Forgive me but is the HUD many are mentioning just lights that either go out or turn red if the PO2 is endangering you? Do you know if the po2 is too low or too high by the lights? Also would you even be able to see a NERD in a complete silt out, isn't it outside of the mask?
It depends to some extent on the unit and the electronics being used.
With the Shearwater HUD on your KISS Sidekicks, there are three LED positions, one for each sensor. Each position has red and green LEDs, and it's set to flash amber (both red and green) at a PP)2 of 1.0. For each .1 over 1.0 it flashes green once. So 2 green flashes confirms you are at a PPO2 of 1.2. Similarly, for each .1 under 1.0 it flashes one red, so 4 red flashes means a PPO2 of .6. Basically, a whole lot of red flashing is bad and a whole lot of green flashing is bad too. 2 green flashes is what you want if you're running a PPO2 of 1.2.
It's pretty simple and it flashes the sequence every 5 seconds, but that also means that with the tome it takes to flash a lot of flashes that if your PO2 is getting down to .2 it's going to be flashing red almost continuously (8 times every 5 seconds) at you, so it's hard to miss or ignore. Same with high PPO2s above 1.4 - lots of near continous green flashing.
The Shearwater NERD is also outside the mask but you can set it basically right up against the mask and it would still be visible in very low viz, and like the Shearwater Petrel display, PPO2s of .4 or less will flash red, and PPO2s above 1.6 will flash yellow, alerting you to an out of range condition, even if you couldn't read the actual number.
You mean other than they are an equipment solution to a skills problem???
I like redundancy and consistency. This way I get two PDCs giving me redundant deco information and am able to see what's happening with my cells. My SF2 is an eCCR using Shearwater's Dive Cann bus system. It has two controllers: the one in the head and the one on my right wrist. If the right PDC dies the one in the head takes over, but then I don't know what my PPO2 or deco status is. A HUD would tell me only what my PPO2 is. With the second Petrel II, I can see cell mV, PPO2 and my Deco Status. Also, I don't feel a need to have lights flashing in my face. I check my Shearwaters quite often and don't need the added reminder.
Given the side mount stuff we're doing and the greater potential, for low viz, I like the HUD. It immediately alerts me if a senor is dropping out, and it also immediately alerts me of high or Low PPO2 conditions even if I am not able to look at my computer at the moment, and in between the times I'll check the computer (every 3-4 minutes during stable depth and workload conditions - more frequently, on ascents and descents). That gives me warm fuzzies.
Since we both already had CCR enabled Predators, we opted to go with a fisher ported external monitoring Shearwater computer to monitor the PPO2 and serve as the primary deco, and then use our non fisher ported stand alone Shearwater computers for back up deco.
I wasn't sure how well it would work at first, since you establish high and low set points and then switch between them on the SA computer, while the EXT computer connected to the sensors bases the NDLs and deco schedule on the actual PPO2 in the loop.
For example, on the SA computer, you might use .7 as the low set point and 1.1 as the high set point. You establish a breathable loop and maintain the .7 at the surface and on initial decent, then once you get to 50 ft or so, switch the stand alone computer to the high set point and start bumping your PPO2 up on the unit to 1.1 as you descend so that you're at 1.1 at your target depth for the dive.
That means the SA computer is computing deco based on perfect maintenance of your set point, while the EXT computer is computing deco based on the set point(s) you've actually maintained on your unit.
In practice however, we've found that even with a manual CCR, we seem to do a very good job of maintaining the desired set point as there is rarely more than a minute or so difference in the deco schedules on the SA and EXT computers. And at any time in the dive you can compare the two computers and see how well you're doing at maintaining an accurate set point, based on the EXT information compared to the "ideal" situation presented by the SA computer.
The SA computer is actually a great tool because you can see if you're falling behind the curve so to speak if you've let the set point drift a little low and let it stay there. Also, the NDLs and deco on the fisher ported computer is based on the current PPO2 in the loop, and if it's momentarily .1 high or low due to a descent or ascent, it can make a significant difference in the projected NDL or deco schedule. However you can still look at your SA computer and see where you'll really be once the set point is back where it should be.
Consequently , I've discovered I like having the SA back up for deco, as it keeps me honest about how well I'm really maintaining a PPO2. I can also look at Marci's SA and EXT computers at any point in the dive and know how well she's been doing at maintain an accurate set point. That's important in terms of ensuring we both have similar deco obligations at the end of the dive. If for example the plan is to maintain 1.1 and one of the team mates maintains an average of .8 instead and incurs a boat load more deco, that will become obvious earlier in the dive when the differences can still be minimized, or at least is allows some discussion to occur post dive about the need to maintain a reasonably accurate set point - which surprisingly is a lot easier to do than it sounds on an mCCR.
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But from a safety perspective, I think it matters less what you've got and more that you understand exactly how it all works and exactly what limitations and failure modes it has, so you can be vigilant about detecting a problem before it gets out of hand.
