Abstract: The invention is a dynamic spot calibration system (60) and process for calibrating, both for sizing and counting, an automatic particle counter (APC) (42). The APC includes a sensor (38) which uses a light source that focuses through a sensing window (48) onto a sensor element. The system (60) includes a probe (62) which carries a spot (64) of known size. A receiver (66) grips one end of the probe (62) and is attached to an electromagnetic driver (70) which, when activated and driven by an oscillator (72)-amplifier (74) arrangement, interposes the spot (64) into, and out of, the fluid passage defined by the sensing window (48). The probe is precisely positioned in relation to the window (48) by an adjustable mounting system including a pedestal (76), a vertically adjustable arm (78), and a graduated helical rack (84).