Second-hand (i.e., sub-minute) timing for deco stops?

[rab]

I'll start by stating that I'm not a tec diver (and might never take that route), but a statement with regard to timing deco stops struck me as odd.

In another thread, someone asked about whether a computer or bottom timer counted/displayed only in minutes or also showed seconds. The idea being that "short" deco stops could be timed more closely. Someone else then suggested that the stop should just be extended by the extra fractional minute to be more conservative. Then, a comment that by extending the stop, you could start on-gassing rather than off-gassing.

OK, long setup for a short question: If you're off-gassing, wouldn't you just reach an equilibrium at that particular depth/pressure? Assuming that the planned time of the stop would actually get you all the way to an equilibrium state, there isn't really a way to start on-gassing at the stop depth, right?

-Rob

 

[theskull]

If you're not, and don't plan to become, a tech diver, then you have no need to spend the time and effort to learn about the different tissue types (some fast and some slow) and their relative absorption speeds. Unless you plan to become a divemaster, in which case it will be part of your required education. If you care, either buy a PADI Encyclopedia of Diving or any book on diving decompression theory and study up.

The only "equilibrium" is saturation, and then you have some serious decompression to do!

theskull

 

[ElectricZombie]

A few seconds is not going to matter.

 

[SDAnderson]

OK, long setup for a short question: If you're off-gassing, wouldn't you just reach an equilibrium at that particular depth/pressure? Assuming that the planned time of the stop would actually get you all the way to an equilibrium state, there isn't really a way to start on-gassing at the stop depth, right?-Rob

Reaching equilibrium at a particular depth/pressure is called saturation. Recreational divers (and for that matter, technical divers) only reach saturation on the surface, if they're lucky. The reason for the guideline about delaying flights after diving is because you won't reached saturation after a dive for many hours - you're still a little gassy for a long time after a dive. The objective for decompression (including slow ascents and safety stops) isn't to achieve equilibrium, it's to allow you to get out of the water safely AND as quickly as possible. Stay too long at a stop and you may very well stop off gassing and start on gassing. Even weirder is that you can be off gassing some tissues and on gassing others at the same time.

Seconds count in technical diving, especially if you're looking at a long deco in 33F water. Don't worry about it if your diving is recreational, just follow the tables or a nice conservative computer program and control your ascents - you'll be fine. Probably.

Steven

 

[Uncle Pug]

Another issue to consider is gas supply. If you start adding in extra minutes at each stop because you are not progressing through you decompression as planned you could end up short of deco gas. Having a timer with seconds showing helps keep the deco schedule and ascent rate on track.

 

[rab]

... The objective for decompression (including slow ascents and safety stops) isn't to achieve equilibrium, it's to allow you to get out of the water safely AND as quickly as possible. Stay too long at a stop and you may very well stop off gassing and start on gassing. Even weirder is that you can be off gassing some tissues and on gassing others at the same time.

Steven

Thanks. This makes a lot of sense to me and I really appreciate your taking the time to give me a useful answer.

-Rob

 

[landfish]

dumb question here but I got to ask
How could you on gas if you are 20 ft breathing 80 percent?
I am a little confused am I missing something here?

 

[SDAnderson]

dumb question here but I got to ask
How could you on gas if you are 20 ft breathing 80 percent?
I am a little confused am I missing something here?

Only if you're doing some very high altitude diving.