Backup Computer for Rebreather Divers

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v101

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Hello -
As a recently trained rebreather diver who is diving a MCCR, I wanted to inquire how many divers use a backup dive computer with their units?

I have been taking two computers with me, however one is wired to a fisher port, and the other is a standalone but set in CC mode, with a Set PP02 level. when I ask other divers, they tend to use this method, and other divers tend to get a 2nd hard wired computer for redundancy. I do find the deco obligations are nearly the same maybe a few minutes longer on the stand alone.

Since I am diving a MCCR, I feel id rather have a redundant computer hard wired to my oxygen sensors, knowing that if primary computer fails, I can always look to the 2nd.

Comments on this?
 
Depending on the linearity of your cells, you may want a standalone computer to have more accurate deco calculations, granted this depends on how deep/long you are planning on diving.
Having a redundant computer to the cells will allow you to stay in CC mode longer in the event of a failure, but you can go to SCR mode to stay on the loop if your monitor computer fails.
You have to decide whether or not the benefit of staying in CC mode is worth the cost of the new computer
 
I see it as two separate issues:

1) redundant loop monitoring

2) redundant deco information.

We chose to go with a Smithers code HUD for redundant loop monitoring. Marci is more inclined to use it to reference her loop PPO2 on a regular basis than I am, but I do greatly appreciate it in small side mount passage, when it gets really silty. In both of our cases, if the primary computer takes a dump, we can still stay on the loop and manage the PPO2 with the HUD, rather than having to revert to one of three other possible options.

We also dive with a stand alone computer set to the same high and low set points we're using on the primary computer. While you can maintain PPO2 with a HUD, it won't do anything for you for deco information. The SA computer also gives you a very good idea of how well you are actually maintaining your PPO2 on the unit, and if you're doing a good job* the differences between the deco obligations on the two will be minimal to non existent.

* The wild card here is sensor linearity. If the sensors are reading low at your chosen set point (1.2 in our case for most dives), then you're actually diving with a higher PP02 in the loop (for deco purposes) than you or the computers thinks, and the deco numbers on both the primary and SA computer will be conservative. On the other hand, your CNS numbers will not be accurate on either computer and your CNS percentage will be higher than the computers think. It's worth knowing how your sensors actually perform at elevated PPO2 in both dry condition, and in wet condition at the end of the dive.

Where I will rely on the SA computer for deco information is if during the course of the dive, two cells start reading incorrectly (As confirmed by a diluent flush and a comparison of dil PPO2 to the sensor readings for O2. I'll manage O2 manually to maintain the PPO2 with the good sensor, but with the full knowledge that the primary computer's deco information will be suspect. Thus I'll complete my deco based on the stand alone computer's recommendations - but still stay in the water to clear the primary computer if needed.

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We dive KISS Sidekicks with the computer and separate HUD for the reasons listed above. But to be honest, if money was not an option I'd spend $2000 on a NERD rather than $800 on a HUD, in addition to a hard wired Petrel. It gets you redundant loop monitoring, and it gets you redundant deco information, and it gets you easy to see numbers in a silt out.

Another issue is whether or not to use a Fischer cable.

Opinions are mixed as Fischer cables are a little fragile, and you need to give them due care in terms of using mineral oil to keep the water out, and ensuring they don't get tugged on excessively. If you scooter, routing them inside the arm is preferable since it avoids snagging the cable at speed, but if you do not, or if you have short forearms, routing them outside reduces the bend in the cable. If your cable goes bad, it's between $350 and $400 to replace on a KISS (although it's only about $250 on a Revo, even though it's a longer cable). The good news is that it's not hard to replace in the field, and if you have a computer failure, it's just the computer going back, and you're still in business if you've got a back up computer.

Hard wired connections are more reliable. On the other hard wired Petrels cost about $400 more than a Fischer ported Petrel and if you have a problem with cable or computer, the whole assembly has to go back for repair. Now you need both a spare computer and a spare cable to keep diving.

