Safety Stops, NDL, and other numbers

[Steve_C]

I firmly believe in safe diving and think computers make me safer. But it is good to keep in mind that the numbers you see are approximations and subject to some variability. It is the built in safety margins that make this ok. That got driven home on a recent dive where I was coming up the anchor line, then across another line, and heading to the hang bar under the boat. In doing so I was between 15 and 20 ft for a bit and my hand was elevated at one point holding a line. I dive two scubapro computers on my left arm. They start the safety stop countdown at 15 ft and continue as long as I am above 20 ft. I looked at my computers and on one of them I had just cleared my safety stop and on the other I was just starting and it showed me with 2:54 to go. The computers are about 3 inches apart on my arm.

 

[nolatom]

Could it be as simple as one computer clicked on at 15 feet, the other would have at 14.9, but you went to 14.95 feet, then lingered at 16-20 feet? first computer started its clock, second one didn't.

Who needs a timer anyway? Just count to 180.

I typically check both my wristwatch and the computer time, since the former gives me seconds, the latter only minutes. Ain't all that much else to do anyway ;-)

 

[Steve_C]

Could it be as simple as one computer clicked on at 15 feet, the other would have at 14.9, but you went to 14.95 feet, then lingered at 16-20 feet? first computer started its clock, second one didn't.

I am sure it was that. Point was not how it happened but that I see all sort of discussions that treat the numbers they are getting as somehow high precision information.

 

[shurite7]

Not sure which SP computers you are using, but nolatom makes a good point. Some computers will end the safety stop countdown if you dip below 15' and start again when you ascend back to 15'. I've also seen others that will start the countdown and continue even if one dips below 15', yet remain counting if you are within a couple of feet of 15'.

 

[Steve_C]

Not sure which SP computers you are using, but nolatom makes a good point. Some computers will end the safety stop countdown if you dip below 15' and start again when you ascend back to 15'. I've also seen others that will start the countdown and continue even if one dips below 15', yet remain counting if you are within a couple of feet of 15'.

 

[Steve_C]

My computers are sort of the same. Same name but a year apart. They both keep counting as long as I stay above 20 ft but do not start until I hit 15.

 

[scubadada]

I believe both my computers start counting down the safety stop when I ascend above 20 ft and do not stop the countdown as long as I remain above 20 ft.

 

[Centrals]

My Uwatec Aladin does not has countdown so I just stay between 5 and 6m for 5mins. And if it was dive deeper than 30m then I will take an extra min at 3m.

 

[EFX]

But it is good to keep in mind that the numbers you see are approximations and subject to some variability.

No. If the computer is working OK the numbers you see are exact to a precision acceptable for safe diving . You may be confusing two different issues. The computer makes calculations based on a model that is an approximation to how the body stores and releases inert gases. The actual calculations are based on water pressure and time that are used in the formulas are exact as long as the program is correctly written (minor bugs notwithstanding).

I am sure it was that. Point was not how it happened but that I see all sort of discussions that treat the numbers they are getting as somehow high precision information.

The output on your dive computer is of a high enough precision to keep you safe. There is no variability in the calculations as long as the input data (pressure and time) is the same. If there is variability when the inputs are constant then the computer is bad. Memory can go bad. The pressure sensor can change over time. You can check the accuracy of your computer regarding depth by measuring the depth with a tape measure and comparing that reading to your computer. Use a different source for time to compare the elapsed time on your computer.

 

[woodcarver]

@Steve_C and @EFX
I think you're both right. The calculations the DC does, are in fact, very precise based on the inputs of depth and time. But I would agree that the precision is not as precise in practice because of other variables, such as the DCs position in the water relative to the body, natural arm movements and hanging on a line or bar, etc.. Is the precision of the calculations that important if you are vertical with your DC at chest level, or horizontal with your hands hanging down, or even if you are doing your SS inverted? As long as the majority of your carcass is somewhere above 20' and below 10', you're in the SS zone.
If it was necessary to be "absolutely" dead on accurate, it would be also be necessary to know exactly where a DC must be relative to your body and for it to always be right there. But what the DC is doing is very precise, based on ITS position in the water column.