Split sensors or not (rEvo, 5 cells)?

[stuartv]

Assume 15/55, 50%, 80% bailouts. Once switched to 50% at 21m, there’s no helium in the mix (aside from your off gassing). On CCR the diluent contains helium and slows down helium off gassing, extending your TTS.

Run some sample dives in MultiDeco and play with switching to 'air' dil at various depths which has a big effect on TTS. Loads of other issues arise as a result (IBCD, CNS, complexity...), but the TTS is interesting.

Okay, I see what you mean. But, I am not experienced enough on CCR and don't have the training yet for that depth (on CCR), so I would have to do some planning. My gut tells me that I probably would not use 50% as an intermediate deco gas. My gut is that I would end up using trimix, to mitigate risk of ICD. But, that's just a gut feeling at this moment.

 

[stuartv]

Yes with both monitoring cells.



Yes. Under "normal" dive conditions, I like to always have two computers calculating deco obligation based on actual PO2 of the cells.

To give an example, if I was in Bikini and my Petrel controller died on dive 1, I wouldn't want to do the rest of the trip with only the NERD2 monitoring cells. Would I still dive? Yes. I would also have my off board Teric.

My logic was partially based on the fact that I replaced my NERD with a NERD2 and had two choices for what to do with the original NERD: keep it or try to sell it.



Perfectly sound logic. I just like the ability to dive with two computers monitoring PO2. On a long trip in a remote location with some reasonably long deco times, I don't like the idea of doing all those dives with only one computer. Would I do it? Yes, absolutely. But, the "cost" of adding another analog cable to the unit is almost zero.



I totally get that and I think most people would take the same approach and I don't think there is anything wrong with that.

But, to me, the cost of adding the additional cable was essentially zero and -- most importantly -- I already had the "spare" NERD computer.

If you are in that situation and you bring along the NERD and your Petrel controller does fail, there isn't anything you can do with that extra computer (except use it as a standalone computer similar to a Teric with an internal setpoint).

- brett

Have you actually dived with both NERDs on your unit? Based on my one, I really don't think I would like the loss of vision that having 2 would entail. But, I haven't tried. If you have, what was it like? You really think you'd be okay to dive the rest of a week like that if your controller died on Day 1?

I understand wanting to have 2 that are both live monitoring O2 sensors. That's why I have a controller and a NERD2 for a monitor.

But, to continue your example, your controller dies during the last dive of Day 1. For Day 2, you're going to dive with 2 NERDs. So, what do you do if your NERD2 dies on Day 2? I mean, your NERD1 doesn't have any of the tissue loading from Day 1. Would you still follow its plan to get yourself out, knowing that it doesn't have all your tissue info?

If I were going on an "expedition" trip, I think I would consider my controller, NERD, and Teric to be enough in the way of "backup". The next things I would pack - before a spare NERD - are spare O2 sensors, spare O2 and solenoid boards, a spare rMS probe, and maybe a pair of spare 4 and 5-pin cables. And maybe a spare loop hose and some scrubber cover O-rings. It's hard for me to imagine a Predator/Petrel and a NERD2 both dying during a week or 2 trip. And as long as they don't both die, I really would not need a spare NERD.

But, I get it for your situation. You have a NERD1 already, and a NERD2. Personally, I'd be trying to sell that NERD1 while it still works and is worth something. I think they don't offer repairs on NERD1 units anymore, so if it dies, it becomes a paperweight. If you were using it frequently, that would be one thing. But, even sitting in a save-a-dive kit, it is at least somewhat perishable.

 

[beldridg]

Have you actually dived with both NERDs on your unit? Based on my one, I really don't think I would like the loss of vision that having 2 would entail. But, I haven't tried. If you have, what was it like? You really think you'd be okay to dive the rest of a week like that if your controller died on Day 1?

Yes, I've tried it. I basically flipped one of them down out of site but still had it on the DSV.

I would be totally okay spending the rest of the trip diving like that.

On a trip to Chuuk in 2018 I had my DiveCan system fail after the first day (the Petrel was still monitoring cells) so I didn't have RMS or a solenoid controller. No big deal. I spent the entire trip running the system manually and just rotating scrubbers after 3-4 hours of dive time. With a CMF, it is really easy to run the system manually.

But, to continue your example, your controller dies during the last dive of Day 1. For Day 2, you're going to dive with 2 NERDs. So, what do you do if your NERD2 dies on Day 2? I mean, your NERD1 doesn't have any of the tissue loading from Day 1. Would you still follow its plan to get yourself out, knowing that it doesn't have all your tissue info?

