Earlier quoted context omitted.
> Any indicator that tells you when a consumable needs I'm not a fan of the blinking reminders either, but it's not a monitor it's just a flashing version of the "remind me in the months" wheel or whatever. The problem they face is a hard one. There are really two timelines you care about, one is how long it stays effective, the second is how long it is safe. The first one especially is highly affected by both usage…
Instead of a timer you could have a counter. The Brita filter looks to last 40 gallons. My Brita pitcher holds 10 cups. 10 cups equals 0.625 gallons. Which means my filter lasts for ~64 fills of the pitcher. So using a counter until 64 would tell me to refill. All you need is a plus sign and some way to reset. Now that I've done the math I might start writing this on a pad next to my refrigerator because I never have…
But it also assumes you go fill-to-empty. In my limited experience they are often refilled from partially empty, whenever convenient. If you are doing this on paper, maybe you should count output instead of input? More work though.