How does a water filter know when its time to change?

I have water filters on my coffee maker, my kitchen sink tap, and my refridgerator.... All of them have some kind of indicator letting me know when I need to change the filter.... which is fine, but how does the indicator know when its time to change? Is it on a timer, is it on a # of use, does it actually detect if the number of impurities after it is filtered?? Is it actually accurate as to the effectiveness?How does a water filter know when its time to change?the directions say it's on some sort of timer based on average persons use, if u have used it less often, you can keep using it for a while after experation.How does a water filter know when its time to change?It could be on a timer, but more likely it senses the amount of restriction to the flow. so as the filter gets plugged it's harder for the water to get through it.How does a water filter know when its time to change?read the exp date

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