Safety Stops, NDL, and other numbers

[Bob DBF]

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.

PADI initiated safety stops as a matter of procedure in 1984 in their S.A.F.E. Diving program. Prior to that the training was to be able to stop your ascent in that range in order to avoid boat traffic or in the case of emergency deco.

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.

The computer continues to track your exact tissue loading, and will show as less tissue loading on the start of the next dive, if you do the stop. In that case the stop is an add on, and taking it is your choice. In the case of some computers, they will penalize you for skipping the stop,but I avoid those computers.



Bob

 

[KenGordon]

The computer continues to track your exact tissue loading, and will show as less tissue loading on the start of the next dive, if you do the stop.

You off gas fastest at the surface, so spending that time at a safety stop will mean a higher tissue load at the same (clock on the wall) time later and so at the start of the dive assuming it starts at the same time. If you want to start the next dive with a lower loading then you need to wait longer before the next dive.

The advantage of the SS is to reduce the peak initial loading on surfacing.

 

[Bob DBF]

I understand, I probably was thinking ahead of my writing and missed a step. Point being that the computer tracks your loading and with an equal surface interval, you will have less nitrogen load at the start of the next dive with a safety stop than without.

Personally, I'm new to the safety stop, and 30 fpm ascent, as my computer has had a dificult time training me over the last seven or eight years.


Bob

 

[EFX]

I understand, I probably was thinking ahead of my writing and missed a step. Point being that the computer tracks your loading and with an equal surface interval, you will have less nitrogen load at the start of the next dive with a safety stop than without.
Bob

I ran several dives through my spreadsheet and came to the conclusion that the medium and slow tissues retain more gas for safety stops with the SI being equal.

 

[dmaziuk]

You off gas fastest at the surface

Unless you believe in one of the models that say otherwise. There's one that thinks when "bubbles get too big" they need to be re-absorbed before you can off-gas efficiently, and one that thinks off-gassing turns linear once your delta pressure gets "too big". In the latter case you don't off-gas any faster, while in the former: you off-gas slower at the surface. Provided you get into that "too big" zone of course.

 

[KenGordon]

Unless you believe in one of the models that say otherwise. There's one that thinks when "bubbles get too big" they need to be re-absorbed before you can off-gas efficiently, and one that thinks off-gassing turns linear once your delta pressure gets "too big". In the latter case you don't off-gas any faster, while in the former: you off-gas slower at the surface. Provided you get into that "too big" zone of course.

This is SB, gas content models rule. Linear offgasing is still proportional to the pressure differential, it is how fast you decide to reduce that differential that is different, it Is still faster with a bigger differential.

If the bubbles get too big a SS is the least of your problems.

Being real for a moment, the simplistic gas content models will actually give you shorter second dive, or require a later second dive if you actually do a safety stop. It will likely be insignificant but you will have a higher calculated tissue load. Those models do not directly work on the risk, but on a set of limits and coefficients that ought to even out at a measure of risk. At the edges they might point the wrong way.

 

[EFX]

Regardless of the algorithm the rate of off-gassing is going to be determined in part by the difference in pressure between the tissues and the inspired gas. To generalize the concept we can simplify the gas content equations to:

Flow = (Pt - Ig)e/r
Flow = (Pt - Ig)n/r

where Pt = tissue pressure, Ig = inspired gas pressure, e = exponential factor, n = linear factor, r = resistance to flow.

returning from a dive of 80 ft directly to the surface gives:

Flow = (80 - 0)/r = 80e/r
Flow = (80 - 0)n/r = 80n/r

returning from a dive to a 15 ft safety stop gives:

Flow = (80 - 15)e/r = 65e/r
Flow = (80 - 15)n/r = 65n/r

Flow rate is pressure divided by resistance per unit time. Clearly, going directly to the surface gives the fastest off-gassing rate. Stopping at 15 ft however briefly may add more inert gas to the slowest tissues. Adding the safety stop reduces the rate to make the dive safer at the expense of greater tissue pressure for the same SI. Thankfully, dive computers keep track of this pressure and use it for any subsequent dives.