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

[Rogerdd]

The deco stops will kick off once NDL goes below 0 (meaning SurfGF is > GFHi, or more precisely shouldn’t it actually be:
‘GF99 if immediately starting surfacing at an estimated ascent rate > GFHi’ as NDL does take into account off-gassing during the ascent?).

When NDL goes below 0, some deco stops are now required but these stops can clear automatically before reaching them (due to the off-gassing happening in real time while ascending at a rate different from the estimated ascent rate, for example different from 10m/min).

On top of the discrete deco stops that are multiples of 3m, there’s a CEIL depth which is the non-rounded minimun depth below which we must stay (as per the model, and based on the pre-defined GFLo for the first stop and GFHi).

-> I assume this CEIL, like SurfGF but unlike NDL, doesn’t take into account the off-gassing that will happen while ascending from current depth to that CEIL value? And that is why we can have a ceiling with NDL > 0 (and no stop)?

 

[BlueTrin]

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

 

[tursiops]

The deco stops will kick off once NDL goes below 0 (meaning SurfGF is > GFHi, or more precisely shouldn’t it actually be:
‘GF99 if immediately starting surfacing at an estimated ascent rate > GFHi’ as NDL does take into account off-gassing during the ascent?).

When NDL goes below 0, some deco stops are now required but these stops can clear automatically before reaching them (due to the off-gassing happening in real time while ascending at a rate different from the estimated ascent rate, for example different from 10m/min).

On top of the discrete deco stops that are multiples of 3m, there’s a CEIL depth which is the non-rounded minimun depth below which we must stay (as per the model, and based on the pre-defined GFLo for the first stop and GFHi).

-> I assume this CEIL, like SurfGF but unlike NDL, doesn’t take into account the off-gassing that will happen while ascending from current depth to that CEIL value? And that is why we can have a ceiling with NDL > 0 (and no stop)?

Sorry, I do not understand your post. Are you saying that CEIL is displayed on an NDL dive?
Shearwater says otherwise...

 

[VikingDives]

I think the question is:

On a Perdix, does the ceiling account for the off-gassing that one will do during the ascent, and if it doesn't, why not? (That's the question I'm answering anyway).

I don't think that there's anything in a Perdix that accounts for ascent, as there's no way for the computer to determine what you're going to do, or how fast or slow your ascent will be.

When I dive with recreational students, I leave my computer in tech mode, with my usual gradient factors. It isn't unusual for my computer to show up to 5 minutes of deco as we get to the end of our (recreational) dive, but I know that based on how we do our ascents in class, by the time I reach the safety stop, I'll have cleared the deco obligation entirely.

I also know if I were to a rapid ascent, I could blow right through the ceiling.

Personally, I don't like to get out of the water with a GF99 that's above 70, and I find 60 preferable, but if there was an emergency I'd take that into account too.

On dives with planned decompression obligations, the difference between the estimate and the actual stop is pretty negligible. The longer the hang time, the less of an effect your ascent will have on your total obligation (unless you're doing something weird like ascending very slowly, or adding in deep stops I guess).

More to the point, I wouldn't want a computer that accounted for my ascent - at least I don't think I would. I think it's more conservative to know that my surfGF is some number if I were to cork from any depth. I was doing a 10 foot stop recently and there were a bunch of OW students and instructors floundering about with minimal buoyancy control and swimming underneath me. One look at my surfGF told me that I didn't want one of them to accidentally drag me to the surface, so I dropped down to 20 feet until they went away.

 

[EFX]

-> I assume this CEIL, like SurfGF but unlike NDL, doesn’t take into account the off-gassing that will happen while ascending from current depth to that CEIL value? And that is why we can have a ceiling with NDL > 0 (and no stop)?

The dive computer makes calculations based on the current conditions of your dive. No calculations are done based on assumed future data (i.e. ascent rates). The dive program is on a loop that repeats every second (this could be faster or slower on some computers). Every second, tissue pressures, NDL time remaining, and ceilings are calculated. Depending on the algorithm used ceilings are calculated for all phases of the dive. For NDL > 0 minutes the ceiling will be negative. This is interpreted as so many feet or meters above the surface of the water which will be conveniently displayed as 0 (the surface) on your dive computer. Deco dives will show ceilings > 0 which are interpreted as feet or meters below the surface.

