Earlier quoted context omitted.
The models do vary, but according to brita all the models with lights are based off of the lid opening. https://www.brita.com/support/faqs/replacement-filters/ Brita® pitchers and dispensers have three different styles of filter indicators—Pitchers WITH Max Fill Line, Pitchers WITHOUT Max Fill Line, and sticker filter indicators. Pitchers WITH Max Fill Line The filter indicator lets you know when it’s time to replace…
Fair enough. I guess I just don't understand how that would possibly work for the model I've got. The lid is at the top, and the electronics are at the bottom, and outside of the water. There is a clear container of water separating the lid and the electronics. The lid is a thin piece of plastic, there's really nothing to it. I'm still convinced it's just a timer for this model, or at best it measures the water comin…
For the pitcher equivalent they document that the indicator is simply a count of the number of pours, where they assume each one is exactly 8 oz, and that the filter lasts for exactly 40 gallons. (i.e. turns red after exactly 640 pours).
For that one, they don't actually document how it works. They do seems to indicate they that they are trying to approximate water used though, since they say it turns red "once you reach 40 gallon[s]". It might be a count of activations of the spigot handle or based on timing how long the spigot handle has been open since last reset (e.g. they previously calculated the flow per second, and use that to count up seconds that up until they reach 40 gallons).