Split sensors or not (rEvo, 5 cells)?

[broncobowsher]

brett,

Very well written.

-Tim

 

[Divermikey]

First, let me say "thanks" for the constructive dialog.

This is a rather long post but hopefully the people who dive a rEvo will find it worthwhile.

I find it helpful to get other viewpoints and opinions on topics like this. I don't think there is a "right or wrong" answer which is what makes it interesting. As I said in my original post, I would post my opinion and thoughts. I've got a "bonus" at the end as well.

IMHO, it doesn't make sense under normal circumstances to split a cell when you have a 5 cell configuration with 3 on the controller/primary and 2 on the backup/monitor. Here is my list of pros/cons:

Pros / Benefits

• Could help with a scenario when the monitor has two connected cells and the controller with three cells fails completely. In this case, the monitor has three cells.
• Could help with a scenario when one of the two cells on the monitor fails (i.e., the monitor could vote out the bad cell and still average the remaining two).

Cons / Costs

• Additional point of failure (splitter)
• Potentially an issue with "connecting" the two completely independent systems and it goes counter to the concept of independent systems
• Additional complexity (remembering which cell is split on each display, thinking about what it means if they are different, etc.)
  • There are ways to alleviate this such as putting the shared cell for both on the left, but it still involves "mental math" when the sensors start disagreeing
• Lose the ability to have AI info in the third PO2 "slot" in the middle row
  • Doesn't affect people not using AI. Also, on a rebreather, I really don't need the AI info all the time anyway and I can always just use a few right button pushes to have it displayed on the bottom row.

My conclusion a few years back was to not split a cell to put a third cell on the NERD display. I'm not claiming that it is the "right" decision, just the one that I have gone with.

However, I do have some additional thoughts.

These are based primarily on "expedition" style trips to remote places like Bikini or Chuuk, etc. I used to do those trips often but haven't in a while due to COVID but hope to get back to them.

Thoughts

• Carry a splitter in your bag on remote trips
  • Note that rEvo sells these with and without isolators (e.g., R436 - Cell Splitter 2)
  • There are failure cases on a remote trip where it can really make sense to split cells. I carry extra cells to avoid that but the "cost" of carrying a splitter is basically nothing

Now, another interesting topic (the "bonus") I mentioned above.

Let's assume that you are in a very remote location like Bikini Atoll with your rEvo. Your primary computer (controller) on the 5 pin wet-mate DiveCan bus takes a crap and is no longer functioning.

Most people would take the third molex connector on the hard-wired monitor cable and connect it to one of the other three cells that was connected to the controller. Now you have a computer monitoring PO2 with three cells. Not a controller, but no big problem either.

I also have a Teric that I use with an internal set point of whatever I'm diving so I have a backup but not connected to the system.

When I started diving in really remote places, I wanted to have the ability to connect a backup computer in case my primary computer failed. I originally had a NERD but upgraded to a NERD2 to get AI and other features (as others have done).

I kept the original NERD and I take it on "expedition" style trips. However, it cannot be used on a DiveCan bus with a 5 pin wet-mate since it is a 4 pin analog wet-mate connector.

Hmmmm...what to do? This was my solution:

“Expedition” readiness with my rEvo

I'm not saying it is right for everybody. In fact, for most people, it just adds complexity and doesn't make sense. However, if you are doing expedition style trips and depending upon your gear, it might make sense.

Regards,

- brett

Great writeup, thanks for the contribution. I always take my splitter cable on any trips.

 

[beldridg]

brett,

Very well written.

-Tim

Thanks, Tim. I appreciate the dialog and the opinions.

- brett

 

[beldridg]

Great writeup, thanks for the contribution. I always take my splitter cable on any trips.

Thanks. Yes, I agree. There is no downside to having a splitter in the bag.

- brett

 

[Heat Miser]

Great writeup, thanks for the contribution. I always take my splitter cable on any trips.

I'm a relative NEWB, but if you have a rEvo expeditions with 2 dreams does a splitter help you? Because 2 dreams can run the revo in in manual across 4 cells?

