A method and device is provided for detecting a flow of particles, particularly paint particles, in a powder transportation line connected to a powder spray gun. The optical transmission device generates an adjustment optical signal used by a reception-side optical amplifier in the optical transmission device at the next node to adjust gain for a transmission loss of the optical signal. The employer may submit one transaction made up of payments collected from multiple employees bound for multiple recipients and the accumulator may receive, translate, batch, and deliver the payments to the multiple recipients. In an alternate embodiment, the image may comprise any available visual andor audio display selected by the user. The side portions pivot on a shaft or hinge mechanism that includes a spring that biases the side portions toward the second position. At least one of the rotor blades includes a section that encounters the airflow that includes an unsteady subsonic airflow having at least a varying angle of attack.