... for clear resins either visual or IR/NIR LED's would be OK.
You can use a "trick" for the flow detection too, with a bit space inside a housing or the outlet into the water-reservoir -- let the water flow through a free spacing, so the water jet will form a "bow" (or arc) and point with a focussed LED or laserpointer through this bow on a sensor on the other side. With flowing water the bow will reflect the beam along its inside, so the light will point down, into the reservoir ... without the water the beam will point straightforward onto the sensor ...
Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [
reprap.org] -- Deutsche Facebook-Gruppe - [
www.facebook.com]
Call for the project "garbage-free seas" - [
reprap.org]