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that's the problem. You don't know. Lots of companies use "customer algorithms" where they pad them with a whole bunch of crap. You'll lose time for short surface intervals, fast ascents, sawtooth dives, etc etc and you can't predict it.
So with the Petrel you get to choose the gradient factors that determine how NDL's and decompression stops are calculated. For recreational divers the important one is GF-High where that number correlates to the theoretical tissue loading when you surface. This algorithm is public and is available in many different decompression planning programs so you can accurately plan dives. The Mares uses a proprietary algorithm and luckily has a dive planner out so you can actually plan your dives, but you have no control over the conservatism of the computer. If you want to dive more conservatively, you have to cut dives short, and even then you don't know. If you set GF high to 85, you are theoretically coming out of the water with your tissues 85% loaded. If that isn't conservative enough, you can drop it to 75, or 70, or whatever, or if you want to be more aggressive you can increase it to 90/95/99 whatever. *PADI tables roughly correlate to 45/95*. You can figure out what gradient factor somewhat correlates to the Mares algorithm, but it's trial and error until you figure out what the approximate GF is. This is true across all of the recreational dive computers btw.
Oceanic Dual Algorithm | Simply Scuba Blog
You can read about the oceanic algorithms here and see where they differ from each other.
https://www.shearwater.com/news/flexible-control-of-decompression-stress/
Little bit about Gradient Factors there
Jim is talking about using VPM with conservatism settings. I don't really like VPM because of the deep stops, but to each his own. I use Buhlmann with Gradient Factors to control mine. With the Buhlmann side you have almost unlimited ability to set the GF's since they are in +_1 increments on both sides. VPM is just one of the five settings. +4 is about where I dive most of the time, but I have bad knees so I'll deal with a little extra deco. In recreational mode you can choose low, 45/95 *similar to PADI tables*, 40/85 which is similar to what ratio deco comes out with, and high of 35/75 which is comparable to what a lot of tech divers use for OC diving. VPM +2 correlates to something like 20/90. Too aggressive for me. This is all way above where you are now, but there are lots of papers written about this if you want to dig into them
So with the Petrel you get to choose the gradient factors that determine how NDL's and decompression stops are calculated. For recreational divers the important one is GF-High where that number correlates to the theoretical tissue loading when you surface. This algorithm is public and is available in many different decompression planning programs so you can accurately plan dives. The Mares uses a proprietary algorithm and luckily has a dive planner out so you can actually plan your dives, but you have no control over the conservatism of the computer. If you want to dive more conservatively, you have to cut dives short, and even then you don't know. If you set GF high to 85, you are theoretically coming out of the water with your tissues 85% loaded. If that isn't conservative enough, you can drop it to 75, or 70, or whatever, or if you want to be more aggressive you can increase it to 90/95/99 whatever. *PADI tables roughly correlate to 45/95*. You can figure out what gradient factor somewhat correlates to the Mares algorithm, but it's trial and error until you figure out what the approximate GF is. This is true across all of the recreational dive computers btw.
Oceanic Dual Algorithm | Simply Scuba Blog
You can read about the oceanic algorithms here and see where they differ from each other.
https://www.shearwater.com/news/flexible-control-of-decompression-stress/
Little bit about Gradient Factors there
Jim is talking about using VPM with conservatism settings. I don't really like VPM because of the deep stops, but to each his own. I use Buhlmann with Gradient Factors to control mine. With the Buhlmann side you have almost unlimited ability to set the GF's since they are in +_1 increments on both sides. VPM is just one of the five settings. +4 is about where I dive most of the time, but I have bad knees so I'll deal with a little extra deco. In recreational mode you can choose low, 45/95 *similar to PADI tables*, 40/85 which is similar to what ratio deco comes out with, and high of 35/75 which is comparable to what a lot of tech divers use for OC diving. VPM +2 correlates to something like 20/90. Too aggressive for me. This is all way above where you are now, but there are lots of papers written about this if you want to dig into them