Decompression Stop Guidelines - What we have to do if got deco alert?

[DevonDiver]

Tom,

Deco is traditionally modeled based on numerous 'tissue compartments' that on/off-gas at varying speeds (Haldanian, Buhlmann dissolved gas modelling).

On a given ascent, one of those compartments will be 'controlling'. i.e. the compartment closest to it's "m-value". That's the maximum value of pressure differential between ambient pressure and the pressure of dissolved inert gas in the compartment

Decompression requires a higher pressure in the tissues than surrounding pressure (super-saturation). The higher the differential, the quicker gas is removed...up to the M-value, whereupon there is a predicted unreasonable risk of DCS symptomatic bubble formation.

Creeping up on ascent causes two effects:

1. Firstly, the differential (gradient) between tissues and ambient is kept very low, which does not provoke effective off-gassing.

2. Secondly, whilst faster compartments may be super-saturated and off-gassing... the slower compartments would not have reached saturation at depth. They will still be on-gassing at your current depth, even as you ascend.

If you you ascend 'riding an NDL', your fast tissues will be clearing out. However, the NDL remains low because slower tissues are still saturating and start to become the controlling tissues.

There's a big difference between a staged level, or distinctly multi-level ascent... and a slow, creeping ascent, especially where it's dictated by staying just 'ahead' of an NDL.

Riding up an NDL is a very inefficient ascent strategy and would leave you far more inert gas loaded on surfacing. I suspect this causes some 'undeserved' bends to happen.

 

[Tom_Ivan]

Cheers

 

[NAM001]

I dont agree with Devons definition that hitting unplanned deco is an emergency. Perhaps its just a legal term or a course diffrentiation in terms. For me the only emergency is OOA. everything else is just an inconvienience.. I know many that onthe second or third dive ran out of ndl time and still had over 1/2 tank of gas in 80 ft of water. Where is the emergency. I agree you have a potential one based on other factors. I would submit that there is only one true M line and if you have a PDC set to very conservative it reaches NDL way prior to the true NDL. When that happens have you really exceeded NDL? one puter says deco and the other says 5 - 10 min till. Isnt this a lot like telling you puter to say out of air at 400 psi. I think i would rather like to see a puter let you set the alert at NDL - x% like NDL15 for 15% propr to ndl or NDL5 fir 5 until ndl ect. Let the actual algorythims work with the real ndl value ad not some adjusted value. tables made it so much easier, no levels of conservatism with a chart. You dive the NDL10% alarm goes off and you head up and do the "required" 3 min safety stop never hitting actual NDL. Beats the heck out of new divers saying that NDL alarm is no biggy cause its probably nanny padded by 5 min or so and they skip all stops.

 

[Storker]

if you have a PDC set to very conservative it reaches NDL way prior to the true NDL. When that happens have you really exceeded NDL? one puter says deco and the other says 5 - 10 min till. Isnt this a lot like telling you puter to say out of air at 400 psi.

No. 0 pressure is 0 pressure. Everyone is OOG when the SPG shows 0. It's either black or white.

DCS isn't like that. Some people bubble easily, some people seem never to get bent. The former ought to use a more conservative algorithm to estimate their gas loading. It's a grey area, and most of us don't know where we are in the greyness since most of us are rec divers who rarely get bent. So we use indicators like age, BMI, physical condition, hydration, etc. which we believe are pushing us closer to or further away from the dark grey/black region, to determine what kind of conservatism we dial in.

When I'm diving at home, I have access to free nitrox. So I use EAN32 all the time. I was fine on air, and I hardly ever had to cut my dive short due to nitrogen loading, since my NDL time and my min gas time pretty much followed each other. But I'm pretty certain that my middle-aged body experiences less decompression stress on EAN than or air, because I hardly ever get lehargic after a couple of deep dives in one day any more. So I might have benefited from upping my 'puter's conservatism when I used air. I just didn't realize it.

 

[DevonDiver]

I would submit that there is only one true M line and if you have a PDC set to very conservative it reaches NDL way prior to the true NDL. When that happens have you really exceeded NDL?

