How long of SI after the computer dies?

[ctref4]

Another thing to consider is this is a new or close to new diver and a computer that has died under water might cause some stress and belief that a bad choice of uncontrolled accent, remember the computer is dead, and failed or wrong depth stops could couse DC or embalisms. A back up computer is a good choice along with a watch to time your accent. Belief that only stops are good and not at the right depths could also be bad. Reasoning behind SSI's belief that 24hrs is a good rule of thum is to completly rule out any N2 in the system. better safe than sorry.

 

[gmhicks5867]

SDI's standard is to wait 24 hours if your computer fails as well. The primary reason here is that the computer does a lot more adjustment to your bottom time than pure tables do. Because of this, if you are on your dives and are following the computer's NDL status, do you know for sure what your max depth was, or your time of dive etc if the computer does fail.

Also if the computer failed on a repetitive dive have you logged your prior dives yet where you could reference the tables.

In both cases if you just dove a table (not wheel) profile your 100' dive may have given you 10 minutes total bottom time but say you did the 100 feet, then moved to 60 etc. With the computer adjusting the NDL based off the profile then you may have actually done a 50 min dive rather than the 10 minutes on the table. Since this is the case, then the straight table would have put you outside the NDL and you would not be able to use the tables for another dive for 24 hours.

 

[Charlie99]

In both cases if you just dove a table (not wheel) profile your 100' dive may have given you 10 minutes total bottom time but say you did the 100 feet, then moved to 60 etc. With the computer adjusting the NDL based off the profile then you may have actually done a 50 min dive rather than the 10 minutes on the table. Since this is the case, then the straight table would have put you outside the NDL and you would not be able to use the tables for another dive for 24 hours.

I disagree.

The question is not whether or not you are outside the NDL as calculated on a square profile table, the question is whether or not you are inside the limits of the model that the table is based upon, and whether using that table to then plan future dives will keep you within the model limits.

Let's take the example of an Oceanic computer, which uses the same DSAT/PADI model upon which the PADI wheel and RDP are based upon. If you are inside NDL limits of the computer, then you are inside the NDL limits of the model.

The Wheel and the RDP track ONLY the 60 minute compartment for pressure groups. One cannot exceed pressure group Z and not exceed the model limits (i.e. have the computer into deco).

It doesn't matter that a multilevel dive would be far, far beyond the NDL as calculated on a square profile table. What matters is that if it was an NDL dive per the MODEL (which is accurately calculated by the computer) that one is still in the NDL regime. And because of how the table is designed, assuming pressure group Z is a conservative procedure.

Charlie Allen

 

[gmhicks5867]

I disagree.

The question is not whether or not you are outside the NDL as calculated on a square profile table, the question is whether or not you are inside the limits of the model that the table is based upon, and whether using that table to then plan future dives will keep you within the model limits.

Let's take the example of an Oceanic computer, which uses the same DSAT/PADI model upon which the PADI wheel and RDP are based upon. If you are inside NDL limits of the computer, then you are inside the NDL limits of the model.

The Wheel and the RDP track ONLY the 60 minute compartment for pressure groups. One cannot exceed pressure group Z and not exceed the model limits (i.e. have the computer into deco).

It doesn't matter that a multilevel dive would be far, far beyond the NDL as calculated on a square profile table. What matters is that if it was an NDL dive per the MODEL (which is accurately calculated by the computer) that one is still in the NDL regime. And because of how the table is designed, assuming pressure group Z is a conservative procedure.

Charlie Allen

If you have record of the times that you dove, especially on a square table and can adjust that then you would be safe. Also yes if you keep track of your depths and times, you can be just as accurate with a wheel on your NDLs as you can be on the computer.

With that said, if the person is diving the computer profile but is not keeping track of the times at various depths then there is a greater risk of being outside the ndl limits of the table or wheel. The main point was that there is too much risk of the diver not knowing the exact data. Also the computers do work off the various models such as the modified Haldanean model in the case of Aeris/Oceanic computers. But these computers can also be set to give a more or less conservative Deco setting (two options). Also all computers are not set using the same models or the same level of conservatism.

An example of that is Suunto computers are much more conservative on the NDL limits than Aeris is when Aeris is using the standard non-conservative model. I have tested this although I did not see how they related to each other when the Aeris was set on its conservative setting.

The other key point is your sampling times. Computers sample much more frequently than you generally would with a wheel. Are you going to do a sample of your dive times and depths at 60 second or less intervals when using a wheel or table? Probably not. Because of this the wheel is NOT going to be as finely tuned as the computer and you can therefore show up in Deco status with the wheel or the table even though they and the computer are using the basic same models.

 

[Gombessa]

I skimmed this article - did Ken Kurtis write it? I don't have the magazine with me, but I was under the impression his prior day's dives were decompression dives, hence the sit-out when the computer died...was this the case (and does it make a difference)?

