A small anecdote about users of your fine product.

In treatment of obstructive sleep apnea, often evidenced as snoring, it is common to slightly pressurize the individual's breathing passages using a "CPAP" machine. Fancy versions turn on automatically by detecting the individual breathing. A water tank to humidify the breathed air is a common attachment.


So one day a user proceeds to refill the water tank. Having misplaced the funnel supplied by the maker of the machine, a larger funnel from the kitchen is used. Water is poured in at a rate the funnel can accommodate.

Suddenly, water spewed out of the funnel.

What happened?
Once the water level in the tank had risen to the bottom of the funnel neck, air pressure rose rapidly. The machine reacted as it was programmed to, so that it responds to the user breathing out, by turning on the air blower.

Hmmm - interesting that the funnel supplied by the maker of the machine is quite shallow, not a standard funnel which might be less costly. Did someone get wet during development testing? ;-)

Suggestion - mold a notch in the top of the filler hole (which is the hose-attach hole for the breathing apparatus), or a bump on the inside of the hole, so that an air passage is left between hole and funnel neck thus air pressure is less likely to rise rapidly.
A less practical solution is to mold a recess in the top of the tank into which the funnel could snap to stay with the machine (The tank has spare volume above maximum water level at one end because of baffling of air input.) Clever, but my first suggestion seems easier and more fundamental.
Extra points: how do you handle decisions between alternatives in product design?