Waste tank level sensor: spontaneous remission?

SidecarFlip wrote:
... It goes on the outside of the tank, not inside and works by induction so it's failure proof. ...

Works by a variable "capacitive reactance" as opposed to induction. Two conductive strips (serving as capacitive plates) are mounted parallel to each other on the outside of the tank where the water inside the tank provides the varying dielectric for an applied frequency etc.