[dom444]

here is some info I found on one of them:

For the sense of the water flow rate on the following equipments: Thermostatic water heater, water purifier, water dispenser, smart card equipment, coffee machine etc.

frequency =26*Q(Q: L/min) , Inner diameter :3.0mm，Flow rang:0.5-5L/min

Maximum current：15mA

further searching says they are made from Polyoxymethylene or POM material
which according to Wikipedia is also used for fuel senders.