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	▲	atoav 6 hours ago
		An art student of mine once needed a way to electrically control precise small amounts of water. We solved that using: `1. Water tank and gravity 2. Medical IV flow regulator¹ 3. Servo hooked up to that IV flow regulator via a 3D-printed part` It worked very well. In medical applications off must be really off, so it was also quite safe in that regard as well. Her 3D-printed part had a little bit too much flex in it, but in principle this works quite well. If it is really, really safety critical I would still recommend a mechanical fallback that protects in case of power loss or when the servo fails open (e.g. bending the hose with the force of a spring if electricity is gone). ¹: see https://www.gvs.com/en/catalog/iv-flow-regulators
	▲	stinos 5 hours ago \| parent [-]
		> If it is really, really safety critical Your suggestions should be fine for hardware failure but I'd be more concerned about software failure: what if a bug in your software makes it unresponsive and stuck in the state with the flow open? Maybe a watchdog or some other system running in parallel checking for a heartbeat or a max amount of time water can be flowing?