An ultrasonic coagulation monitor for measurement of prothrombin times and related coagulation times includes at least one piezoelectric crystal operated at a fixed frequency near its resonant frequency. A sample of material to be analyzed is placed in acoustic communication with the crystal. The voltage across the crystal is monitored in order to detect a voltage change, preferably a drop, that occurs upon coagulation of the sample. A timer is included that is turned off at a predetermined voltage drop corresponding to a particular stage of coagulation desired to be timed. In an alternative embodiment of the invention a second crystal is included and is mounted a predetermined distance from the first crystal to define a cavity therebetween. One crystal is driven by the frequency oscillator circuit and the material the coagulation of which is to be measured is placed in the cavity. A voltage sensing device senses the voltage across the second crystal and controls a timer as in the first embodiment.
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