Diving without computers

[Storker]

There are two types of stops - stops that are recommended no matter what the dive profile (safety stops), and stops that "bring the risk back to 'normal' for this diver on this dive", as you put it. You can distinguish between staged decompression stops and mandatory safety stops if you like, but they both do the same thing (bring the risk back to normal), and skipping either increases your DCS risk. This is not the case for an optional safety stop, the reason for which is not dive specific and which we discussed upthread.

Can some of the confusion in this thread come from the fact for some of the stops that "bring the risk back to 'normal' for this diver on this dive", the risk connected with skipping the stop is outside accepted limits, but still significantly lower than one, but for others, the risk is so high that you're more or less guaranteed to be bent like a pretzel if you skip them?

BTW, your post was one of the most succinct definitions of a "mandatory safety stop" I've seen so far. Thanks.

 

[KenGordon]

I believe that you are making an arbitrary distinction. There is nothing physiologically different between what you are calling a mandatory safety stop and a regular deco stop, it's semantics.

All stops are imposed by the limits of tissue N2 overpressure (as opposed to total N2 loading). On a deep, long dive you are at risk of bubbling if you go directly to the surface at the standard ascent rate because you have so much N2 loaded that you can't offgass fast enough ascending at 30 FPS. On a dive within NDLs you could increase your DCS risk with an ascent rate violation. That's why the computer generates a mandatory stop. The computer can't tell if you are out of shape or dehydrated or have a PFO, but it can tell if you ascend quickly enough to cause a transient overpressure spike.

There are two types of stops - stops that are recommended no matter what the dive profile (safety stops), and stops that "bring the risk back to 'normal' for this diver on this dive", as you put it. You can distinguish between staged decompression stops and mandatory safety stops if you like, but they both do the same thing (bring the risk back to normal), and skipping either increases your DCS risk. This is not the case for an optional safety stop, the reason for which is not dive specific and which we discussed upthread.

That is why I said

There are stops. The purpose is mostly to reduce the probability of a bend. Whether you need these depends on the profile and how much risk you want to take.

Actually there is no difference between any of these stops. They all control risk. Sometimes the risk starts out comfortably low and people might disregard them out of hand, sometimes not.

The distinction is made in the manuals of computers. Hence RTFM.

BTW, you understand that not all NDLs come with the same risk? So the benefit of a safety stop on one dive on a table may be the same as a compulsory decompression stop on the same table.

 

[doctormike]

Can some of the confusion in this thread come from the fact for some of the stops that "bring the risk back to 'normal' for this diver on this dive", the risk connected with skipping the stop is outside accepted limits, but still significantly lower than one, but for others, the risk is so high that you're more or less guaranteed to be bent like a pretzel if you skip them?

BTW, your post was one of the most succinct definitions of a "mandatory safety stop" I've seen so far. Thanks.

Glad you like the post!

I don't think that it's a great idea to try to quantify the degree of risk among the various required (mandatory) stops. In one dive you go over NDLs by 2 minutes, incur a 5 minute deco obligation at 20 feet, you do 4 minutes and surface. In another you rack up an hour of deco and go straight to the surface. Obviously the risk is far greater in the second dive, but if it's this hard getting recreational divers to sort out mandatory stops and optional safety stops, having different levels of mandatory stops would REALLY muck things up!

 

[Storker]

I don't think that it's a great idea to try to quantify the degree of risk among the various required (mandatory) stops.

Personally, I agree completely. Doing that would, IMO, incur an unacceptable risk of normalization of deviance.

 

[dmaziuk]

My next question is, are we better off with identical dive computers, sharing one USB cord, or is having different models, as proof against one model's failure, worth the extra accessories?

I think the bottom line is DC failures are very rare. Just because having two of the same kind may not change them from "very rare" to "astronomical" -- merely to "very very rare" -- doesn't make them any less reliable.

The other side of it is if you all dive together and stay as close as you should, identical computers should be within a few minutes of each other: not as close as as your own backup, but if you're not pushing the limits, how close do you need them to still be "close enough" in case one fails.

In my case, I got a package price that would more than double if I went for 2 different models + extra accessories, so that was easy: the bang was clearly not worth the buck.