Dirkadiver
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Redundancy is a beautiful thing.
Using tables as backup to computer
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halocline
Contributor
Well, since the tables use 10ft max depth intervals, you don't need to know your exact max depth, just come close enough to be able to intelligently choose which row on the table you're going to use. If my computer were to fail on a dive, how I would handle it would depend on the nature of the dive; if I were anywhere near NDL, or given the profile, if it was a dive in which I could expect to be near NDL, I'd do a longer safety stop. I usually do one anyways, and it absolutely has improved the way I feel after several days of diving. How I'd handle subsequent dives on the same day would also depend on the situation. If the dives were not anywhere near NDL, AND I had a reliable, trusted buddy, I'd consider consulting the tables using a best estimate of max depth/time on the dive in which the computer said bye-bye, and then do a second dive with said buddy, who would clearly understand that I was using him/her for depth info on that dive. I dive without a backup depth guage, but with a watch. That would work for me on a typical 2 tank dive, so that the computer failure did not happen on a repetitive dive, making estimates with the tables more complicated...you'd have to remember your SI to the minute, which I wouldn't. If it happened on the 2nd dive, I'd probably be through for the day, unless the dives were so shallow that the whole thing was academic.
Keep in mind this; computers vary wildly in how they compute NDL and deco requirements, and none of them are proven to be any safer than the others. Consequently, adhering strictly to a particular computer's NDL info (or it's translation into tables) is no guarantee of safety. Further, lets say that the broken computer was a suunto, and that subsequent dive behavior would have sent the computer into deco, had the diver still had a working computer. Another diver using an oceanic might do exactly the same profile and have no required deco. As such, using good, common sense dive practices like very slow ascents, good profile planning and execution, and staying hydrated, etc...probably is better insurance against DCS. One last thing; when entering into mandatory deco from NDL diving, I believe the initial required stop is always at 10-15 ft. It's only when you REALLY bust the computer's algorithm that you are required to do deeper stops. So, as an added safety precaution, an extra long safety stop mimicks what your computer would be telling you to do if it had survived the dive and you had in fact exceeded NDL by a small margin.
In short, I take the computer's calculations with a grain of salt, and instead trust my awareness of the profile and dive behavior, coupled with a nice long stop, to try to maximize my dive safety. Plus, I'm paying for the damn dives, plane ticket, hotel, etc....and I'm going to enjoy that extra 5-10 minutes underwater!
Keep in mind this; computers vary wildly in how they compute NDL and deco requirements, and none of them are proven to be any safer than the others. Consequently, adhering strictly to a particular computer's NDL info (or it's translation into tables) is no guarantee of safety. Further, lets say that the broken computer was a suunto, and that subsequent dive behavior would have sent the computer into deco, had the diver still had a working computer. Another diver using an oceanic might do exactly the same profile and have no required deco. As such, using good, common sense dive practices like very slow ascents, good profile planning and execution, and staying hydrated, etc...probably is better insurance against DCS. One last thing; when entering into mandatory deco from NDL diving, I believe the initial required stop is always at 10-15 ft. It's only when you REALLY bust the computer's algorithm that you are required to do deeper stops. So, as an added safety precaution, an extra long safety stop mimicks what your computer would be telling you to do if it had survived the dive and you had in fact exceeded NDL by a small margin.
In short, I take the computer's calculations with a grain of salt, and instead trust my awareness of the profile and dive behavior, coupled with a nice long stop, to try to maximize my dive safety. Plus, I'm paying for the damn dives, plane ticket, hotel, etc....and I'm going to enjoy that extra 5-10 minutes underwater!
There is some interesting information and advice as well as some not so hot advice floating around.
Dive computers have lower NDL's as in effecf the multilevel feature means that every dive is what would be termed a "square profile" on a table - ie: staying at max depth for max time. You don't do this with a table because it leaves no fudge factor or margin for eoor or any variation due to a number of factors (water temp, exertion, differences in individual metabolism, moon phase, etc.) So...computer NDL's are lower to build in this conservatism, but overall you get longer bottom times compared to a table as you seldom dive a truly square profile anyway.
That said if your computer dies during the first dive of the day, you should be tracking your dive time and max depth close enough to be able to tell if you are still within the normal (table) NDL, that you should have momorized. In that case finish the dive as a table dive and to the next one like any repetetive dive.
A failure on a repetetive dive is more complicated as your computer most likely did not give you a repetive dive group or RNT after the first dive and SI, and without knowing that it is hard to say exactly where you are at on a table. So your option is to basically call the dive and ascent, do a safety stop and get out (since you should have been looking at your computer often enough to know whether you were near the deco limit or not - and you should have been cross checking what the computer told you with a little common sense to assure what it was telling you was at least in the ball park.) Once on the surface you can try to reconstruct the dives prior to this one on a tabel and see where it leaves you. But unless you are using a wheel or a multilevel table technique that can be difficult as assuming square profiles will put you into pretty serious dco situation on the table that you did not really have. And even with a wheel, you need a very accurate memory of depths, times, etc and those depths will have to fall within the parameters required by the wheel.
