Question Understanding Decompression Stops: Confusion with Shearwater Peregrine

[Njord fr]

Hello everyone,

Last week, for the first time, I did a decompression dive. I wanted to exceed my NDL to see how my Shearwater Peregrine would handle the stops. As shown in the graph below, I went through stops for 3m, then 6m, and finally 9m. However, when we began our ascent, the stops started to disappear...

I must have misunderstood how decompression stops work because I thought that once the computer displayed them, we had to follow them strictly. But it seems that the ascent rate has something to do with it?

Could you help me understand this better?

Thank you!

 

[Alekseolsen]

You "Deco ceiling" is the shallowest you can get in the water (rounded to nearest 3m) without going beyond your computers GF settings. When you have a slow ascent/stop deeper, you are off gassing even though your are deeper than the ceiling level. Effectively you have done the deco-stops, just somewhat less efficient by beeing deeper than the ceiling depth.

 

[inquis]

The computer assumes you will ascend at 10 m/min, so off-gassing before arriving at the stop will be somewhat limited. While the off-gassing was less efficient for your slower ascent, you took enough time that the stop cleared.

The problem is there are multiple tissues with different gas transfer rates. If you ascend too slowly, one of the slower tissues may become the limiting factor, prolonging the stop time and requiring more gas -- possibly more than you're carrying.

This is precisely why deco dives should be planned and executed according to the plan. This ensures you have the gas to do the deco time. You also need redundant equipment to be able to handle failures without surfacing (since directly surfacing with a deco obligation is just trading one problem for another).

I would urge you to seek proper training before continuing your experimentation. An approach of "just follow the computer" won't always work, and you may not enjoy the outcome when it doesn't.

 

[Learner Diver]

How much did you exceed your NDL, and at what depth?

 

[ABay Diver]

Related question, what’s the relationship between exceeded ndl and deco? Straight line, i.e. exceed by a minute and a minute of deco or something else? If the latter, why, whats the deco theory behind it?

 

[tursiops]

what’s the relationship between exceeded ndl and deco?

No simple relationship. Deco time accumulates much more quickly than the time exceeding NDL.
You can see that by playing with MultiDeco, or Subsurface, or examining deco tables like those from the USN.

 

[JMarc]

I would urge you to seek proper training before continuing your experimentation.

Seconded. And a dive below 40m with stops at 9m is probably not the best first step.

BTW, the fact that you got a first stop at 9m makes me wonder if your GF settings are up to date with the current understanding. ISTR that some Shearwater have presets emphazing deep stops more than currently recommended when on air. An additional reason to get training and continue to stay informed.

 

[Njord fr]

How much did you exceed your NDL, and at what depth?

As shown in the graph, first deco 3m/1min at 16min around 35 meters, then 2 minutes later first 6m deco stop, then 2 minutes later first 9m deco stop.

 

[Njord fr]

The computer assumes you will ascend at 10 m/min, so off-gassing before arriving at the stop will be somewhat limited. While the off-gassing was less efficient for your slower ascent, you took enough time that the stop cleared.

The problem is there are multiple tissues with different gas transfer rates. If you ascend too slowly, one of the slower tissues may become the limiting factor, prolonging the stop time and requiring more gas -- possibly more than you're carrying.

This is precisely why deco dives should be planned and executed according to the plan. This ensures you have the gas to do the deco time. You also need redundant equipment to be able to handle failures without surfacing (since directly surfacing with a deco obligation is just trading one problem for another).

I would urge you to seek proper training before continuing your experimentation. An approach of "just follow the computer" won't always work, and you may not enjoy the outcome when it doesn't.

Thank you for your answer.
Do you know any books to learn more about decompression ?

 

[Wibble]

The Shearwater is working perfectly. You effectively off-gassed whilst playing around in the shallow part of the reef.

Example of last weekend's deco dive to a wreck (max 50m/165ft). You can see where the ceiling is and how the diver (moi) stopped below them, moving up as the ceiling reduced. The NDL line's shown where this would be the NDL time which quickly reached zero as the body tissues absorbed the nitrogen (and some helium in this case). Being an open water dive, the hour's deco portion was done mid water with only a SMB reel bouncing up and down for company.

BTW the last ceiling stop is at 3m/10ft. Being it's the sea and how much buoyancy changes, it's safer to skulk around 4.5m/15ft to give a little safety margin.