Abstract: A flow measurement device for assessing free surface fluid flow that includes a void container defining a void chamber and including an upper opening configured to receive the free surface fluid flow, a measurement container defining a measurement chamber in fluid communication with the void chamber via at least one through opening, an exit orifice defined in the measurement container and configured to allow fluid to drain from the measurement chamber at a known outflow rate based on fluid level, and at least one capacitive sensor configured to measure a level of fluid in the measurement chamber. A fluid path is defined through the upper opening, into the void chamber, through the at least one through opening, into the measurement chamber, and through the exit orifice.