Yes, you have. Because you made a decision to set your algorithm based on a self-awareness of your DCS pre-disposition for that dive.

Pre-disposition is an individual thing, and can vary from dive to dive. The basic DCS predisposing factors are well educated in recreational scuba training. Coupled with that is the personal awareness of the individual... what they learn about their relative DCS tolerances from their diving and/or medical history.

What one person may call 'high conservatism' could be a baseline (not conservative at all) for another diver.

What happens when you really exceed YOUR no-stop limit... and don't/won't/can't remedy that excess inert gas supersaturation before surfacing... is that you get bent.

 

[scubadada]

If the computer you're using to control your dive says you're in deco, then you're in deco, and you should satisfy the obligation.

 

[Diver-Drex]

I have trouble with the thought process here.

... if you have a PDC set to very conservative it reaches NDL way prior to the true NDL. When that happens have you really exceeded NDL? one puter says deco and the other says 5 - 10 min till. Isnt this a lot like telling you puter to say out of air at 400 psi...

If you chose your computer and set it up you have made the decision to follow what it says. If you want less conservative settings than set it up that way and then follow the computer. This isn't hard at all, I don't understand where this is coming from. PDCs don't have snooze buttons. The alarm says it's time to get up you get up.

 

[NAM001]

Im not sure i agree when yo can have 2 identical computers and one says deco and the other says not. all because of configuaration. The choice issue is not a factor. so many will run liberal to EXTEND THEIR DIVE TIME, as if it warts off deco limits and prevents gong into deco. The only thing setting does is change when the PDC go;'s into deco. I had an occasion to dive with someone. I had my LP95 tank filled to 3k and the kid with an AL80 was with was fiddling with the computer. I found out that he was changing his computer to more liberal so that he would not have to come up to soon and rob me of my dive time. Nice intent but bad logic.

 

[NAM001]

No. 0 pressure is 0 pressure. Everyone is OOG when the SPG shows 0. It's either black or white.

DCS isn't like that. Some people bubble easily, some people seem never to get bent. The former ought to use a more conservative algorithm to estimate their gas loading. It's a grey area, and most of us don't know where we are in the greyness since most of us are rec divers who rarely get bent. So we use indicators like age, BMI, physical condition, hydration, etc. which we believe are pushing us closer to or further away from the dark grey/black region, to determine what kind of conservatism we dial in.

When I'm diving at home, I have access to free nitrox. So I use EAN32 all the time. I was fine on air, and I hardly ever had to cut my dive short due to nitrogen loading, since my NDL time and my min gas time pretty much followed each other. But I'm pretty certain that my middle-aged body experiences less decompression stress on EAN than or air, because I hardly ever get lehargic after a couple of deep dives in one day any more. So I might have benefited from upping my 'puter's conservatism when I used air. I just didn't realize it.

STORKER You are speeking at an intelligence level that the majority of those that use PDC's do not have. For so many the PDC is a depth gage and SPG if an AI model. Most do not have a clue what is on their wrist other than that. What ever is on the main display is all the puter has to offer. that means depth , time, dive time , time remaining ,and ascent rate. A lot of bad assumptions are made from that stuff. Its not uncommon to make a dive and your buddy thumbs beause he hit NDL and doesnt understand why you did not. Then they find out that if they change the conservatism to less they can stay down longer. You and I know that means they can stay down longer before the alarm goes off. Comfiguration doesnt change the NDL tme.

 

[NAM001]

No. 0 pressure is 0 pressure. Everyone is OOG when the SPG shows 0. It's either black or white.
.

Perhaps bad example but the premis is that just because you set a computer to react AS IF it does not change the facts of whether YOU ARE.

You can get by with a liberal setting on a drift dive. but may have to go to conservative if you are working. Most users dont understand those things. They function with a one rule fits all. max conservative is max safe. which means max liberal is max dive time on a 2 tank dive. LOGIC STARTS GETTING A LITTLE DANGEROUS THERE.