For £85,-- ish you can buy an ultrasonic flowmeter from Gentech. It is a self-contained little black oblong box, with 3/8" BSP male thread connectors on either side, that runs on anything from 5V to 12V. It has no moving parts, and emits pulses corresponding to flow rate. You could hook it up to a fan header if you wanted.
Of course, the £30,-- Swissflow SF800 is smaller mechanical flowmeter, which does exactly the same thing and has a very low friction impeller, causing no significant wear.
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"There is a thin line between magic and madness"
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