The reason computers are programmed this way is a theory of safety. It's important to remember that THERE IS NO GOOD WAY TO DO THIS, so they picked one of several bad and awkward options. Essentially, it reverts to 21% to force you to check your gas mixes and to make sure your computer settings and your actual gas mixes match. The thinking is: you put down your dive gear for a couple hours. Did you check your settings before your next dive?
Before anyone gets all huffy, and make all kinds of bold statements about how careful they are, and how experienced, etc., ask yourself have you ever forgotten the milk or lost your car keys? People make mistakes. There are all kinds of scenarios you could come up with where you could poke holes in this solution, about how it's awkward and does not apply to you, but the simple fact is that it defaults to force the divers to go double check their equipment. It's a mechanism put in place by manufacturers to minimize risk. It's not an ideal solution, but that is the reason for it.
As to manuals, I applaud anyone who takes the time to read the manual, every diver should. That being said, some are better than others, some are just downright terrible, but speaking from long experience dealing with manuals and how people use them, you cannot write a manual that everyone can understand. Even people who read them don't really READ them. I cannot calculate how wealthy I would be if I had a nickel for every conversation I've had that went something like this:
"This is a piece of crap, I can't make it do this and it's nowhere in the manual! this things sucks, your company sucks and you suck!"
ME: : The feature you are asking about is on page 6, paragraph 3 of the manual. It provides a step by step process to do what you want. In the meantime, let me walk you through it, and later you...
"I have the book right here! It does not say.. Oh...... Well, it's not listed anywhere."
ME: It's the first entry , first page on the table on contents. Just flip the cover, it's on the first page. In the meantime, let me walk you through it to make it easier so we can get you started...
**CK YOU! and hangs up the phone.
I could retire. No one likes to shown they are wrong, and there is no graceful way to help them if they don't listen.
I checked by setting mine to a different percentage, and indeed it set itself back to 21%.
I am not sure if it's actually better that it doesn't reset during a dive series or not. In my case, I have been diving nitrox, and a day later I went on air, but it was still on my last setting of 32%. So I had to set it to 21%, no big deal.
However, what I find potentially dangerous when thinking about this is that you usually don't test air for O2 content, so it doesn't start the "I test my nitrox for O2 content and then have to set my computer accordingly" routine. You could jump in and find yourself in water breathing air, but computer is still set to whatever it was the day before (because you are still in the dive series), so NDL time it is showing would be wrong and for following dives nitrogen load will be wrong, too. So it's either abort dive or dive tables I guess.
The other way round, if I were to jump in with Nitrox but computer is (re)set to air, the only thing to worry about is max bottom, but nitrogen load won't be higher than whatever the computer is calculating, even for following dives.
---------- Post added February 19th, 2014 at 01:42 AM ----------
I would also put the Uwatec Aladin 2G in that group... by itself it might be a few bucks more expensive, but considering the cost of the data cable for the others, I think the Aladin (uses infrared) is pretty competetive.
