher than sea level.

 

no im not suggesting that. I believe the comment i read was that many rec computers do not have conservative settings. That may be so if the computer is an ancient one. I have not seen one for years that does not have safety/ consevative settings.

 

I just pulled the trigger on a Perdix AI and while it was my intention to get one soon, I did it now because I no longer trust my Mares Nemo Wide. I knew the Nemo was conservative, but on my last few dives it has been giving me much shorter NDLs compared to the other divers I was with which were very experienced divers. These divers were hugging the reef more than I (photogragher and searching for lion fish) so had deeper profiles and still I always had to ascend to a shallower depth long before they did because I got within a few mins of my NDL. They are using regular rec computers, I think Suunto. After one 2nd dive, I saw one of the divers looking me because I was so shallow (compared to them). I gave him the 'ok' sign and continued following them from above. After he asked why and said he know it can't be air. I told him I was getting close to deco which surprised him as he was no where near deco at that time.
On a dive last week, it did the same. On the 2nd dive I got to about 4 NDL, so started to get shallower, but it wasn't giving me much 'credit'. At 37ft it was still showing me 4 mins NDL. As I approached 30ft it went to 14mins. Then when I got to 29 it showed 99mins. I decided to stay there a bit then slowly started to descend from 30ft and as I approached 40ft I was back at 2 mins NDL. I just went back up and stayed there watching my buddies on reef for quite a while.
¯\_(ツ)_/¯

 

I reached out to Mark Powell after the Navy study. He responded as follows:

Hi Aziz, that is a great question. My views are much closer to Simon Mitchell than Bruce Wienke although to be fair to Bruce both Simon and myself used to have the same views as Brice and have only changed in recent years as a result of the latest research. I have attached an article that goes into more detail on this. This will be included in a future edition of the book.

Delving Deeper into Deep Stops - Divernet

 

I think that is ( if I understand your statement) a work around perhaps to not haveing user adjustable settings or in some cases the conservatism ( from what I have been able to reaserch) used the 0 conservative value of NDL and then reduced it by x amount for the conservative levels or used say a depth reading of 100 ft and provided the NDL for deeper depths to create conservatism through tth NDL component. GF's now are very common and concervatism setting accessable by the user is also very common no matter how the computer accomplishes the calculation. I prefer actual GF's being used as opposed to doing things like take the M value NDL x 95% for low 90% for med and 90% for high etc. Surely other methods have been used in the past but a GF is by definition a percentage of the un padded M value. I was surprised how this had been done in the past. Mow I cant speek for computers that do nto use GF's I have no clue how the rgbm thing is handled when it comes to conservatism.

 

this is the comment you seem to have misinterpreted...

 

Wow I havent heard this for some time. Im no expert at all just lightly read. What I can gathered form the reading I have done which I can not cite. most computers if equally set up are about the same for the first dive. after that the numbers go crazy. due to how they use residual and from what compartment and the definitions of those conpartment groups.

 

Hello @CaveSloth
Every other response here has talked about NDLs and time planning. Mostly good stuff, knowing about how long you can stay at any one or even a combination of depths is a good idea. But even staying within NDLs that requires a gas plan. For example, if you stay right up until the NDL at 90ft on an AL80 and then your buddy goes OOA... do you have enough gas to share and ascend?

Doing a plan ahead of time is the best time to decide on what your minimum gas is. Some dives are NDL limited, some are gas limited, some are limited by both.

 

Sorry I read it as they do not have settings availiable. My fault everyone