@beldridg like the website/blo!, going to Bikini next year too. Keen to follow some of your good/experienced advice

 

[Wibble]

We've been discussing the 'new' Revo configuration: three cells to the Petrel controller on the wrist, and two cells to the mouthpiece-mounted Nerd which is a backup only (i.e. doesn't control the solenoid)

The original config used the Revo Dreams which monitored one or two cells (I don't think it was more). Dreams were backup only and had a HUD each (flashy lights in front of the eye).

 

[Heat Miser]

We've been discussing the 'new' Revo configuration: three cells to the Petrel controller on the wrist, and two cells to the mouthpiece-mounted Nerd which is a backup only (i.e. doesn't control the solenoid)

The original config used the Revo Dreams which monitored one or two cells (I don't think it was more). Dreams were backup only and had a HUD each (flashy lights in front of the eye).

OK isn't there redundancy with having the Petrel and then the two dreams for Bikini, because you can link 4 cells to the 2 dreams. I understand the Heads Up Display on the Nerd is aesthetically good. But does it offer you better redundancy than the stock standard Petrel + 2 dreams.

So if we can a gree a splitter doesn't help someone with a petrel + 2 dreams I think we have reached a first time consensus on Scubaboard.

 

[beldridg]

OK isn't there redundancy with having the Petrel and then the two dreams for Bikini, because you can link 4 cells to the 2 dreams. I understand the Heads Up Display on the Nerd is aesthetically good. But does it offer you better redundancy than the stock standard Petrel + 2 dreams.

So if we can a gree a splitter doesn't help someone with a petrel + 2 dreams I think we have reached a first time consensus on Scubaboard.

Yes, I believe the original idea behind the two dream configuration is that you have a lot of redundancy for monitoring PO2 and it might make sense to split cells in that case (I really haven't thought it about enough because I learned on Dreams but bought my unit with a NERD).

But the question about dreams vs. NERD has a different aspect than just monitoring PO2.

IMO, and for me personally, there isn't enough redundancy in terms of deco calculations.

Let's say that you are in Bikini with a Petrel and two dreams and your Petrel goes and you are 200' on the Nagato and on day 2 of your trip. Now what?

I carry a Teric on every dive with an internal set point and it is only usually a minute off my Petrel in terms of deco. For that dive, I would use the dreams to monitor PO2 and my Teric to calculate my deco and pad it (and assuming I had a buddy, I would use the more conservative of their deco and my Teric).

But, now you have another 6-7+ days of diving without a computer calculating deco based on your actual PO2 (assuming the Petrel failure was "real" and not just a dead battery).

There are plenty of people who dive a CCR with one computer monitoring PO2 and a second computer on an internal set point. Let's face it - complete computer failures are really rare.

For me, I prefer the redundancy of having two computers calculating deco based on actual PO2, especially on long trips to remote locations where one complete (but unlikely) computer failure won't leave you with ONLY a computer with an internal set point.

I'm fine with being called "paranoid" and I do realize that there is a cost to the extra complexity but I have made those decisions explicitly while aware of those tradeoffs.

- brett

 

[beldridg]

I'm a relative NEWB, but if you have a rEvo expeditions with 2 dreams does a splitter help you? Because 2 dreams can run the revo in in manual across 4 cells?

@beldridg like the website/blo!, going to Bikini next year too. Keen to follow some of your good/experienced advice

See my other reply about dreams & splitters.

Glad you like the blog. If you dive a rEvo, I have an equipment tag that includes some posts about rEvo (along with other miscellaneous stuff):

Equipment – Wrecked in my rEvo

I'm not advocating you do what I did but it might make interesting reading.

- brett

 

[lermontov]

Y


I carry a Teric on every dive with an internal set point and it is only usually a minute off my Petrel in terms of deco. For that dive, I would use the dreams to monitor PO2 and my Teric to calculate my deco and pad it (and assuming I had a buddy, I would use the more conservative of their deco and my Teric).

- brett

the danger is having to constantly monitor your hud and flying manual - which would be ok if you did it all the time, likely though is as soon as you get task loaded youll revert to your default -which is letting the controller do the work - always a concern when esp if you diving < 18% 02