Abstract: An approach for counting particles suspended in a flow of gas or liquid in instruments that direct the flow through an illuminated region. Pulses are detected when the signal is below a threshold amplitude and moves above the threshold amplitude. This movement above the threshold creates a dead time during which only one pulse is detected until the signal amplitude moves sufficiently below the threshold such that a subsequent particle creates a distinct pulse. After counting the number of pulses, and determining the measured live time that the signal is below the threshold value, an initial particle concentration is calculated, and the calculation corrected for coincidence by calculating an actual live time as a measured live time minus a constant multiplied by the number of distinctly counted pulses, where the constant has the units of time. From this, particle concentrations in a volume can be determined.