I like the NERD and I love the concept. However, I also know three separate divers who have had NERDs go black on them during dives, one of them has had this occur more than once. Based on that, I would not recommend a NERD as your only method of monitoring the loop. If you go that route, polish and practice your skills bailing out to SCR mode, and have another SA computer along for backup deco information.
I'd replace a HUD with a NERD in a heart beat, but I would not use it to replace a hard wired or Fischer ported Petrel, if you plan on technical CCR diving.
To be fair the NERD 2 may not have this issue, but I'd certainly explore what Shearwater has done differently in the NERD 2 to prevent it before I made it my only method of monitoring the loop.
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You did not say what unit you are diving, so this may or may not be relevant. We opted for HUDs in addition to a Fischer ported computer on our KISS Sidekicks, but that required a splitter on each cell, and those splitters caused a problem for us, until we worked out the bugs.
The problem was that the potting compound used to cover the splitters degraded, probably due to the caustic nature of absorbent in the presence of moisture. This would create some current loss from the cell, with a resulting drop in the readings in one or more cells once the moisture level in the head increased (due to condensation), particularly when there was any significant amount of absorbent dust in the head. Once we figured that out, I coated the relevant portions of the splitters with additional epoxy, which resolved the problem, at least until the underlying epoxy eventually lifted off entirely. Then I just re-potted the splitter, and we've been good to go since. We were not the only divers with this problem, and I've been advised that KISS has addressed the problem.
We actually took a three part approach to the problem. In addition to re-potting the splitters, we added sponge between the cells in the head to absorb moisture and keep it off the splitters and contacts, and we keep the head as free as possible of any absorbent dust.
The point here is that we got a HUD to back up a computer that has been very reliable and has not, to this point, failed. However, in the process of preventing one possible total failure mode, we added another partial failure mode - and another maintenance requirement to the unit as a whole to prevent it. Absent a HUD, and the splitters required to support it, we would have had essentially zero issues with sensors dropping out. Someday the HUD may pay off in terms of redundant loop monitoring, but until then it and the splitters are another item to maintain.
We're split on what we'd do if we did it over again. Marci likes her HUD and uses it frequently to monitor the loop PPO2, while I tend to refer to my Petrel almost exclusively. That could be partly due to Marci preferring I lead in small, very silty passage, with the result that she spends more time in low viz than I do. I'm not sure I'd opt for the HUD if I did it over again, given the reliability I've had with my Fischer ported Petrel, and the potential failure mode introduced by the splitters.
There's no free lunch, so choose carefully, then plan accordingly. But unless you really perceive a need to add redundant loop monitoring I'd lean toward just doing what you are doing now - primary computer displaying cell PPO2, and a SA computer for redundant deco information.