We opted for Fischer cables but I also rigged a strain relief to carry any sudden loads in tension to prevent pulling the collar off the Fischer cable (no need to ask me how I know this), and we carry a spare cable the spares/tool kit.
 
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For the past 10 years I've been diving ccr with 2 redundant computers. In over 1000 hours of ccr diving I've actually never had to rely on my backup hard wired unit. That's. it to say tomorrow my primary won't crap out, it if it does, I'm going to bailout and deco out on my perdix.

This year I swapped to a kiss orca spirit lte and use a single nerd with a Fischer cable. I've discontinued the use of a second "wired" device. I have over 200 hours on it with nary a problem. Also in all of those dives, I've never had more than a few minutes of difference between deco between my nerd and my Perdix. (Boy auto correct does not like the word Perdix.)

The key to successful rebreather diving IMO is preventative maintenance
  • Use good cells (vandegraph) and an even cell rotation
  • Maintain your mole connections!
  • Maintain your Fischer cable according to Curt McNamees recommendations
  • Check all hoses, loop, LP, HP, keep in good shape
  • Keep everything simple
Tony
 
If you have the budget for it, Shearwater's NERD would satisfy both the loop and deco monitoring concerns. I love using it and looking forward to upgrading to the NERD 2.

I also use a standalone too but that's because I encounter more deco time than most. I have plenty of dives just using the standalone when running my mod-smither's code HUD, and that worked fine. But once you go NERD, you don't go back. :wink:

My vote: start a lemonade stand to get a NERD 2!
 
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.
 
From what I have seen/ heard is there is very little difference between hardwired and stand alone. I have 2 stand alones and never been bent on a rebreather.
 
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.

Our mesophotic team probably has over 500 collective hours using the NERD and have had 1 failure in that time from a flooded battery compartment. Since the NERD 2 is rechargeable, hopefully that failure point will be eliminated in the future.
 
From what I have seen/ heard is there is very little difference between hardwired and stand alone. I have 2 stand alones and never been bent on a rebreather.

That's how it should be.

I have a mCCR and I use the comparison between the SA computer and the wired computer actually reading the cells to grade how well I maintain the planned set points. As long as I make the switch from low to high set point at about the same time I bump the PPO2 up to 1.2, and then maintain a reasonably accurate 1.2 on the wired computer, the NDLs and the deco stops are identical.

If not, it's on me for not accurately maintaining the proper set point. But, I've found that you don't have to be dead on, and you really have to screw it up fairly badly for a period of time to get more a few minutes difference.

Similarly, if you are running an mCCR with two SA computers, and are doing a good job maintaining the set point, then you're not really losing anything relative to a hard wired computer. The hard wired computer, reading actual values, in essence becomes important if for some reason you do a seriously poor job managing the loop PPO2. And that should be a seldom to never event, as after not that many hours, maintaining the set point and knowing pretty much exactly how much O2 to add and when becomes second nature.

The cell performance matters however. "Consistent" is not the same as "accurate" if your cells are not reading properly. From a deco perspective the good news is that low reading cells seem to be more common than high reading cells (in my experience at least). In effect, if the cells are reading .05 or .1 low, then a set point of 1.2 will produce an actual loop PPO2 of 1.25 or 1.30, since either you or the unit is using the low cell readings to maintain the target PPO2. That's on the conservative side for DCS, but it'll mean a higher CNS than you're planning, or the computer is showing. Which is another reason not to run at a PPO2 of 1.3, particularly when the error is probably going to increase at higher PPO2s.

In contrast, if you've got an odd set of cells that are reading high, then your actual loop PPO2 will be lower than the computer is using to calculate the deco, and that can lead to inadequate deco.
 
My buddy and I run the same gasses, same deco model, so his comp is my backup, and my comp is his backup. If we get separated AND my comp craps out, well then I wing it. Use SMB for depth measurement, and count.
 

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