Deep Stops

This has been an interesting thread - the so-called "Pyle" stops are pretty much well accepted I think. Re Trimix there is a need for deeper stops due to the He in the mix.

Have a look at the stuff coming out of DAN's Dive Explore and Safe Dive projects which are monitoring real world diving practices. The early indications are that deep stops are more important than ascent rates. Deep stops are not just for Trimix are not just for Trimix.

What are your views on the value of a stop at the gas switch stage on accelerated deco. For example switch to EAN50 at 20/18m but make this into a "stop" by a slow gas switch one buddy at a time??

I have been using deep stops in all my decompression dives for the last 6 years. I average avout 60 such dives a year. Typically these are dives in the 30 to 45m range with bottom times of 30 to 50 minutes. My technique has varied in response to reading stuff, hearing stuff and doing stuff. My current technique has been stable for a little over 2 years.

I refuse to use any table with BSAC written on it for decompression diving. My plans are based on Buhlmann tables and I use Proplanner to check the schedules.

I use the Pyle method of halfing the travel distance to the first shallow stop. Deep stop times are always two minutes. Ascent rate between deep stops is always 10m/min. The ascent rate between shallow stops (mandatory stops) is always 3m/min. The final shallow stop is always made between 4.5 and 5.5m depending on surface swell. I include a 3m, 3min safety stop on all dives.

The gas switch is made at the MOD of the deco gas. I use a deco MaxPPO of 1.6 (and don't give a fudge what BSAC happen to say). A 2 min stop is tacked onto the switch to extend the time breathing at Max PPO. The 2 mins starts after both divers have completed the gas switch.

I use Air for all dives shallower than 45m. I use 80% as a deco gas for all mandatory decompression dives. Generally you will see me carrying a deco stage on most dives deeper than 25 meters.

Air + accelerated deco on 80% gives similar ascent times to an optimised Nx mix without accelerated deco. It has the advantage of avoiding the hassle of having to know what your max depth is going to be on the day and is very cost effective in terms of gas fills.

Personally I do not accelerate deco on mixes <70%. I have used 50% for accelerated deco in the past but had the same type of sub-clinical symptoms associated with using air. My explanation (no one elses) is that I feel a PPO < 1bar is inadequate for accelerated deco. With a 50% mix the PPO drops below 1 bar shallower than 10m, the depth range which accounts for the bulk of your stops.

I played with gradient factors for a while but have since abandoned them. I feel the deep stops are not deep enough and the shallow stops are too long.

YMMV

Regards
Matt

I played with gradient factors for a while but have since abandoned them. I feel the deep stops are not deep enough and the shallow stops are too long.

GFHigh > 100%

Scotty

:=I played with gradient factors for a while but have since abandoned them. I feel the deep stops are not deep enough and the shallow stops are too long.

GFHigh > 100%

Sounds like your moving into guinea pig territory there.

IIRC 100% refers to the % of Buhlmann's M-Value that the model will permit. Setting GFHi > 100% will allow a compartment to supersaturate above the limit which has been accepted as 'safe'.

Regards
Matt

Sounds like your moving into guinea pig territory there.

IIRC 100% refers to the % of Buhlmann's M-Value that the model will permit. Setting GFHi > 100% will allow a compartment to supersaturate above the limit which has been accepted as 'safe'.

True, but if you accept that the 'bend and mend' approach is wrong by adding deep stops then you don't need the over long shallow stops.

Scotty

>> GFHigh > 100%
>>
> Sounds like your moving into guinea pig territory there.

Not really. Many people do this (not me, I hasten to add - I'm a congenital coward). There's sufficient material published for it to be done in comparative safety.

> IIRC 100% refers to the % of Buhlmann's M-Value that the
> model will permit.

Not quite - that's that maximum over-pressure that Buhlmann's (final) published criteria will permit; the model allows you to do what you like with an M-value - it's just a parameter.

IIRC, Buhlmann's original derived values were very much higher than those currently used. He modified them because his test divers were getting bent; there's no magical property about the numbers he eventually chose, they're just the tool he used to control the DCS incidence. It is extremel;y likely that much of this incidence was down to the ascent procedure used which, by "modern" criteria, was provocative.

By performing a less-provocative asscent, then, it should be possible to allow a greater tissue saturation[1]. And the theory does seem to work...

> Setting GFHi > 100% will allow a compartment to
> supersaturate above the limit which has been accepted
> as 'safe'.

The difference, really, being that the "safe" level was only derived by provocative testing. Consideration of any instantaneous level as "safe" implies the use of a saturation-only model; as soon as we introduce any sort of bubble model, then it is bubble size that defines what is "safe", with tissue saturation being merely one factor that influences bubble size.

And if we're doing saturation-only models, deep stops are evidently a waste of time... ;-)

Vic.


[1] Tissue saturation is, of course, a mathematical concept in this context, so it's not really worth thinking *too* hard about the underlying biology, which always turns out to be far more complex than we'd imagined anyway...

I agree with what you said. Deep stops cannot be argued, their track record speaks for itself, in my opinion. On another note, a good friend of mine had repeated skin bend incidences over a period of about two years. They appeared to worsen each time they occurred. Finally, he took a major CNS hit that nearly killed him. A year later he took another, similar but worse CNS hit. I wonder if repeated skin bends could be an indicator of deeper, underlying problems?