| Re: Solenoid?
I'm going to take a completely different tack on this from AD_Ward.
IMHO, you need to consider a few things here FIRST:
What is the consequence of a stuck open (or failed closed) valve? This comes down to how much time you have from the event until something really bad (e.g. too high PO2) occurs. It also comes down to what sort of redundancy (is it manual, electrical, etc) do you have in place for such failures. Basically, is a failure a nuisance or a life-threatening event? This is determined by a number of things, such as the IP you're running at, the flow rate of an open valve, etc. Much of this is interdependant with other things (a valve that flows little must be "on" for more time overall, thus draws more power and has more impact on battery life, etc.) Price is also a part of this, because at a low price a diver keeping a spare valve around is not a big deal (if it can be swapped easily); at a high price or with limited availability that's unreasonable, thus, you need much higher reliability.
Once you figure THAT out, you can then determine if you can tolerate failures or not, and how often you can tolerate them.
BTW be aware that there's more than one risk with solenoids - if they or any part of their wiring are inside the loop you have to pay very close attention to electrical issues because at depths under 70' you're likely to be running a FO2 in the loop that will support greatly accelerated combustion. Thus, avoiding potential ignition risks becomes a critical matter.
This is very similar to most of the other design decisions, really. Mechanical items break - this is a reality. The questions come down to how often do they break, how much do they cost to fix, what risks come when they break, etc.
I recommend looking at the entire system and doing this analysis piece-by-piece..... with the solenoid as part of it......
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