I agree, but it's probably worth some discussion of the purpose of a lights out drill in a modern cave class - whether it is CCR or OC.
I think most cave divers and instructors would be hard pressed to recall the last fatality, or even the last incident where a team encountered total light failure. Modern primary lights are very, very reliable, and many modern LED back up lights put out more light than the primary light I started with and given burn times of 4-5 hours at that intensity and approach 30 hours at lower intensities, automatically stepping down to achieve that as voltage in the battery drops. When you consider you have 2 primaries and 4 back ups (minimum) on a team of two the odds of having to come out of a cave with no lights are right up there with winning next Wednesday's Powerball jackpot.
The purpose however is to teach the skills needed to deal with a silt out. A potential problem for an instructor in a lights out drill is that modern computers provide a fair amount of light, so I suspect some instructors still prefer to cover masks as they apparently can't trust students to close their eyes, and/or don't like the limited ability to monitor the student(s) with brief, dim flashes of light.
Whether you'd want to operate with no access to your primary display or HUD is probably unit specific. For example, on our KISS Sidekick mCCRs, there's no O2 solenoid to stick open, and if the O2 MAV were to stick you'd hear it, and shut off the O2 to avoid a high PPO2 condition, and if it had already occurred, you'd do a dil flush to get a known to be good volume of gas in the loop, even if you didn't trust the sensors or couldn't see the displays.
Low PPO2 conditions take several minutes to develop, unless you are also rapidly ascending and a roll off of the O2 (which we've had happen) is a non event as you note the PPO2 going .1 low either on the HUD or on a routine check of the computer every 3-4 minutes, hit the manual add valve to correct it and notice there's no O2 bleeding into the loop when you press the button. If there are no scooters or open circuit divers around, you'll probably note the lack of sound of gas flowing through the orifice as soon as the roll off occurs. And, just like OC side mount, you should be checking for a roll off anytime you encounter a situation where a roll off could have occurred.
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In that regard, I don't think using a covered mask for a few minutes would lead to my imminent demise, however I personally don't think covering a mask at any time makes any sense in a rebreather course. At a minimum though I'd want to be confident that the instructor would right over my shoulder monitoring the loop and maintaining an awareness of the loop and the configuration of the rebreather.
Mask covering is also pretty pointless as it doesn't reflect any real world scenario. In truly zero viz, you can still plant the HUD right up against your mask and still see the flashes, and with a missing make, you can still see the HUD happily flashing the PPO2 information from the sensors. You'd also really have to screw up to have a total silt out with truly zero viz for more than 50 feet or so. Even if you come through a few hundred feet of small, silty side mount passage and then turn the dive and retrace your route, that zero viz will have improved to at least some viz over most of the distance, and just a few inches is enough to easily see a HUD.
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We've been diving under a limited sign off for cave diving under some limited penetration distance and bailout conditions, following our technical CCR cert and pending completion of our full CCR cave cross over on our next trip. Consequently, I can't speak to full cave CCR classes specifically, and I suspect it might be a different for a CCR diver taking cave training for the first time as opposed to a full cave diver taking technical CCR training in a cave. However, running the unit without electronics is covered in technical CCR course and you have some options. Assuming the depth is reasonably constant and you're running it with minimum loop volume, you should be able to continue to maintain your set point just by adding O2 as needed as you metabolize it with no reference to the electronics. That however assumes you can maintain a steady depth and know it's the same depth. That isn't something you can do with a mask covered, unless the line you are on is at a constant in depth. Another option is to run it as an SCR, using the SCR operation to extend the off board bailout gas. The PPO2 during the dive and the deco won't be as efficient, but it's something that can be done even with variations in depth.
We added a HUD to our KISS Sidekicks in part for redundant loop monitoring, but also in part to ensure that we could monitor the loop in low viz conditions in silty, tight, side mount passages. It works as we've been in some very limited viz (a few inches) a few times in short sections of tight, silty passage, and you can still see the HUD happily blinking the loop status.
In that regard, the only time we're not going to be able to monitor the loop would be in the event of an electronics failure, and that's a whole different set of drills, options and responses.
