Stand alone secondary computer use

[rddvet]

I recently finished my first CCR class and during the class we obviously discussed decompression on the rebreather in depth. We also briefly discussed the use of a secondary stand alone computer as a backup if you're primary handset craps out. Half way through the course I got sick and it put a little damper on some of the things we wanted to cover in further depth. Eventhough we discussed using a secondary backup, we didn't actually have time to sit down and go through setting up the Shearwater backup that I brought with me as a standalone backup. Now that I'm back at home and getting comfortable on the unit, I went to setup my backup and am a little confused. Shearwater has you set a hi and a low setpoint and they can be set to automatically adjust at a set depth. My rebreather is fully manual and the HUD is set to a p02 of 1.0, so I generally maintain between 1.0 and 1.2 depending on the dive. At the surface I'm obviously not necessarily concerned with maintaining a 1.0 before descent. I think I'm overthinking it, but I'm trying to wrap my head around how I should set the set points if I'm typically maintaining a 1.0 for most dives. The stock setpoints in the shearwater are 0.7 and 1.3 I believe with an auto switch around 66ft. What I'm overthinking is: should I just set my hi and low setpoints to both be 1.0 since that's what I maintain on a typical dive? Or should I keep the low at 0.7 since I'm generally running my p02 a little below 1.0 from the surface until mid-descent?

I've also had a few people tell me that they run their backup in OC mode with whatever BO gas they're carrying. The thought process being that if the crap hits the fan you may forget to switch a computer to BO mode, thus affecting the deco obligation. Doing that seems silly to me.

Before anyone says anything about asking the instructor, I plan to but I bug him with enough stuff as it is, so just wanted to see if it's way simpler than I'm making it out to be before bugging him.

 

[rjack321]

I set mine to switch at 25ft I think but I can't remember if that is both ascending and descending which I think are set independently.
low is 0.7
high is 1.2 or 1.3 depending on the dive. (1.2 for longer cave dives, 1.3 for colder shorter wreck dives)

Running a setpoint of 1.0 seems really low to me unless you are doing 6 hour dives already? Even for 4 hr dives I run 1.2. Over 4 hrs I drop to 1.1 but my max is around 5 hrs still, longer is cold and boring at that point. I have not had a dive that necessitated dropping to 1.0 yet and I have 4 years and a bit over 200 CCR hours now.

 

[victorzamora]

I have mine at my target setpoints for "bottom=low" (1.1) and "deco=high" (1.4) and I run my CCR manually with no auto-switch on my Petrel.

What I did (and do) is check the CNS clock and the deco obligation of both computers a few times throughout the dive. I was much more diligent about this early on as I wasn't sold on the standalone computer, but feel like I'm now pretty consistent.

My two computers are usually within +-1% on CNS and +-2min on deco obligations. Obviously I just fall to the level of the more conservative computer, but that's WELL within what I'd call "acceptable margins of error" between measured and standalone.

 

[northernone]

If you're running the loop at a ppo2 of 1.0 consistently from the surface to the bottom, yes I'd set my backup to a constant ppo2 where the high and low match your best guess so your deco obligation and oxygen exposure is estimated as accurately as possible.

Cameron

 

[lv2dive]

I’m with Cameron - when I run my backup SW I run it in CC but I don’t have it switch automatically based on depth. (The automatic switching makes me think of Cochran dive computers -bonus points if you understand what I mean!)

 

[broncobowsher]

If you set the computer to the lower side of your setpoint window it will add a little extra Nitrogen loading. If you run 1.0 to 1.1, set it to 1.0 and call it good. Yes, you won't be counting full CNS, but with a controller failure you are not likely to be back in the water anytime soon anyway. CNS is ignored with a controller failure, you are getting out of the water ASAP. You should be counting a little extra Nitrogen loading than actual but that is a safety cushion.

You can still program bailout gasses and do a gas switch on the computer if you need to bail.

Is the controller a Shearwater as well? If so you just set both computers the same. Just that the stand alone will assume the PPO2 instead of actually knowing what you are breathing.

 

[lv2dive]

Richard I think 1.0 to 1.2 is fairly standard starting point for a lot of N FL instructors and divers. For me, I start with 1.0 for the bottom time and go up or down from there depending on the dive. I don’t ever dive higher than 1.2 on the bottom (in caves). I bump it up for deco - not everyone does that - but then again I’m not doing deco longer than 5 hours*.

