The old way in industrial automation is a float sensor, which is basically a reed switch with some buoyancy on the end. With substances that are corrosive, or

The old way in industrial automation is a float sensor, which is basically a reed switch with some buoyancy on the end. With substances that are corrosive, or where there's a cross contamination concern, you drop down to load cells or a rangefinder, both of which are more expensive and fussy. :)