Perdix CEIL, does it take into account the estimated off-gassing on ascent (with an assumed ascent rate)?

[stuartv]

Interesting qualifier... I think the most compelling evidence is this:

1. Shearwater has stated that surfGF assumes a teleport ascent.
2. In my logs at the time sample when reported NDL clicks from 1 minute to 0 minutes, surfGF equals the configured GFhigh.

Since teleporting takes no time, I conclude that Shearwater's NDL calculation assumes zero off-gassing.

When you are viewing your logs, what are you viewing them in? Shearwater Cloud? Subsurface? On the dive computer?

And how do you know what the software showing you the log is showing you a logged NDL and SurfGF rather than the software calculating those things?

 

[inquis]

And how do you know what the software showing you the log is showing you a logged NDL and SurfGF rather than the software calculating those things?

That's a good question. I'm looking at the logs in Subsurface, which shows both the reported NDL and the Subsurface-calculated NDL (which match to within one 10 sec sample period). I'm certain the Subsurface-calculated NDL shown is the "no-offgassing" variety.

FWIW, subsurface can compute the off-gassing version of NDL in the planner. On a dive to 100 ft on EAN32, there's more than 3 minutes difference between them. That's not something that's lost in computation roundoff.

 

[Rogerdd]

Just a quick update, still waiting for an answer from Shearwater…

 

[dmaziuk]

That seems weird, is that really possible to have a CEIL but no stop? My understanding is that the current stop is ceiling rounded to a multiple of 3m

yes

 

[dmaziuk]

Interesting qualifier... I think the most compelling evidence is this:

1. Shearwater has stated that surfGF assumes a teleport ascent.
2. In my logs at the time sample when reported NDL clicks from 1 minute to 0 minutes, surfGF equals the configured GFhigh.

Since teleporting takes no time, I conclude that Shearwater's NDL calculation assumes zero off-gassing.

It's the fastest tissue compartment that controls the initial ascent (which in the case of a no-stop dive takes you all the way to the surface). It is also the compartment with largest M0 i.e. it allows for greatest overpressure. I suspect if you run the numbers, you'll find that for a 5-minute TC you'll be looking at very deep short bounce dives past the NDL, for the off-gassing during ascent to make appreciable difference.

 

[jborg]

yes

View attachment 811974

That’s a lot of DC warnings.

 

[inquis]

is that really possible to have a CEIL but no stop? My understanding is that the current stop is ceiling rounded to a multiple of 3m

No, it's not, and your understanding is correct (always rounded up). Whether you clear that deco obligation by stopping at the stop depth or 10 ft lower or even leisurely ascending through 30 ft lower, etc. than the stop depth is immaterial. The stop depth -- that which you shouldn't pass -- changes after sufficient off-gasing.

Being at the stop depth yields more efficient off-gasing, but you may choose to be less efficient in order to see something cool. (E.g., @dmaziuk 's multi-level profile).

 

[dmaziuk]

That’s a lot of DC warnings.

It's a Cressi Leonardo. It's their distinctive feature.

 

[dmaziuk]

Being at the stop yields more efficient off-gasing, but you may choose to be less efficient in order to see something cool. (E.g., @dmaziuk 's multi-level profile).

That would be true if a) going straight up was the plan (it wasn't) and b) look closer: say, I have "the worst" of the ceiling at 25 msw. If I were to teleport to the surface, my 5-minute TC would've been over its M0 value by some -- not very large, considering the ceiling -- number.

However the ascent to the surface from 25 msw at 10 m/min takes 2.5 minutes which happens to be half of my leading tissue compartment's half-time, which means it would have off-gassed to 1/4th of its current loading (just over M0) minus 1 (sea level pressure).

The point of this thread is whether that would be enough to take it below its M0. I suspect it could have been on that profile and I'm pretty sure it would be on deeper "bounce" profiles.

 

[inquis]

The point of this thread is whether that would be enough to take it below its M0.

My take is the point of this thread is whether CEIL assumes an ascent. IMO, it doesn't, so it's on the diver to manage this grey area you're describing. (I agree with everything you've said, BTW.)

My advice for the OP would be one of these:

• If not trained in deco procedures, keep NDL nonzero. Ascend a bit as needed to achieve that.
• Otherwise, plan the multi-level dive with a direct ascent contingency. Verify the gas needed for the leisurely ascent is sufficient to cover the mandatory stop if a direct ascent was required at any time. (I agree for a bounce dive it probably will.) Ensure sufficient redundant equipment.