Well, as you said in another post, if the NERD2 dies the day after the Petrel dies then we likely have a more serious problem.

But, in that case, remember that I also have the offboard Teric that is using an internal setpoint. If that did happen though, I would follow the more conservative of the Teric and the NERD1 and pad my deco.

If I were going on an "expedition" trip, I think I would consider my controller, NERD, and Teric to be enough in the way of "backup". The next things I would pack - before a spare NERD - are spare O2 sensors, spare O2 and solenoid boards, a spare rMS probe, and maybe a pair of spare 4 and 5-pin cables. And maybe a spare loop hose and some scrubber cover O-rings. It's hard for me to imagine a Predator/Petrel and a NERD2 both dying during a week or 2 trip. And as long as they don't both die, I really would not need a spare NERD.

Most people would agree with you.

On expedition style trips, I bring 2 spare O2 cells, a spare solenoid, an Apeks 1st stage rebuild kit, etc. I also do bring a spare loop hose section, a spare DSV, etc.

I don't need an extra 4 pin cable because it is already installed in the unit.

If you want, send me a PM and I'll send you my checklist of spares that I bring on trips like that.

- brett

 

[rjack321]

How would you have a shorter TTS on BO?

Run some side by sides

Beyond about 65-70m your TTS is shorter on OC (also shorter on BO). Assuming you arent doing the dil switch magic wand.

Bailing does not mean your deco is somehow going to be longer or more complicated - except for needing to vent your loop as it expands.

 

[Wibble]

Okay, I see what you mean. But, I am not experienced enough on CCR and don't have the training yet for that depth (on CCR), so I would have to do some planning. My gut tells me that I probably would not use 50% as an intermediate deco gas. My gut is that I would end up using trimix, to mitigate risk of ICD. But, that's just a gut feeling at this moment.

It's not really about experience with CCR, it's about an understanding of decompression and planning theory.

The brilliant thing about CCR is trimix is sooooo cheap! You can pretty much use it for every dive and have a clear head. Any dive below 25m/80ft should be a mix dive. When diving lower than 35m/110ft mix should be considered as obligatory simply because it makes the dive safer and you remember more.

It's worth doing a 'pot' dive (recompression chamber) to at least 40m/130ft to see just how 'off yer face' you are on nitrogen. Like a giggling bunch of schoolkids and incapable of simple problem solving.

I remember doing a quarry dive to only 45m on air (actually it was 23%). We dropped straight down a shot line to a little boat. I was off my face and utterly incapacitated with fixation on the depth and gas pressure... A real shock, literally checking my gauge and computer. Only when we ascended to 30m/100ft did it fully abate.

One does learn how to cope with narcosis, but it's insipid as it dulls your senses, so you really don't know how poor your judgement is as you rely on your training and unconscious competence (muscle memory). Even on OC I used to set my personal nitrox limit to 40 metres / 130ft, at which point I'd dive with helium. Nowadays (if ever I could go and dive again -- locked down prisoner), I'll dive with trimix for all but the shallowest of dives. The £5/$7 extra gas bill isn't a problem.

 

[stuartv]

Run some side by sides

Beyond about 65-70m your TTS is shorter on OC (also shorter on BO). Assuming you arent doing the dil switch magic wand.

Bailing does not mean your deco is somehow going to be longer or more complicated - except for needing to vent your loop as it expands.

I'll have to run some plans and check it out.

What I'm hearing is that I could plug my BO deco gas to my offboard Dil port on my MAV and, when I ascend to the right depth, do a dil flush with that to shorten my ascents. Of course, I'd also have to program that BO gas into my computer as a second Dil. I always wondered by Shearwater gave the ability to define more than one dil....

To be clear! I am NOT going to actually do that. I'm just talking about the potential practical application of what you're saying.

My guess (not having run the planning yet) is that the difference would be single digits of minutes. Not worth the complexity, I think, but still an interesting thought experiment.

 

[Dsix36]

You can change dils and do a dil flush but in all reality it is not very practical and usually the benefits of less deco are very minimal. The chances of other serious physological issues from a sudden gas switch needs to be considered too and there is always the chance of plugging in the wrong gas and killing yourself.

In all my years of diving both deep oceans and caves, I have only done a dil switch and flush one time. It was when a 5 hour planned cave dive went bad and turned into more than 15 hours. I manged to get out in less than 10 hours and live to tell about it.