Abstract: Valid signals from a cell in a centrifuge rotor are distinguished from extraneous signals by passing the signals through circuits which compare the amplitude and duration of the signal with the minimum expected values for those signals. Signals having the desired amplitude and duration are passed to the appropriate recorder which can be selected by a switching mechanism actuated by a circuit triggered at the release of the signal from the duration comparison circuit. The duration comparison circuit counts signals emitted from the rotor as it rotates and hence will operate over a wide range of speeds of rotation.