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

[EFX]

HOWEVER, I believe the NDL calculations do. And the Perdix user manual would seem to agree saying that NDL calculations account for ascent rate, which they would only need to do if they were accounting for off-gassing during the ascent.

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There is nothing in the Important message box above (and in information from other versions of the Perdix manuals that I've looked at) that indicate that an "assumed" ascent rate applies to calculations for NDL. For what its worth Erik Baker does not use an assumed ascent rate in his calculations for deco stops and times. I didn't use the rate in my calculations for NDL and deco in the spreadsheet. My calculations compare favorably to MultiDeco and the Perdix dive planner.

The SubSurface code is available and I am looking through it to determine if they use an "assumed" ascent rate in the calculations. If someone else can look through the code and give us an answer at least we have another source toward answering the questions posed here. Granted, it's not Shearwater but I believe the information would be useful.

 

[stuartv]

There is nothing in the Important message box above (and in information from other versions of the Perdix manuals that I've looked at) that indicate that an "assumed" ascent rate applies to calculations for NDL. For what its worth Erik Baker does not use an assumed ascent rate in his calculations for deco stops and times. I didn't use the rate in my calculations for NDL and deco in the spreadsheet. My calculations compare favorably to MultiDeco and the Perdix dive planner.

The SubSurface code is available and I am looking through it to determine if they use an "assumed" ascent rate in the calculations. If someone else can look through the code and give us an answer at least we have another source toward answering the questions posed here. Granted, it's not Shearwater but I believe the information would be useful.

The ascent rate is right there on the Subsurface Planner screen. You set it to what you want. I think it defaults to 33ft/min.

 

[inquis]

The SubSurface code is available and I am looking through it to determine if they use an "assumed" ascent rate in the calculations.

Setting the planner to NDL, Subsurface extends bottom time such that the (instantaneous) ceiling clears just as you surface. Consistency would suggest the NDL calculation when analyzing a dive would also include off-gassing during ascent.

 

[inquis]

Consistency would suggest the NDL calculation when analyzing a dive would also include off-gassing during ascent.

It appears consistency is overrated. After changing the ascent rate(s) in the planner, the computed NDL for a downloaded dive at a given timestamp is unchanged. The Profile NDL seems to ignore the ascent while the Planner NDL (Recreational mode) seems to include it.

I suspect this choice to have been intentional, as the Profile NDL is computed as the mouse moves and needs to be quick.

 

[inquis]

There is nothing in the Important message box above (and in information from other versions of the Perdix manuals that I've looked at) that indicate that an "assumed" ascent rate applies to calculations for NDL

That's not my interpretation of the Important message box wording. Leaving out extraneous words: "NDL is a prediction that assumes an ascent rate of 10 mpm/33 fpm."

 

[stuartv]

It appears consistency is overrated. After changing the ascent rate(s) in the planner, the computed NDL for a downloaded dive at a given timestamp is unchanged. The Profile NDL seems to ignore the ascent while the Planner NDL (Recreational mode) seems to include it.

I suspect this choice to have been intentional, as the Profile NDL is computed as the mouse moves and needs to be quick.

How do you conclude that the Profile NDL is computed as the mouse moves? As opposed to computed when the parameters are changed, so moving the mouse just changes what value is shown from a pre-calculated table?

 

[inquis]

How do you conclude that the Profile NDL is computed as the mouse moves?

That is my suspicion, and I vaguely recall seeing calls to computational routines in the mouse movement callback function. However, that's irrelevant.

A more obvious illustration is to plan a Recreational mode dive with, say, a 20 fpm ascent across the board. Then position the mouse -- still in the planner -- where the ceiling first appears. In the Profile Info window "NDL: 1 min (calc)" is shown slightly before that instant, while "In deco (calc)" shows slightly after.

TLDR; in Subsurface, the Profile NDL ignores off-gassing, but the Planner NDL includes it.

 

[EFX]

The ascent rate is right there on the Subsurface Planner screen. You set it to what you want. I think it defaults to 33ft/min.

We've already discussed that a predetermined ascent rate is used for some data but it doesn't mean that the rate is used in the calculations for NDL and deco. You need to look at the actual calculations to prove it one way or the other.

 

[EFX]

I checked the NDL calculation in Subsurface and could not find any code that uses an assumed ascent rate.

 

[Rogerdd]

To sum it up, with Shearwater:
- The mandatory deco stops will appear once NDL becomes less than 0.
- SurfGF is the gradient factor assuming a teleportation to the surface (no time to off-gas)

But how are both NDL and CEIL calculated? The doc mentions an ascent rate of 10m/min, but it doesn’t clearly say which parameters are using an ascent rate.

NDL:

NDL < 0, is it when:
1- SurfGF is > GFHi
Or
2- [estimated GF99 at the surface if going to the surface now at an estimated ascent rate of 10m/min] > GFHi

Basically, does NDL take into account the off-gassing that will take place on ascent (projected to be 10m/min)?

CEIL:

Similarly is CEIL the depth at which the GF99 is equal to GFHi assuming:
A- an instantaneous ascent (teleportation) to that depth
Or
B- an ascent of 10m/min

Shearwater came back to me, so we have a definite answer at last.

‘To summarize and address your current questions, if I understand correctly the answers would be B and 2. All decompression predictions, including calculation of the ceiling are based on an ascent rate of 10m / min. The only exception I believe is surface GF which is a tool that assumes an instantaneous ascent.
I hope this info helps and please have a great day.
Kind Regards’