Perdix vs Subsurface deco/NDL calculations

[jborg]

So I was just down in Sharm and did some dives. After importing to Subsurface I notice that it has a somewhat different view of NDL times than the Perdix had at the time (and which the Shearwater app shows). For one dive, with both set to GF 45/85, the Perdix at one point says 8 minutes NDL while Subsurface says in deco (although just; GF 86%). One thing that's obviously different is that the Perdix had realtime data, while Subsurface just has the 5 second data points to work with, but this was a quite slow dive and even so I would expect them to be closer to each other than this?

 

[Blackcrusader]

Thanks for this post quite interesting. Real time data for a multilevel dive is always going to be different than the best of dive plans. This was discussed when technical divers planned dives using tables and their computers in Gauge mode did not match up due to variances in diving that a dive plan cannot foretell.

People find out that really no matter how they plan a dive there are variances that won't show up so plan the dive and then dive to that written plan or allow yourself to change the plan to match your real time with the Perdix. Ascent rates often don't match the plan, you might be on gassing when you think your should be off gassing.

Hope the difference wasn't subsurface using air and the Perdix on nitrox 32%?

 

[Belzelbub]

I seem to recall something Dirk wrote a little while back about this. I believe the gist of it was that when Subsurface calculates NDL & Deco ceilings, it may use a different sampling rate than the DC, so the results may vary a bit.

I'm away from my computer that has Subsurface installed at the moment, but I believe in the settings/preferences there are a couple of toggles you can check. One is to show DC NDL/Ceilings, the other is to show Subsurface calculated ceilings. Looking at your info box, it appears you have the calculated NDL/Ceilings on, but the DC NDL/Ceilings may be off.

 

[jborg]

DC reported ceiling is actually on, but the DC never reported a ceiling, the minimum NDL was 4 minutes at one point. The gas matches, 31% in both Perdix and Subsurface. It's also interesting to see that the point of minimum NDL differs quite a bit between DC reported and Subsurface calculated. Here MacDive shows an NDL graph:

At the marker (22:30 into the dive) NDL has started increasing again according to Perdix/MacDive, up to 8 mins from the minimum of 4 minutes a little earlier.

In Subsurface 22:30 (the red marker) is actually the point of minimum NDL (max GF) which looks like it coincides more with the average depth graph.

I guess this could all be due to differing sample intervals, but it feels more like differing half lives on compartments or something...

 

[scubadada]

I hope you discover an explanation, that is a significant, large difference.

@dirkhh

 

[Belzelbub]

DC reported ceiling is actually on, but the DC never reported a ceiling, the minimum NDL was 4 minutes at one point. The gas matches, 31% in both Perdix and Subsurface. It's also interesting to see that the point of minimum NDL differs quite a bit between DC reported and Subsurface calculated. Here MacDive shows an NDL graph:

View attachment 714791
At the marker (22:30 into the dive) NDL has started increasing again according to Perdix/MacDive, up to 8 mins from the minimum of 4 minutes a little earlier.

Curious. On the MacDive image, what is the greenish graph displaying? I'm trying to make sense of it, but can't identify what it could be.

 

[jborg]

On the MacDive image, what is the greenish graph displaying?

That's NDL time ("NDT" in MacDive). It's too bad Subsurface doesn't show a graph like that, it would have been easier to correlate.

 

[scubadada]

That's NDL time ("NDT" in MacDive). It's too bad Subsurface doesn't show a graph like that, it would have been easier to correlate.

Yes, too bad Shearwater doesn't show it either.

Naively, I would think 2 download apps would display the same data, unless implemented differently. I wondered the same thing about different brands of computer running Buhlmann ZH-L16C with the same GFs. I tested this with my Teric and my Dive Rite Nitek Q and they were virtually identical

 

[Belzelbub]

That's NDL time ("NDT" in MacDive). It's too bad Subsurface doesn't show a graph like that, it would have been easier to correlate.

Thanks. That makes sense now. Axis for that must be reversed with 99 (or whatever the max is) near the top. I guess that's why it wasn't making sense to me.

 

[inquis]

I think it's down to Subsurface having to stair-step depth changes and the granularity of displayed times. When I am gradually descending, the subsurface-calculated NDL is greater than that of the dive computer, sometimes more than 10 minutes greater. When I am at a consistent depth, the calculated value is within a minute of the DC's value. When I am ascending, the calculated value is shorter than the DC reports.

The consistency in NDLs when depth is consistent suggests agreement with the fundamental algorithm (half-times, # of compartments, etc.). The general direction of difference is what I would expect with subsurface being forced to operate on stale depth information. In the OP's case, the time in question is during an ascent section.