I voted for AUTO on/off.
having said that, a number of issues come to mind:
1. what are the problems of running the humidifier with no water? does it represent a fire hazard, electrical hazard, hot plastic fumes hazard, etc? In the same breath I should stipulate that whether it's manual or automatic doesn't bear on that issue, except that if there are safeguards built in to protect from any such imaginable hazards, then it is only a question of convenience. I would choose to conveniently have it turn itself on and off. That is not to say that one couldn't manually turn it on then watch it melt because there was no water in the reservoir, it would just seem less likely to happen.
2. The above also presupposes that the thermostat can operate perfectly with or without water in the reservoir to distribute the heat. My initial reaction is that it is a "given", (it has been tested and certified by UL and the FDA, they MUST'VE thought of that, yes?) Otherwise i'm back to "manual".
3. Some people like to pre-heat the HH to start it hot and wet right out of the gate. I personally don't think those few minutes make much difference. But since we're redesigning this, it might be good to have it both ways: manual "ON" to pre-heat, then automatic "REALLY ON" with first puff.
4. Another engineering consideration: with the "auto on" feature there is probably a tightly-coupled "automatic-forgetting-to-check-the-water-level" feature. I would suggest a water-level detection and response feature. (There are lots of ways to check the fluid level with NO MOVING PARTS). Granted, it would add another $5 to the cost of manufacturing, (AND raise the retail price another $500). Engineerically speaking, one could probably "infer" the water level using the existing equipment, by calculating the temperature response curve for a given applied voltage to the heating elements.(?) But better ways to do it.
5. More HH RE-design considerations: Add a choice of audible alarms, or downloadable "ring tones": ("THE WA-TER LE-VEL IS DAN-GER-OUS-LY LOW PLEZE ADD WA-TER NOW". or "Please suh, more water", or "YO! BRING ME DA WATUH! and SOON!). or whatever... At least something detectable within the audio range of the intended user. Especially for the "AUTO ON/OFF" user. And a volume level control. Come to think of it, I bet they could blast the message up the hose where it would actually do some good. ("HELLOOOOOOOOOOO.... LOOOOO... ooooo, THIS IS YOUR HUMIDIFIER........ FIER...... ierrrr... rrrr........ *gasp* *choke*)
6. (This is getting slightly off theme, but now that I have given it some thought, I AM gonna hafta connect a short hose from the HH to a (slightly modified) boom box "IN", and plug the mask into the boom box "OUT". Has anybody tried that? If not, I intend to patent it.) If this is going to function as a message system, I can think of LOTS of practical uses for this, and not just soft lullabyes for going to sleep (i.e. gentle strains of Pink Floyd and Led Zeppelin, as mikemoran would say.). "HEELLOOO! THIS IS THE CONTROL UNIT! Was that a "CENTRAL" or are you really swallowing your tongue??? take your time..."
7. I would think the HH AUTO ON/OFF feature, even if it didn't appeal to the "early-on pre-heat" audience, would appeal to the "middle-of-the-night-get-up-and-go" crowd who, for reasons of forgetfulness or laziness (me for example) don't remember to turn the HH back ON. And therefore aren't getting the full benefit.
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