No stop limit vs. No stop time remaining

[JMarc]

The dive program simply processes real-time data based on sensor input, stored dive profile parameters and past data. An assumption implies that there is a possibility of another course of action that the computer cannot possibly know.

The assumptions are built into the program and were made by its designers. They had other courses of actions and they chose one based on more or less conscious assumptions and always difficult to take into account prejudices.

It is usually harmless to anthopomorphize the system formed by the computer and its program and give him attributes of its creators. That's usually harmless for most practical purposes as long as you are aware of the shortcut taken.

 

[dmaziuk]

M-values themselves are probabilistic, derived from bent goats and burly 19th century caisson workers. If you are a surly 19th ct caisson goat diving 100/100 to the NDL, they should reflect your chances reasonably well. If you're a 21st ct retiree diving 50/70 and surfacing with GF 42 -- who knows. We assume it's "safer" that way, but we don't actually know.

 

[EFX]

The assumptions are built into the program and were made by its designers.

So, let's keep the conversation focused on the calculation of NDL. I'm currently diving at a depth of 60 ft. I glance at my computer to discover that my NDL is 30 minutes. At that point in time I have three options: (1) I can stay at the current depth, (2) I can go deeper, or (3) I can go shallower. Which of these three assumptions is built into the program?

 

[Jcp2]

Is this a trick question? If you stay at the same depth the NDL remaining will run down in real time. If you go deeper the NDL time will run down faster than real time. If you go shallower the NDL time will run down slower than real time.

 

[Zef]

Is this a trick question? If you stay at the same depth the NDL remaining will run down in real time. If you go deeper the NDL time will run down faster than real time. If you go shallower the NDL time will run down slower than real time.

it doesn't run down faster or run down slower. When you change depth, the computer recalculates the NDL for the new depth taking into account factors such as previous depths reached during the dive and time spent at those previous depths. The computer displays its constant recalculations until one ends the dive.

-Z

 

[dmaziuk]

So, let's keep the conversation focused on the calculation of NDL. I'm currently diving at a depth of 60 ft. I glance at my computer to discover that my NDL is 30 minutes. At that point in time I have three options: (1) I can stay at the current depth, (2) I can go deeper, or (3) I can go shallower. Which of these three assumptions is built into the program?

How about this one: you just jumped in the water and are going down. Your current depth is 60 ft and calculated NDL is 30 minutes, however, you're still descending to your planned bottom at the sand at130' from where you plan to ascend checking out the nooks and crannies, all the way up to the wheelhouse at 80'.

Does your calculated NDL of 30 minutes has any relevance to your planned dive?

 

[JMarc]

So, let's keep the conversation focused on the calculation of NDL. I'm currently diving at a depth of 60 ft. I glance at my computer to discover that my NDL is 30 minutes. At that point in time I have three options: (1) I can stay at the current depth, (2) I can go deeper, or (3) I can go shallower. Which of these three assumptions is built into the program?

I don't know, I haven't looked at the code of any dive computer. (1) is the intended interpretation of the number.

The computation could be different; for instance the designers could have assumed that the diver isn't that stable, or the pressure sensor imprecise, and added three feet to the current depth for the computation of the NDL. I know of no documented case of such safety margin, but there are small discrepancies when you compare traces of dive computers with the computations made by desktop software and that's one plausible origin for them. Which has documented precedent in the way tables where computed.

 

[tursiops]

How about this one: you just jumped in the water and are going down. Your current depth is 60 ft and calculated NDL is 30 minutes, however, you're still descending to your planned bottom at the sand at130' from where you plan to ascend checking out the nooks and crannies, all the way up to the wheelhouse at 80'.

Does your calculated NDL of 30 minutes has any relevance to your planned dive?

Why should it? How could it? Why do you ask? Are you confused? Are you bored?

 

[Jcp2]

I don't know, I haven't looked at the code of any dive computer. (1) is the intended interpretation of the number.

The computation could be different; for instance the designers could have assumed that the diver isn't that stable, or the pressure sensor imprecise, and added three feet to the current depth for the computation of the NDL. I know of no documented case of such safety margin, but there are small discrepancies when you compare traces of dive computers with the computations made by desktop software and that's one plausible origin for them. Which has documented precedent in the way tables where computed.

I think my Cressi has a MOD safety factor of 2-3 feet less than the calculated MOD for pO2 of 1.4.

 

[tursiops]

I think my Cressi has a MOD safety factor of 2-3 feet less than the calculated MOD for pO2 of 1.4.

See this thread: Cressi Leonardo - MOD vs PO2 discrepancy
and especially this post in that thread: Cressi Leonardo - MOD vs PO2 discrepancy