 

[Charlie99]

......... Because of this the wheel is NOT going to be as finely tuned as the computer and you can therefore show up in Deco status with the wheel or the table even though they and the computer are using the basic same models.

One can easily plan a multi-level dive on a wheel that would be far into deco if calculated as a square profile dive on the RDP. Even so, it is a valid procedure to take the ending pressure group after that dive,(as calculated on the wheel) and then plan the next dive on the RDP. Do you agree with this? Do you see that this is still a safe procedure, even though the 1st dive is well into deco on the table? If you don't agree with this, then further discussion is meaningless.

Moving from a computer to the table is an analogous situation. If you are within NDL on the computer, then all compartments of the model are within their limits. Obviously, if all compartments are within their limits, then the 60 minute compartment is within its limit. This is the same as saying that you are at or lower than pressure group Z.

Since the controlling compartment for all dives greater than 45' depth are faster than the 60 minute compartment, they will offgas faster than the 60 minute compartment during the SI. This is why the table and the wheel can get away with just tracking the 60 minute compartment for repetitive dives. This is why assuming the maximum allowable loading in the 60 minute compartment (pressure group Z) is sufficient for transferring to the table, independent of the loading levels of all faster compartments (provided that they were all within limits -- i.e. you were still within NDL on the computer).

======================

It is important to keep in mind that the computer, the wheel, and the RDP are all based upon the same model, but don't track loading with equal accuracy. The table, due to the limitations of its format does a very poor level of tracking. Just because a multilevel dive would be far into deco on the RDP does not mean that the dive exceeds the model limits.

 

[jeckyll]

Charlie99: It's probably also worth pointing out that you can get quite different results with different computers.

 

[gcbryan]

Let me suggest a different twist on this. Forget the computer for a minute. There are two places for a diver to off-gas nitrogen: Underwater and on the surface. The "Surface Interval" is the time a diver is off gassing on the surface. As to underwater off-gassing, there are alternatives to the popular 3 minutes at 15' that can effect how much N2 you surface with, and therefore effect the amount of surface off-gassing one needs to be concerned about.

Say for example that you do a dive to 100', but instead of going straight up to 15' for 3 minutes, you do a deep stop, and then some stops at 10' intervals along the way up. And then, say you take 6 minutes to get from 20' to the surface. Would you come out "cleaner" than someone who just does a 3 minute stop at 15'? Would this then effect the amount of surface-off gassing you would need to take into account on the next dive?

Well, sure it would.

What I am suggesting here is that the amount of time on the surface between dives is really less important than the amount of time spent cleaning up before you get out. Blindly following some agency "rule" that is so general that it's rarely applicable to any specific diver isn't how I would suggest approaching it. I would prefer to think it through.

It depends on what you mean by "cleaner". I believe offgassing is an exponential process. Spending a few more minutes at a deep stop or at 10' increments would slow down the formation of bubbles (a good thing) but would not appreciably speed up offgassing. Offgassing takes place fastest on the surface and in any event since it's an exponential process you can't stay down long enough to make much of a difference.

You can reduce bubble formation but nitrogren will fully come out of solution only after 6 half-lives. Since most dives are no more than 1 hour you should be ok after 6 hours. If you do enough dives in one day then I suppose the slower compartments could be controlling.

I just don't think it's accurate to speak in terms of taking longer stops speeding up offgassing. Using 100% O2 as a deco gas does of course prevent ongassing during it's use in the first place but that was not in the discussion here. Since we're talking about thinking through things I just want to make sure we're thinking. Please correct any facts I've misstated.

 

[FranzKafka]

I am conservative. I dive with 2 computers for this reason exactly. But my life, or a life spent in a wheelchair is not worth the cost of blowing a day of diving. I do not want that risk (however minimal). What if the computer died for a reason?

Obviously a lot of you have TONS more experience than me and could assess the situation better. You all would be 100% capable of making the right decision.

But for vacation divers like myself I always assume it is best to err on the side of caution. Remember there are a lot of newbies lurking the boards here. They need to know every situation is different.

 

[gmhicks5867]

Charlie I am not disputing the idea that you move from the wheel to the square RDP between dives and back. What I am saying is that when you are diving a computer that even though it calculates your pressure with the same basic algorithms as do the the wheel and tables, the computer is taking samples more frequently and therefore adjusting the NDL time much more frequently than you would with either of the other manual methods.

Because of this if your computer fails and you do not have the exact times or depths, it would be very difficult for a person to safely revert back to the manual tables. Now if you did log your exact bottom time and depth and did have detailed information on times at various depths then you possibly could revert back. Regardless of the fact that you may be able to do this, Agency recommendations are thre that you do not do this as you cannot be sure that you did not exceed NDLs.

Add this to the fact that all computer brands do not give you the same amount of time underwater just as two wheels may not show the exact same data would lead to the argument that the diver with the computer should err on the side of caution and not dive again for 24 hours just in case they violated NDLs.