Background
One of the reasons I chose the rEvo rebreather was the amount of redundancy built in. You can configure the rEvo in many different ways, but mine is configured with 5 oxygen cells. Three are wired into a Shearwater Petrel2 via a 5 pin wet-mate connection on a DiveCan bus. The Petrel serves as my “controller” and my primary computer. In addition, I have a NERD2 which serves as a secondary computer and it is “hardwired” on a 4-pin wet-mate connection through a PG7 gland into two different cells.
Effectively, I can have either computer completely fail OR have 1 or 2 cells fail and still safely complete a dive. I also have a “offboard” Shearwater Terric which runs an internal setpoint in case everything on the rEvo completely fails.
Many would consider this amount of redundancy overkill; however, they aren’t executing multi-hour dives to depths that require a reasonably large chunk of decompression.
The “Problem”
When I was planning for my trip to Bikini, I started going through a lot of “what-if” scenarios in my head. One of those scenarios was “what if my Petrel controller fails on day 1 of the trip?”
If that happened, I would connect a third cell to the NERD2 and then run my unit “manually” for the rest of the trip. I had DiveCan problems on my trip to Chuuk a couple years ago and ran the unit without the functions of the DiveCan bus (solenoid, RMS) but the O2 sensors connected to the Petrel worked fine.
However, in this case, it would mean I would be down to one on-board computer and I would have a single point-of-failure. Given the remoteness of a Bikini expedition and the time and effort involved in getting there, I started thinking about what to do about that situation.
The Solution
When I originally purchased my unit, I bought it with the original version of the NERD. A while ago, I decided to upgrade to the NERD2 for a few different reasons (which is a topic for another post). That left me with a “spare” NERD. I had talked to a few fellow divers about selling them the original NERD but it just never worked out.
So, I had a “perfect” situation in that I had a spare computer. However, both of my NERD computers were “hard wired” on a 4-pin wet-mate cable to the sensor tray. So, if my Petrel computer on the DiveCan bus with 5-pin connectors broke, I couldn’t just “plug in” my spare NERD.
At this point, I contacted Pieter at rEvo to discuss some options (and also ask why both computers don’t simply use DiveCan bus connections — but again, that is a topic for another post). He recommended running a spare “analog” cable with a 4-pin wet-mate connector through an unused hole in the entry port on the top of the rEvo case.
I purchased the R581 4 pin wet-mate to trip molex, including cable gland (80 Euros) and routed it through the exhale lung and now it is there in case I need it. I also bought end caps for the cable to protect it.
If my NERD2 fails, I simply plug in my NERD1 and I’m ready to dive (note that I use AI sensors on my unit with my NERD2 but I have the Teric which also reads them).
If my Petrel2 fails, I plug one of the molex connectors from my NERD2 into a third sensor and then wire up two of the molex connectors in the backup cable to the NERD and I’m ready to dive. It would take me no more than 30 minutes to make the changes.
Here are some photos of what it looks like:
Photo showing the routing of the extra wet-mate cable through the case.
Photo showing the triple molex connectors in the exhale lung with the O2 sensor tray
R581 – 4 pin Wet Matable Connector 43″ to Triple Molex including Cable Gland
