Please help me understand my computers conservatism : Nitek Q and Buhlmann ZLH16
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flots am
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00wabbit
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Yes it is a computer for all levels of diving. Currently I am using it on recreational dives so I intend to keep everything within the Ndls. I just don't want it set so conservative that it "puts me in deco" when it really isn't necessary. After reading a bit and playing with the plan mode and comparing to tables I am going to set it to cons1.
At 100ffw it gives my 9mins ndl at cons0
At 100ffw it gives me 16mins at cons1 - this is more in line with tables but still a few minutes conservative.
At 100ffw it gives my 9mins ndl at cons0
At 100ffw it gives me 16mins at cons1 - this is more in line with tables but still a few minutes conservative.
Nirvana
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I will try my hand at giving a concise explanation.
The Bühlmann algorithm works by calculating how saturated different compartments (bodily tissues) are with inert gas. It predicts a certain maximum value of saturation for each compartment at each depth. It works with the assumption that if none of your compartments go above 100% maximum supersaturation, you will not develop DCS.
The settings on the Niteq Q refer to how close to that theoretic maximum supersaturation you are willing to go before a deco stop is mandated. The first number dictates the level of supersaturation allowed on the leading compartment in the first stop. The second number indicates the supersaturation when you get to the surface. Therefore, GF 30/80 means you will do your first stop at a depth your controlling compartment is at 30% supersaturation. When you get to the surface, it's supersaturation will be at 80%. Remember that the amount of inert gas that can remain in solution without forming bubbles is proportional to the ambient pressure, which explains why the supersaturation percentage increases as you go up.
The settings on the first number (30 in the example above) indicates how deep the first stop will be. It is common for technical divers to choose 30 or thereabouts for this value. The second number indicates how close to the limit of the supersaturation you are willing to surface with. I have known of people choosing values above 100 (115, 120) due to, among other factors, a strong belief in their fitness and resistance to bubbling. Most people (myself included) seem to prefer values bellow 90.
The Bühlmann algorithm works by calculating how saturated different compartments (bodily tissues) are with inert gas. It predicts a certain maximum value of saturation for each compartment at each depth. It works with the assumption that if none of your compartments go above 100% maximum supersaturation, you will not develop DCS.
The settings on the Niteq Q refer to how close to that theoretic maximum supersaturation you are willing to go before a deco stop is mandated. The first number dictates the level of supersaturation allowed on the leading compartment in the first stop. The second number indicates the supersaturation when you get to the surface. Therefore, GF 30/80 means you will do your first stop at a depth your controlling compartment is at 30% supersaturation. When you get to the surface, it's supersaturation will be at 80%. Remember that the amount of inert gas that can remain in solution without forming bubbles is proportional to the ambient pressure, which explains why the supersaturation percentage increases as you go up.
The settings on the first number (30 in the example above) indicates how deep the first stop will be. It is common for technical divers to choose 30 or thereabouts for this value. The second number indicates how close to the limit of the supersaturation you are willing to surface with. I have known of people choosing values above 100 (115, 120) due to, among other factors, a strong belief in their fitness and resistance to bubbling. Most people (myself included) seem to prefer values bellow 90.
The behavior of computers running algorithms designed for technical diving can be a bit puzzling when doing NDL dives. My X1, for example, does not believe there are any NDLs at 50 feet or above, although various tables list them.
The very good Rebreather World article linked above sums it up in the last paragraph. Basically, the lower the first number on your gradient factor choice is, the earlier in your ascent you are expected to slow down or do stops; the lower the second number, the more time you are going to be asked to spend from that offgassing point to the surface (the more of your nitrogen you are asked to offgas before surfacing). What the "best" choice is is not at all obvious, nor is there consensus. Marroni's work suggests very strongly that there is a GF Low that is TOO low -- in other words, slowing your ascent too deep is a bad ascent strategy. Since most DCS in recreational divers is neurologic, that suggests that there is definitely a GF high that is too high . . . but what that is is not at all clear. Most technical divers use something in the 70 to 85 range, and most of them don't get bent . . . that doesn't at all mean those are the best, or the only choices.
The existence of multiple algorithms rendering different no-deco times and different decompression schedules just shows that there is a lot we don't know about decompression and how it ought ideally to be done.
The very good Rebreather World article linked above sums it up in the last paragraph. Basically, the lower the first number on your gradient factor choice is, the earlier in your ascent you are expected to slow down or do stops; the lower the second number, the more time you are going to be asked to spend from that offgassing point to the surface (the more of your nitrogen you are asked to offgas before surfacing). What the "best" choice is is not at all obvious, nor is there consensus. Marroni's work suggests very strongly that there is a GF Low that is TOO low -- in other words, slowing your ascent too deep is a bad ascent strategy. Since most DCS in recreational divers is neurologic, that suggests that there is definitely a GF high that is too high . . . but what that is is not at all clear. Most technical divers use something in the 70 to 85 range, and most of them don't get bent . . . that doesn't at all mean those are the best, or the only choices.
The existence of multiple algorithms rendering different no-deco times and different decompression schedules just shows that there is a lot we don't know about decompression and how it ought ideally to be done.
jcaplins
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Nuribromanski
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to understand what gradient factors are about you should have an idea what m-values are
http://www.cavecountrydiving.com/storage/Understanding M-values.pdf
Shearwater used to have 30/85 as factory setting on their Predator, the Petrel comes with 30/70 as default, so they obiously decided 30/85 was a little aggressive.
bruce said he is looking at moving his personal SWs to 40/70 or 40/75 last i saw. if u can stand it, the RBW deep stops threads R good reading
remeber that computer a in deco is computer b with 10 minutes of ndl time left. u could just bring the gas, and do the stops as Q calls 4 them. they will be quite short as long as you stay within ur buddy's ndls.
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