Split sensors or not (rEvo, 5 cells)?

[beldridg]

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.

Fair enough.

That is really a shame because a clogged or off CMF should normally not be an issue and should give the diver plenty of time to react. It takes quite a while for a loop to go from 1.3/1.2 to hypoxic. I'm not sure what rebreather each was using, but I'm assuming there was a way to manually add O2.

- brett

 

[beldridg]

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.

Yes, I agree and that is exactly what I said I would do in one of the other posts on the topic:

Let's say that you are in Bikini with a Petrel and two dreams and your Petrel goes and you are 170' (corrected from my original 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).

However, that was only part of the situation. The real question isn't what do you do on the dive where it fails.

The real question is: what do you do for the rest of the trip?

The original question posed was something like "is a configuration with a Petrel and 2 dreams enough for an "expedition" style trip?"

For me personally, it isn't and the primary reason is that if the Petrel fails, you are "stuck" for the rest of the trip doing some reasonably serious diving with no way to calculate your deco based on the actual PO2 of the cells. I wouldn't want to do those dives with only a standalone computer set to an internal set point.

I think that is the biggest reason to prefer a NERD over two dreams (or any other HUD for that matter).

- brett

 

[beldridg]

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?

I use the most conservative between the Petrel and the NERD. The Teric is there in case the entire unit craps the bed. I do check the Teric to make sure it is working and calculating deco, but I don't "actively" use it for my deco schedule.

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?

They are basically identical in most cases. I will often run the NERD on a different gradient factor for a variety of reasons (e.g., I will set it to whatever GF my buddy is running). In that case, it can be different and I will follow the more conservative.

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

I would personally prefer to have a backup computer to connect to the rEvo if your Petrel fails.

If you already have a Perdix or Teric (from your note it sounds like you do), then I don't personally think you need redundancy for that. But if you only have one computer calculating deco connected to your cells and it fails and you don't have a backup plan, you have some hard decisions to make for the rest of the trip.

Another way to say that is that if I'm going to spend money and I already have one "off board" computer, I'd rather spend it building a backup plan for my single "on board" computer.

Does that make sense?

- brett

 

[Wibble]

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.

That is really a shame because a clogged or off CMF should normally not be an issue and should give the diver plenty of time to react. It takes quite a while for a loop to go from 1.3/1.2 to hypoxic. I'm not sure what rebreather each was using, but I'm assuming there was a way to manually add O2.

One understands when you're task loaded that monitoring may be a little less rigorous. There seems to be quite a few hypoxic rebreather incidents where oxygen is turned off or fails. In very general terms, why would one not be monitoring for this especially given the "always know your PPO2" mantra, especially on a manual unit? Also all units have HUDs and wrist mounted computers which would be flashing warnings.

 

[grantctobin]

Also all units have HUDs and wrist mounted computers which would be flashing warnings.

All units?

 

[rjack321]

Fair enough.

That is really a shame because a clogged or off CMF should normally not be an issue and should give the diver plenty of time to react. It takes quite a while for a loop to go from 1.3/1.2 to hypoxic. I'm not sure what rebreather each was using, but I'm assuming there was a way to manually add O2.

- brett

At a constant depth. Try ascending from 60ft to 15ft while distracted thinking running manual is no big deal with no CMF or solenoid. Your starting ppO2 of 1.2 turns into 0.6 without any metabolism at all. And you vent a bunch of O2 from the loop too so it breaths down that much faster. Oh crap you drop something or there's silt or current and you never notice how sleepy you're getting.

One understands when you're task loaded that monitoring may be a little less rigorous. There seems to be quite a few hypoxic rebreather incidents where oxygen is turned off or fails. In very general terms, why would one not be monitoring for this especially given the "always know your PPO2" mantra, especially on a manual unit? Also all units have HUDs and wrist mounted computers which would be flashing warnings.

Done victim blaming yet? CMFs clog, batteries fail, they shut off their O2 to breathe down the loop a little, they think "I can do this" and try to run their unit manual but are distracted by something else and go hypoxic. It absolutely happens.

 

[Wibble]

Not blaming victims, trying to learn from others' experiences and perspectives, as should every diver.

Ascents are challenging with so much going on.

 

[beldridg]

At a constant depth. Try ascending from 60ft to 15ft while distracted thinking running manual is no big deal with no CMF or solenoid. Your starting ppO2 of 1.2 turns into 0.6 without any metabolism at all. And you vent a bunch of O2 from the loop too so it breaths down that much faster. Oh crap you drop something or there's silt or current and you never notice how sleepy you're getting.

Not to mention that maybe an ADV fires with a big slam of hypoxic mix after maybe you vent a little too much gas and need to get some loop volume?

I totally get it. I run manual on all my ascents and deco stops (granted, with a CMF and a low set point and solenoid).

