Shearwater NDL Replacement Field Persistence


I recently read the release notes for the latest Shearwater software release (v91) and was excited — yes, I know, no comments from the peanut gallery.

One of the standard information fields on a Shearwater computer (all dive computers really) is the NDL or No Decompression Limit field. This is the amount of time left at the current depth before decompression stops become “mandatory.” For most divers, the idea is to never let this get down to zero and they generally keep it 5 minutes or more and then do a 3-5 minute “safety stop” at 15-20 feet just to add a buffer.

From the Shearwater Manual

When conducting decompression dives, once that field is zero, the user gets to decide what to put in place of the NDL field instead of just “0.” Options exist like Delta+5 (how much longer will be decompression be if I spend 5 more minutes at this depth), Max Depth, Ave Depth, etc.

From the Shearwater Manual

Surface Gradient Factor (SurfGF)

Back in January 2019 in Version 65 of the firmware, Shearwater added a new data field called Surface Gradient Factor (or SurfGF).

The SurfGF field shows what your “gradient factor” would be if you were to surface immediately. For me, it is a real-time measure of risk on any given dive. Basically, if your SurfGF is 100 and you go to the surface immediately, you are right on the edge of the curve to getting decompression sickness (of course, it is only a mathematical model).

Divers set something called a “GF Hi” which is basically a percentage of that number at which they will surface and it controls your decompression obligation. It is a way of building in a safety margin. My GF Hi is usually set at 80% but I usually wait a bit longer and surface at 70-75%.

On one of my computers, I chose to have the “Surface Gradient Factor” or SurfGF be displayed when NDL reaches 0 and decompression stops are required.

New “Feature”

The limitation that used to exist is that once I cleared my decompression obligation (lets say at a SurfGF of 80), then the NDL display would revert back to NDL 99 minutes (i.e., no more decompression required). If I wanted to wait longer on the last stop and monitor my SurfGF, I would have to keep “right clicking” through the menu structure to see my current SurfGF.

Hence, for a while now, I have been asking for them to make the SurfGF field persistent (see #7 on that list) even after clearing your decompression obligation.

The Version 91 release notes had this little gem:

It looked like they might have given the user the ability to do that!

I checked with Shearwater support and, as of right now, V91 is only available on the Perdix 2 and the Petrel 3. According to the support team, they are “planning on including many of the features of this firmware update to our older computers” but they “can’t say for sure when these will be added, however it is in the pipeline.”

4 thoughts on “Shearwater NDL Replacement Field Persistence

  1. Would be a great feature to keep persistent display after deco has cleared but sadly I won’t be holding my breath on getting this for my NERD2 any time soon. Shearwater have growing track record of abandoning firmware development on ‘old’ models (even though NERD2 is current and should be their flagship product). These days they put their nearly all their effort into churning f/w on their lower end mass market models. Things haven’t been the same since Bruce sold the company, they’ve lost much of their customer service (and quality) in favour of going after $$. I still recommend Shearwater when others seek advice but set lower expectations.

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