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A two-port monolithic crystal filter (MCF) viscosity sensor has been developed using high coupling langasite material and advanced packaging techniques. The present work details the evaluation of quartz and langasite MCF elements with varying shapes, contours and electrode designs. Additional work on protective coatings has made the sensor reliable for a wide range of harsh conditions. The study has resulted in a candidate design on 10.25 mm blanks, housed in a TO-8 header with support circuitry contained in a 3/4" NPT bolt. The sensor elements are presently operable over the MIL temperature range and packaging improvements are underway to allow operation to 200degC or higher.