Safety Stops, NDL, and other numbers

[tcpip95]

@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.

Steve, you raise a great point. As a relatively new diver, I know to "use" the dive computer, but don't "rely" on it. IOW, plan the dive, and use the computer to verify the dive. I found it interesting about how it calculates 12 separate "tissue compartments" and then uses the most conservative calculation.

I pulled this out of my Genesis ReACT Pro manual:

MULTIPLE TISSUE TRACKING
The ReACT Pro tracks 12 tissue compartments with halftimes ranging from 5 to 480 minutes. The NiBG always displays the controlling compartment that is the only one important at that time.

Think of the NiBG as transparent displays laid on top of one another. The tissue compartment that has filled up fastest is the only one that can be seen from the top.

At any particular point, one tissue compartment may be absorbing nitrogen, while another that was previously higher may be off gassing. One compartment hands over control to another com-partment at a different depth.

 

[KeithG]

..., The computers are about 3 inches apart on my arm.

How accurate are their "depth" (pressure) sensors?

Diving is not a highly accurate endevour. Hence no real need for "weights and measures" accuracy Weights and Measures Regulations.

It's not like your are using them to buy meat at your local grocery store https://www.nist.gov/sites/default/files/documents/2017/04/28/hb-155-final.pdf

So you can expect some observable deviation across units of the same model. My PADI dive tables use 10 foot compartments (if you round that is 5 foot accuracy). I would be very happy if my various dive computers were within 12 inches of each other. But they may not be?

Is Your Dive Computer Correct?: Undercurrent 05/2013

 

[Neilwood]

My guess would be a slight inconsistency in the pressure sensors. Enough to mean that they read the depth fractionally differently but close enough in reality to be safe.

 

[CT-Rich]

The computers, if identical should match up pretty well if they are sitting on the same arm. The one closer to the wrist will have the slightly greater range but the basics should be virtually identical when comparing the dive profile on a computer. The physiologic differences between divers is going to be much greater than those between the computers of the same brand. QC for humans is pretty bad . I expect ScubaPro to do much better.

 

[АлександрД]

I can suspect, that reason was:
You started you safety stop.
You been near bottom level of safety stop range.
One swing of your hand put one of the computers below minimal safety stop depth, and it starts counting safety stop time again, from the beginning. Another one just continue safety stop time.

and according to the topic title: comparison of different NDLs

 

[Blasto]

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.

I don't think that should be considered an argument against computers. If anything, it exposes the awkwardness of safety stops in PDC controlled diving.

Safety stops on computers are a tack-on. The computer spends the whole dive tracking your exact tissue loading of each compartment, it knows exactly what your saturation is and where... then it has to count down an arbitrary amount of time at an arbitrary depth, regardless of everything it knows.

If table diving had never existed, say if the demand regulator was invented after the computer, nor would the safety stop. This isn't to advocate against doing the safety stop, just for treating it as what it is, a historical tradition rather than an optimal practice.

It's difficult to design a PDC that wouldn't have this issue, because it's a matter of requirements.
Suppose the book defines safety stop as 15' to 20' - then, if it's 1" shallower or 1" deeper than the allowed range, is it still a safety stop? But that just extends the range. Is it "half a safety stop"; does it give 50% time credit or (100 - deviation*19.35)% time credit?

Once you do fuzzy math, you're basically creating a new model; and if you do that, you might as well make it a modern one and count down based on Buhlmann 50/30 (sic). So computers stick to all or nothing, as the only option that won't cause any unexpected behavior. Except when one computer ends up an inch above and the other an inch below the threshold...

 

[Steve_C]

I don't think that should be considered an argument against computers. If anything, it exposes the awkwardness of safety stops in PDC controlled diving.

The computer spends the whole dive tracking your exact tissue loading of each compartment, it knows exactly what your saturation is and where...

Not really. The computer takes its data and tracks the tissue loading of each compartment as predicted by a mathematical model inside it. The computer takes no measurements of my actual loading. But its model is based on data drawn from a number of people and so is usually pretty reasonable for most people. Among its data is the % of my mix which is almost never exactly what I enter due to variance in O2 sensors. Note that the computer has no knowledge of the size of the compartments. But then the number and behavior of compartments is a mathematical approximation to a body also. If done right it is a good approximation to a complex organism.

It's difficult to design a PDC that wouldn't have this issue, because it's a matter of requirements.
Suppose the book defines safety stop as 15' to 20' - then, if it's 1" shallower or 1" deeper than the allowed range, is it still a safety stop? But that just extends the range. Is it "half a safety stop"; does it give 50% time credit or (100 - deviation*19.35)% time credit?
Once you do fuzzy math, you're basically creating a new model; and if you do that, you might as well make it a modern one and count down based on Buhlmann 50/30 (sic). So computers stick to all or nothing, as the only option that won't cause any unexpected behavior. Except when one computer ends up an inch above and the other an inch below the threshold...

Agree

 

[Blasto]

The computer takes its data and tracks the tissue loading of each compartment as predicted by a mathematical model inside it. The computer takes no measurements of my actual loading.

Yes, of course, the PDC just knows its model's parameters. Easy to forget the distinction. But that's a lot of parameters for that 16-dimensional continuous model (or a different complex one) to a fairly good of precision. The safety stop rule is a 1-dimensional 2-value discrete model: do it if you went deeper than D.

I'm contemplating dropping the safety stop when in charge of the dive and doing a low flat GF instead - could lower post-dive fatigue by probably a negligible amount through better use of these few minutes. OTOH, the SS is universally recognized and a good time for some drills.

 

[shurite7]

the SS... and a good time for some drills.

This is something I've been telling my students and encouraging others to take up.

 

[Steve_C]

I'm contemplating dropping the safety stop when in charge of the dive and doing a low flat GF instead --- OTOH, the SS is universally recognized and a good time for some drills.

I am sure if I tried to skip it I would get some very upset divers who view it as religion. I am amused how I will see folks do a careful safety stop and then shoot to the surface when done.

Unless it is rough. I like SS. I find them relaxing. You just hang there and watch what drifts by, schools of fish come through, a cuda gives you the old eyeball, or like two weeks ago a pod of dolphins checked out the folks on the hang bar.