capacitance sensor

Use a sensor that has wetted parts suitable for the liquid to be measured, and calibrate the instrument range on that liquid only.

BEWARE OF THE PROBLEMS OF USING CAPACITANCE! If the liquid changes, or its concentration or mixture ratio changes, then the sensor will no longer give an accurate measurement. Those who have disregarded this factor sometimes find themselves with spillage from over-filled vessels, which may be significant for safety reasons if capacitance is the only viable measurement technique. If spillage cannot be tolerated, then the about the only ways out are either radar, or putting the vessel on weigh cells.