 

[EFX]

I think the question is:

On a Perdix, does the ceiling account for the off-gassing that one will do during the ascent, and if it doesn't, why not? (That's the question I'm answering anyway).

I don't think that there's anything in a Perdix that accounts for ascent, as there's no way for the computer to determine what you're going to do, or how fast or slow your ascent will be.

That's correct. At the current time, there are no dive computer options you can download that mimic crystal balls.

 

[iointerrupt]

There very much are things in the Perdix that assume an ascent rate.

https://www.shearwater.com/wp-content/uploads/2019/01/Perdix-Operating-Instructions-Manual.pdf

Page 15: "Note: Deco calculations assume 33fpm (10mpm) ascent rate"

As well as page 21, TTS.

Discussed more on page 41, "DECOMPRESSION INFORMATION ACCURACY".

That said, while it says TTS depends on the assumed rate, it doesn't make that note for CEIL. So perhaps ok to assume CEIL doesn't.

 

[Rogerdd]

No calculations are done based on assumed future data (i.e. ascent rates).

That's not really correct. As mentioned by @iointerrupt some numbers are calculated using an estimated ascent rate (10m/min for the Perdix). TTS is one of them for example. My understanding is that NDL is as well. My question was about CEIL, and I believe that unlike NDL, it is not calculated using an ascent rate. It’s an instantaneous ceiling.

For NDL > 0 minutes the ceiling will be negative. This is interpreted as so many feet or meters above the surface of the water which will be conveniently displayed as 0 (the surface) on your dive computer. Deco dives will show ceilings > 0 which are interpreted as feet or meters below the surface.

I think that's where we would need to clarify our definition of NDL. If we agree that the CEIL is not calculated using a projected ascent rate:

If your definition of NDL>0 is when SurfGF is <GFHi.
--> Then I agree that we can't have a ceiling with an NDL>0.

But to me and that's where we differ, the NDL takes into account the off-gassing that will happen on the ascent assuming a specific ascent rate that may or may not be what the actual rate will be (and that's why some deco stops can clear off or appear while ascending as the computer is constantly recalculating in a loop, as you said).

NDL>0 means we can ascend directly to the surface without any stops.
Directly here doesn't mean instantaneously. So my definition of NDL>0 is when the expected surfacing gradient factor if the diver started ascent now at the projected ascent rate is <GFHi
and in this case we can indeed have a ceiling with an NDL>0 (NDL>0 meaning no stops).

That seems weird, is that really possible to have a CEIL but no stop?

with my definition of NDL in the paragraph above, mandatory stops/deco mode (NDL<0) means we must have a ceiling, but conversely having a ceiling doesn't necessarily mean we have mandatory stops/are in deco mode.


I did some searching and found some posts agreeing with my understanding:

 

[crofrog]

All of the NDL tables that I'm familiar with include the ascent time as part of their calculations for NDL.

For instance the NOAA 32 table has a surface GF of ~101% at the end of the bottom segment for the 100ft / 30min max no-stop time, and assumes an ascent rate of 30fpm and you surface around 92% gf

The navy air table of 100ft for 25 minutes has a surface GF of around ~117% at the end of the bottom segment, and at 30fpm you would actually surface around ~105% gf

 

[inquis]

Send an email to Shearwater for the authoritative answer. My take based on comparison to another Buhlmann 16C planner: the NDL time shown ignores future/potential off-gasing or equivalently, assumes an infinite ascent rate.

A quick check with a Buhlmann planner like Subsurface shows that at 110 ft on EAN32 there's about 2.5 minutes between a non-zero ceiling appearing and the time at which initiating a 30 fpm ascent would not break the (continually updated) ceiling. The Shearwater NDL planner time is very close to the "ceiling appearing" time and about 2 minutes shorter than the "assumed ascent" NDL. It's possible that the NDL displayed during an actual square profile dive is calculated differently from the planner, but I doubt it.