An apparatus to detect and measure suspended particles in a molten metal where the suspended particles have an associated noise frequency range has in addition to the electrically non-conductive barrier for immersion in the molten-metal, a device to move the molten metal through an orifice in the barrier and electrodes, a time varying excitation source that generates an AC current at a predetermined range of excitation frequency that is out of the predominant noise frequency range to generate an AC signal that is representative of the measure of the suspended particles. A detector detects and quantifies from the measurement AC signal the suspended particles. The detector can be a synchronous detector.