Resonator sensors namely piezoelectric benders are used for sensing fluid viscosity and density. The benders are inserted into a liquid to be monitored. An input voltage is applied to the bender causing it to vibrate in the liquid at a resonant frequency. Output voltage amplitude or frequency are monitored to determine changes in viscosity and density, respectively. The device may be used as a liquid level sensor and to control alteration of the viscosity of the fluid as, for example, by connection with a supply of ink or solvent in an ink jet printing device.