I'm not trying to "blame" anybody. My only point in my last response was that it is a shame in those two cases since it should have been preventable. Could it happen to me? Yup.

(NB - I think we've strayed pretty far off of the original topic which was wether or not it makes sense to split cells in a rEvo when you have 5 cells and two computers).

- brett

 

[stuartv]

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 clear on what you're saying here about keeping the NERD1. Are you saying that if your controller totally died on Day 1, you would dive the rest of the trip with 2 NERDs on your rEvo?

And that is so that you will be prepared in case your Petrel dies and then your NERD2 ALSO dies on the same trip?

And you want to be diving with 2 NERDs (presuming your Petrel is dead) so that if the NERD2 dies, the NERD1 will be there giving you an ascent plan based on live monitoring?

I think for your expedition planning, I would be satisfied to be starting my diving with a working controller, NERD2, and a Teric running on a fixed setpoint. If the Petrel died, then continue my remaining dives with just the NERD2 and Teric, flying it manually. If the NERD2 then died, get out using the Teric, unplug the NERD2, plug the NERD1 into that cable, and keep diving with it and the Teric. I would also really think about taking a day off if my controller AND my NERD2 died in the same week. If for no other reason that, as soon as you add an unused computer to your mix (e.g. the NERD1), it does not have your tissue loading info in it, so you couldn't really rely on it for deco calculations anyway. But, really, if you have 2 Shearwater computers die on your in the same week, that might be the Universe telling you to take a break and think about what you're doing.

I would not add a second 4-pin cable to my unit, so that I could dive with 2 NERDs at the same time.

 

[stuartv]

I have configured my rEvo with its Predator controller, a NERD2 (w/AI), and 5 sensors.

I use a splitter so that each computer gets 3 cells to monitor. My protocol is the controller is on my left arm and it shows sensor #s 1, 2, and 3. My NERD2 shows sensor #s 3, 4, and 5. Simple and easy to remember. I can see them both at the same time and know that the values in the "middle" (i.e. rightmost on the Predator and leftmost on the NERD) are the same sensor (#3).

I use the splitter that has a diode in each leg to prevent any "cross-talk" between the DiveCAN side and the analog side.

I have had at least 2 separate conversations with Richard Morton, at Dive-Tronix, about this. He is both the US Shearwater repair and service tech and also a rEvo factory-authorized service tech. He told me that there were some concerns with using splitters years ago, but they had worked the bugs out of that and it is fine now. I've read people talking about splitters that have a resistor in them. When I was talking to Richard, I specifically asked if it has diodes and he said yes. That said, that is something that I think even Richard might say "I think so, but don't hold me to that." So, don't take it as gospel on that.

If my controller dies, I want my ascent plan (from my NERD) to be based on live cell monitoring that includes voting logic. I.e. not just based on 2 cells.

If you have a long deco obligation, your controller is dead, and your monitor only monitors 2 O2 sensors, do you just trust them for that final hour (or two, or three)? I mean, even if they continue to read close to each other the whole time, do you just trust them? Or do you start doing periodic dil flushes to verify that they are reading correctly? Do you occasionally spike the O2 just a little, to make sure they haven't gotten current limited?

Maybe it's just my inexperience diving a CCR, but if I had a bunch of deco racked up and only 2 cells that I could "see", that would make me nervous. A LOT more nervous than if I had 3. And I definitely would not want to be feeling a need to start doing periodic dil flushes or spiking my O2. If it was a dive that racked me up that much deco, I'd be worried about wasting that much gas and potentially running out. Would that mean I would end up not doing it as much as I should, and get into trouble that way?

As far as having the AI info displayed in the middle row... meh. If it's a dive where I am not using my compass, then I just scroll the bottom row of the NERD over to show the AI info. If I'm using my compass, then I'll just hit the button to scroll over and check the AI every now and then, and then switch back to the compass.

Tangent (Shearwater Feature Wish):

What I wish Shearwater would implement - and I have posted this before - is a way to have the monitor (whether a NERD or a Petrel) monitor 3 cells, but only use 1 field in the middle row to display ppO2 info. I would have it display the actual ppO2 value it is using for its deco calculations. Presumably, that is some kind of average of the sensor values that haven't been voted out. I want it to just show me that one ppO2 value, but then use Yellow and Red color-coding in that field to let me know if I have a sensor that has gotten out-of-range from the other 2, or if one has gotten voted out. With this, they would also need to make the bottom row where I could right-press to get to a display that showed all the individual sensor values in the bottom row, if I wanted to see them.

I'm only talking about having this for a Monitor. I'm okay with having the Controller work like it does now (always shows all 3 sensor ppO2s in the center row).

With the fact that a lot of people apparently dive with just a blinky-LED HUD that only displays 1 sensor - and only as blinky lights - having my NERD monitor only show one "aggregated ppO2" value does not seem like a huge safety compromise.