Not looking for level, just flow. Short version, determine whether water flow for tub filler faucet was on or off. Don't need to know the exact flowrate. Only need to worry about the 99% everyday scenario where there will be modest flow rate. Guessing a threshold of 1L/min would suffice, meaning whenn flow was at that rate or above output is positive or switch closed etc.
Specific use... send signal to automation controller to turn on lights over tub when water turned on. Keep them on until occupancy sensor says no more motion in bathroom.
Just a thought exercise at this point in thinking through ways I might want to leverage a lighting system and automation controller. Trying to figure out what is possible, impossible, or just impractical, and what planning needs to happen to make something work.
For example, in this case the tub filler hardware will mount on a wall that is shared with shower on the other side. Even if I were to find a suitable switch and getting suitable signal to a controller isnt difficult, the switch needs to be serviceable. Meaning not enclosed in the shared tiled wall. Would have to find some upstream access point that could allow for service from an adjacent closet, attic etc, and talk to the plumber about whether that was possible or even allowable by code.