Split sensors or not (rEvo, 5 cells)?

[beldridg]

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

If your controller dies that would happen no matter what type of secondary monitor you have (HUD, Dreams, NERD, etc.).

If a diver can't run their unit manually while task loaded then maybe they shouldn't dive a rebreather (btw, that isn't a statement about you).

On a side note, I always go to my low set point when I start my ascent and manually control my PO2 during deco so I end up flying manual for a portion of every dive and I can't even remember the last time I was using a non-hypoxic mix.

- brett

PS - I also spent an entire trip in Chuuk manually controlling my unit since my DiveCan bus died on the first day. The CMF makes it a lot easier once you get to depth. I recommend everybody practice running manually on occasion just like practicing other skills.

 

[lermontov]

. I recommend everybody practice running manually on occasion just like practicing other skills.

agree - i sometimes do a few dives on the occasional trip on OC sidemount too for the same reason. unfortunately were are not all 'A grade' and on a long deco stop it wouldnt be hard to lose concentration and forget were now on manual. - its happened.

 

[broncobowsher]

I thought the idea behind the "expedition" model was 2 dreams and 1 Petrel, you could loose any one and still have 2 forms of monitoring. So on your expedition you could have a failure and not just finish your dive, but start another dive and still have a redundant PPO2 monitor.

 

[grantctobin]

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

If you’re at 200’ on the Nagato, you need new depth sensors on all of your computers

 

[beldridg]

If you’re at 200’ on the Nagato, you need new depth sensors on all of your computers

Yes, you are right. 170' deep on the Nagato....

- brett

 

[beldridg]

I thought the idea behind the "expedition" model was 2 dreams and 1 Petrel, you could loose any one and still have 2 forms of monitoring. So on your expedition you could have a failure and not just finish your dive, but start another dive and still have a redundant PPO2 monitor.

Yes, you have redundant PO2 monitoring. That doesn't mean you have redundant deco obligation computations based on that PO2 monitoring.

If you have one Petrel and 2 dreams and the Petrel craps the bed, then how do you calculate your deco obligation?

- brett

 

[beldridg]

agree - i sometimes do a few dives on the occasional trip on OC sidemount too for the same reason. unfortunately were are not all 'A grade' and on a long deco stop it wouldnt be hard to lose concentration and forget were now on manual. - its happened.

Yes, but hopefully with CMF it makes it a lot less likely.

Once you are at a deco stop and your PO2 is at whatever level you set manually, it should stay there without any (or minimal) intervention due the CMF of (from memory) about 0.65 liters/min.

If you haven't tried it and you have a rEvo, you should.

- brett

 

[rjack321]

IIf a diver can't run their unit manually while task loaded then maybe they shouldn't dive a rebreather (btw, that isn't a statement about you).

Come down off your pedestal there chief. In the past month alone, there have been 2 hypoxia events where CMFs were clogged or off. One diver died, the other had CPR on the beach and was within a hair's breath of dying.

It can happen to you too.

If you have one Petrel and 2 dreams and the Petrel craps the bed, then how do you calculate your deco obligation?

You don't need live monitoring to know your deco. A standalone running a fixed setpoint is fine and even over 90+mins of deco is within about 5mins of live monitoring.

 

[Heat Miser]

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.

Point noted, so on a normal dive when all three computers are working, I guess you use the most conservative deco profile of all three computers?

Out of interest how often is there a difference in deco between the nerd and the petrel, assuming they are on 2 different cells each, with 1 common cell?

I think you have just talked me into buying or borrowing a second perdix/teric for my trip.

 

[broncobowsher]

Out of interest how often is there a difference in deco between the nerd and the petrel, assuming they are on 2 different cells each, with 1 common cell?

With no shared cells, my typical Truk dives ~2½ hours would have both computers surfacing about 30 seconds apart. All but once the Petrel had the slightly longer deco time. Presume that was due to it being slightly deeper on my wrist as compared to my face.