Abstract: A method for measuring changes in the composition of fluids, for medical diagnostics or quality control of fluids, comprising: One) placing a drop of fluid upon a quartz crystal; Two) generating ultrasonic vibrations of a predetermined frequency in the crystal; with the vibrating wave directed into the drop of fluid in a vertical direction relative to the area of the crystal upon which the drop is placed; Three) measuring voltage changes over a bridge circuit connected to the crystal, over a time period during which total or partial evaporation of the liquid occurs; Four) comparing the measurement result to previously obtained results or to predetermined data; Five) detecting changes in the composition of the fluid, from the results. detecting changes in the composition of the fluid, from said results.