Stand alone secondary computer use

[broncobowsher]

@rddvet mine is set for manual switch from 1.0 to 1.4. I try to run 1.1 *the least annoying HUD setting on my Meg*. My backup computers are set at 1.0 to give a bit more conservative deco and since I run between 1.0 and 1.1.
The "high" setpoint is 1.3 for deco and I switch when I start deco. That one is a bit complicated since I try to do as much of my deco at 10ft, so have a max of 1.3. Rare to have any big gap in the two, always less than 5 minutes.

I would consider starting out with a loop full of O2 if I were you. Easier to deal with your 1.6ppO2 check at 20ft, but also minimizes risk when using hypoxic dil.

@broncobowsher if you can't remember to change your computer from CC to BO you shouldn't be diving a CCR. That should be ingrained in your bailout procedure. Shut DSV/BOV and switch to OC. Turn off O2. Change computers. Signal buddy you've bailed out and O2 is off.
If you are diving at depths where your dil ppO2 may be .7 or .8 and that is a massive difference in deco obligations.

Where did I say anything about not remembering to change from CC to BO?

I have been taught (and practice) changing ALL my computers from CC to BO when I bail. Wrist first, NERD second. Much more likely to see the wrist while bailed and others can see it as well. NERD gets changed as a backup but since it isn't in my eye anymore it takes too much work to look at compared to wrist (while bailed out).

But this thread is about what to set a stand alone computer to while on a rebreather. The poster is new to rebreathers, can't expect a perfect stable PPO2 100% of the time. But there is a target. Better would be a window. Personally I aim to stay in the 1.2 to 1.3 window. Try to stay close to the 1.3 but the solenoid will fire if I let it drop to 1.2
What would I set a completely stand alone computer to calculate at? The 1.2 PPO2. If I manage to dive at 1.3 what errors will this cause?
Not counting all the CNS. Problem? No, your controller died. You are not going to be diving until the controller is fixed. Don't need to worry about stacking up too much time.
Adding extra deco time. Problem? No, just like diving nitrox on air tables, you are just a little conseritive. In the water a little longer, but safe.
Bending the computer. Problem? No, it is a Shearwater, it will clear itself. With the controller working and saying all is good the stand alone might ask for an extra minute at that depth. If you can, what's and extra minute going to do to the plan? (this could get sketchy if you are doing extreme diving where you are into multiple hours of deco, but for general use it is a non-issue. Get more training, better gear).
Did I miss any?

What you don't want is the other way around. If you set the SA at 1.4 and dive 1.2 you are not counting all of your on-gassing. Like breathing air and using Nitrox tables.

 

[stuartv]

If you can afford it, NERD is a good backup. It knows your real PO2, it’s great to have all that info at HUD level, especially for photographers. And you can use it even if you have to go off the loop...

Speaking just from a practical, mechanics point of view, is it really feasible to keep using the NERD after you bail?

You normally dive your CCR with just a single handset controller and a NERD?

If you have to go off the loop, wouldn't you just switch your handsetcontroller to BO mode and follow that instead of trying to use the NERD? If the handset dies, then fly manually but stay on the loop, using the NERD?

Is there a legit risk that the handset would die AND you'd have to go off the loop?

 

[stuartv]

With just a Petrel and NERD, what do you do with them if you have to go to SCR mode for some reason? (i.e. something happens where all your sensors are dead) Maybe that would be the one scenario where you'd actually use a standalone backup?

 

[doctormike]

Speaking just from a practical, mechanics point of view, is it really feasible to keep using the NERD after you bail?

You normally dive your CCR with just a single handset controller and a NERD?

If you have to go off the loop, wouldn't you just switch your handsetcontroller to BO mode and follow that instead of trying to use the NERD? If the handset dies, then fly manually but stay on the loop, using the NERD?

Is there a legit risk that the handset would die AND you'd have to go off the loop?

Yes, you are right, those two failures are unlikely. The following is based on the only CCR that I am certified on (JJ), not sure what other units do for a controller, but I'm using a Petrel. The point is that the NERD replaces both the SA backup and the HUD (with more functionality).

If your CCR stops being a CCR and you go off the loop, chances are that the controller is still going to be working as a dive computer, it just won't have current PO2 information. You bail out to open circuit, switch your controller to OC, follow its ascent plan and forget about the NERD (although you need to switch it to OC as well, since if you go back on the loop you want it to have accurate deco information).

On the other hand, if the controller dies, the CCR will switch to low setpoint. Then you can finish the dive maintaining whatever PO2 you want manually by watching the NERD. Sort of like that video of Leon destroying his Meg at 215 feet, although I think that with no controller the Meg is maintaining 1.2.

With my CCR, I would only need to use the NERD off the loop for deco if I had to bail out and also had a failure of the controller, which is two failures, so as you point out, unlikely. More of an issue for a unit where you aren't getting deco information from your controller, and the NERD is your only deco computer. But I suppose most people with that sort of configuration are going to have a SA. Don't know, don't have that kind of experience.

 

[broncobowsher]

It takes what, 3 button presses to change the NERD to BO? While not my highest priority it is still worth doing. Take care of urgent things first.