Realistically if you are in multiday/multidive situation and your only computer dies, you are sitting out for 12 hours.
Alternatively, if you brought along an inexpensive hockey puck computer to slip in your pocket as backup, you are still good to go. In my experience, even diving nitrox with a nitrox computer on repetetive dives, most inexpensive air computers only need a few minutes at 10-15 feet to get out of the deco they are in. And you should be diving conservatively and doing a 5 minute safety stop anyway in order to exit in the green on repetetive multiday dives with your nitrox computer anyway. If you are doing that, you will not be violating your air computer (but it's a good idea to check it before surfacing to make sure to ensure it is still viable as a backup.)
Dive computers have lower NDL's as in effecf the multilevel feature means that every dive is what would be termed a "square profile" on a table - ie: staying at max depth for max time. You don't do this with a table because it leaves no fudge factor or margin for eoor or any variation due to a number of factors (water temp, exertion, differences in individual metabolism, moon phase, etc.) So...computer NDL's are lower to build in this conservatism, but overall you get longer bottom times compared to a table as you seldom dive a truly square profile anyway.
That said if your computer dies during the first dive of the day, you should be tracking your dive time and max depth close enough to be able to tell if you are still within the normal (table) NDL, that you should have momorized. In that case finish the dive as a table dive and to the next one like any repetetive dive.
A failure on a repetetive dive is more complicated as your computer most likely did not give you a repetive dive group or RNT after the first dive and SI, and without knowing that it is hard to say exactly where you are at on a table. So your option is to basically call the dive and ascent, do a safety stop and get out (since you should have been looking at your computer often enough to know whether you were near the deco limit or not - and you should have been cross checking what the computer told you with a little common sense to assure what it was telling you was at least in the ball park.) Once on the surface you can try to reconstruct the dives prior to this one on a tabel and see where it leaves you. But unless you are using a wheel or a multilevel table technique that can be difficult as assuming square profiles will put you into pretty serious dco situation on the table that you did not really have. And even with a wheel, you need a very accurate memory of depths, times, etc and those depths will have to fall within the parameters required by the wheel.
Realistically if you are in multiday/multidive situation and your only computer dies, you are sitting out for 12 hours.
Alternatively, if you brought along an inexpensive hockey puck computer to slip in your pocket as backup, you are still good to go. In my experience, even diving nitrox with a nitrox computer on repetetive dives, most inexpensive air computers only need a few minutes at 10-15 feet to get out of the deco they are in. And you should be diving conservatively and doing a 5 minute safety stop anyway in order to exit in the green on repetetive multiday dives with your nitrox computer anyway. If you are doing that, you will not be violating your air computer (but it's a good idea to check it before surfacing to make sure to ensure it is still viable as a backup.)
Charlie99
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My approach to the computer failure problem is relatively simple ----
If all of my previous dives were within NDL and I was still within NDL when my computer failed, then I assume that I am within the NDL limits of my dive table.
I calculate future dives as if I exited the water in the maximum pressure group.
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This works because the repetitive groups or pressure groups are based upon one and only one compartment of the model --- the 60 minute compartment for the PADI table, and the 120 minute compartment for USN and USN-based tables (NAUI, SSI, YMCA, etc.). My dive computer tracks multiple compartments, including the 60 and 120 minute compartments. If I have not exceeded the surfacing limit for the 60 minute compartment (i.e. my computer has NDL time remaining, which means that all compartment loadings are less then their surfacing limits) then I will not have exceeded pressure group Z on the PADI table.
Charlie Allen
If all of my previous dives were within NDL and I was still within NDL when my computer failed, then I assume that I am within the NDL limits of my dive table.
I calculate future dives as if I exited the water in the maximum pressure group.
----------------------------------------
This works because the repetitive groups or pressure groups are based upon one and only one compartment of the model --- the 60 minute compartment for the PADI table, and the 120 minute compartment for USN and USN-based tables (NAUI, SSI, YMCA, etc.). My dive computer tracks multiple compartments, including the 60 and 120 minute compartments. If I have not exceeded the surfacing limit for the 60 minute compartment (i.e. my computer has NDL time remaining, which means that all compartment loadings are less then their surfacing limits) then I will not have exceeded pressure group Z on the PADI table.
Charlie Allen
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If you and you buddy did the same exact dive and was together except for the giant stride and the 2 second a head went under the water are when it broke the surface how is that different how is that dangeous?
I see air to air, nitrox to nitrox and even air to nitrox on air tables are fine. Being they dove together for all dives. Please explain?
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