[lancesells]

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 any idea when to switch the filter. Paper is preferable to electronics in my life.

[bewaretheirs]

At one point Brita distributed a spring-driven mechanical ratchet counter that would sit on top of the filter cartridge; it had some sort of diaphragm arrangement that would sense the water level and "tick" once per refill cycle and tell you when it was time to swap in a new filter.

[ska]

I wonder why they go rid of it. Was this shipped with every filter or was it a think you reset and put on again? Do you recall if it could tell the difference between a partial fill and full fill?

What you really want is to measure the volume through the filter, but I can't think of a way to do that cheaply and mechanical only.

[bewaretheirs]

It was reusable. You would turn it back to the start to reset it when you'd pull it off the top of the old filter and stick it on top of the new one.

I don't recall whether it came with the pitcher or as a bonus item included in a filter-multipack. It just counted one "tick" on a ratchet per fill/empty cycle, no attempt to measure volume or fractional fills.

[lozenge]

They probably consider the electronic version more "modern". Even though it contains a non replaceable, non rechargeable battery.

[ska]

That definitely works better than a timer, but still has to be adjusted for the water quality in your house.

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.

[lancesells]

Didn't think of water quality. I'll have to see if I can test it. I live in Brooklyn in a superfund site so it'll be interesting to see.