 

[rddvet]

Just to follow up, I've set my secondary to a low set point of 1.0 and a high of 1.4, representing bottom segment and deco. Interestingly, I had to talk to shearwater because diving it this way won't allow auto set point switching. I thought I was doing something wrong. So I had to set it fro manual switch. Which is fine. If I don't remember to switch it on the dive, I'm an idiot.

 

[tbone1004]

@broncobowsher must have been an incorrect tag, my bad. alt. you were responding to someone and I was agreeing with you but it got edited, who knows. I've been teaching high schoolers all weekend with like 6 hours of sleep each day so my brain is fried. I think it was something about leaving a computer in b/o mode on your dil mix while diving and me saying that that's retarded.

I dive 1.1 setpoint because anything else is super obnoxious on my HUD, or most huds for that matter. My standalone stays at 1.1 or 1.0 depending on the dive. If I'm somewhere with relatively stable depth, 1.1 is fine and it's within a minute or two of my Predator. If I'm diving a cave with some wicked depth changes, then I leave it at 1.0 because I'm going to be pretty consistently lower than 1.1 with all of the ups and downs. Again, stays within a couple of minutes on long dives.

If you bend it, who cares, it's a Shearwater, so there's no issues. It just gets angry with you, then gets over it. If you don't want to bend it, who cares, an extra 5-10 minutes of deco never killed anyone.

@stuartv and @doctormike on the NERD thing, if you have a BOV, why would you ditch the loop? If I had a pair of Shearwaters on the rig though, I wouldn't have a standalone computer. No point.

@rddvet my switch points are the same. Much easier to have the standalone "low" be your "dive" setpoint and "high" as my deco setpoint

 

[doctormike]

With just a Petrel and NERD, what do you do with them if you have to go to SCR mode for some reason? (i.e. something happens where all your sensors are dead) Maybe that would be the one scenario where you'd actually use a standalone backup?

But the NERD and the Petrel controller are two separate dive computers, with two separate power sources. Dead sensors wouldn't affect either of them as dive computers. As per Leon's video, you could cut both DiveCAN cables with your shears and still have two dive computers. So not sure why a SA would be better than the NERD for running your ascent (the NERD really IS a SA in that respect). Even with a SA, you still have the same question - what do you tell your dive computer that you are breathing so that it calculates your deco correctly in SCR mode?

And this is an interesting question... It's pretty much beyond my level. There isn't a lot out there about SCR mode, and even less about planning deco if you have to do it. This thread is one of the longest discussions that I have seen. You should realize how rare the situation would be where you would be doing significant deco in SCR mode. It's basically an alternative to BO if you lose your O2 or if you lose your electronics. You want to stay on the loop, for at least part of the dive, and there are a few situations where that might help.

So while you could just calculate deco based on your dil (e.g. set your computer to BO with dil as your gas instead of of your offboard BO, and changing it if you plug richer deco mixes into the loop on ascent), most people recommend adjusting for the SCR mode PO2 drop with a small factor to increase your deco. To be honest, this is pretty complex, and while I can see the utility of practicing the technique, I'm not at the level where I would be relying on it in a situation with a big deco obligation. Better to just carry enough BO.

 

[doctormike]

@stuartv and @doctormike on the NERD thing, if you have a BOV, why would you ditch the loop? If I had a pair of Shearwaters on the rig though, I wouldn't have a standalone computer. No point.

Right, that's the point I was making. When I got a NERD, I ditched the SA because the NERD is my backup for deco. And as Stuart pointed out, if you need a backup DC to calculate deco, chances are you haven't ALSO needed to bail out - two failures. So it's sort of a non-issue. But the point is whether you are on the loop, off the loop with a BOV, or off the loop with a DSV, the NERD is still readable. It's just a bit less convenient in the last scenario.

 

[stuartv]

But the NERD and the Petrel controller are two separate dive computers, with two separate power sources. Dead sensors wouldn't affect either of them as dive computers.

Scenario: Imagine my CCR has a Predator controller and a NERD. And it has 3 sensors.

Imagine that I somehow get water into the O2 sensor tray and it screws with some of the sensors. Through doing a dil flush and checking pO2 readings, I am able to determine that both computers are in a state where they are each reading 1 good sensor and voting logic has voted that 1 out. So, both computers are getting bad input regarding pO2 in the loop. But, between the 1 sensor on each that is good, I am able to fly it manually with confidence in staying on the loop.

AFAIK, there is no way to tell the Predator or the NERD to only use the 1 sensor (which has been voted out), instead of the other 2 sensors. And there is no way to switch it to "Standalone mode" where it ignores all sensor input and lets you tell it your SP. So, the deco calculations are going to be wrong. But, if I had a standalone computer, it would be correct because I would flying the unit manually, maintaining a SP that matches what the SA computer thinks.

I do realize that is a pretty far-fetched scenario. But, for a unit with only 3 sensors, and given the discussion that has been posted elsewhere here on SB about someone dying from having only 1 good sensor and it getting voted out, it doesn't seem THAT far-fetched. And yes, I do realize that even if this happened, you'd probably be able to simply reduce your SP and have the 2 "bad" sensors work well enough to get you home.

I'm just explaining why I said earlier about having dead sensors and if that would be the only case where you'd really want a SA computer.