Droplets from a liquid stream are directed between a pair of apertures and a light source, which is strobed at a selected frequency and directed toward the apertures. In one embodiment, the apertures are spaced less than a wavelength of the droplets apart and the light source is strobed at a frequency other than the frequency of the droplets. The time between when a first of the apertures is blocked by a droplet in the stream as indicated by the light being broken during a strobe and the time when a second of the apertures is blocked by another droplet when the light source is strobed is counted. This period of time will give the velocity of the droplets, and correction of the velocity is made, if the velocity of the droplets is not at the desired velocity, by changing the pressure, for example, of the manifold supplying the liquid stream until the desired velocity is obtained. In another embodiment, the apertures are spaced from each other a wavelength or an integral of the wavelength of the droplets, and the light source is strobed at the same frequency as the frequency at which the droplets are generated. The strobing frequency has its phase changed relative to the frequency of drop generation and the pressure of the stream changed until both of the apertures are blocked simultaneously. Any deviation from this indicates that the velocity of the droplets has changed and correction must be made.