*to be fair, I'm not doing *any* deco right now but a couple of years ago... and hopefully again soon.

I set mine to switch at 25ft I think but I can't remember if that is both ascending and descending which I think are set independently.
low is 0.7
high is 1.2 or 1.3 depending on the dive. (1.2 for longer cave dives, 1.3 for colder shorter wreck dives)

Running a setpoint of 1.0 seems really low to me unless you are doing 6 hour dives already? Even for 4 hr dives I run 1.2. Over 4 hrs I drop to 1.1 but my max is around 5 hrs still, longer is cold and boring at that point. I have not had a dive that necessitated dropping to 1.0 yet and I have 4 years and a bit over 200 CCR hours now.

 

[rjack321]

I’m with Cameron - when I run my backup SW I run it in CC but I don’t have it switch automatically based on depth. (The automatic switching makes me think of Cochran dive computers -bonus points if you understand what I mean!)

Why would you not let the backup manually roll over to your chosen bottom setpoint? For me the low is basically surface mode. The "high" (1.2 or 1.3) is the rest of my dive, it clicks over at somewhere between 10 and 25ft on descent. I might have short spikes or dips around that average but they work out to nothing different ascent-wise. I would not want to waste bottom time (or worse forget) to mess with my backup at the bottom since I already run live deco on my other shearwater anyway.

 

[rjack321]

If you set the computer to the lower side of your setpoint window it will add a little extra Nitrogen loading. If you run 1.0 to 1.1, set it to 1.0 and call it good. Yes, you won't be counting full CNS, but with a controller failure you are not likely to be back in the water anytime soon anyway. CNS is ignored with a controller failure, you are getting out of the water ASAP. You should be counting a little extra Nitrogen loading than actual but that is a safety cushion.

You can still program bailout gasses and do a gas switch on the computer if you need to bail.

Is the controller a Shearwater as well? If so you just set both computers the same. Just that the stand alone will assume the PPO2 instead of actually knowing what you are breathing.

I'd be bending my backup if I did that. For my diving, with both my live deco and my standalone on 50/70 GF I would have about 10-12 more minutes of deco at 1.2 vs 1.3 on the standalone.
eg if I run 1.3 on the wreck and follow my live deco shearwater then my standalone set at 1.2 will be bent around the 20ft stop and continue to be bent all the way to the surface. Usually the deeper stops are close enough that I can clear them within a minute but the longer slower tissues are not clearing as readily and it adds up to a total of ~10-12mins more typically spread across the 30-10ft stops.

 

[rddvet]

Running a setpoint of 1.0 seems really low to me unless you are doing 6 hour dives already? Even for 4 hr dives I run 1.2. Over 4 hrs I drop to 1.1 but my max is around 5 hrs still, longer is cold and boring at that point. I have not had a dive that necessitated dropping to 1.0 yet and I have 4 years and a bit over 200 CCR hours now.

It's fairly standard with many cave CCR insturctors to recommend a 1-1.2 p02 and the manufacturer of the unit recommends 1.0 for longer exposure dives and 1.2 for a typical shorter cave dive. I actually was a little surprised when I first learned it. To prove a point we sat down and calculated several typical dives I'll be doing on CCR (not much heavy trimix at this point) and the difference in deco between 1.0 and 1.2 was minimal. My instructor's point was he would much prefer a lower CNS % as opposed to doing a little bit of extra deco because eventually most people will use their CCR for much longer dives than accomplished OC>

I have mine at my target setpoints for "bottom=low" (1.1) and "deco=high" (1.4) and I run my CCR manually with no auto-switch on my Petrel.

What I did (and do) is check the CNS clock and the deco obligation of both computers a few times throughout the dive. I was much more diligent about this early on as I wasn't sold on the standalone computer, but feel like I'm now pretty consistent.

My two computers are usually within +-1% on CNS and +-2min on deco obligations. Obviously I just fall to the level of the more conservative computer, but that's WELL within what I'd call "acceptable margins of error" between measured and standalone.

Thanks, that helps. I think I will set the low for my typical 1.0 and the high for deco at 1.4 and see how I like that. I was severely overthinking it. I was thinking too much about the lower p02 I start with at the surface and allow it to build to 1.0 on descent. That's really such a small part of the dive it really shouldn't factor in much.