Abstract: A sedimentation recorder including an optical tracking head controlled to track the liquid-solid separation boundary in a tube containing a sample of liquid-solid mixture. The tracking head is controlled by a pulse operated servomotor which responds to the output of a photodetector mounted in the tracking head. A two-pen plotter is provided to record the sedimentation behavior of the sample. One pen is connected to the tracking head and continuously records the position of the liquid-solid separation boundary to provide a settling curve. The other pen is driven by a rate of change mechanism and records the approximate rate of change of the separation boundary position. The rate of change mechanism operates by measuring the excursion of the tracking head during each of a series of successive time intervals. The difference between each excursion measurement and the preceding measurement is calculated and each difference value is converted into a corresponding number of pulses.