A device for capturing a predetermined volume of a predefined portion (e.g. first void) of a liquid flow, comprises an inlet, an outlet, and a guide with an element displaceable in a first position for capturing the first liquid portion in a reservoir, and in a second position for blocking access to the reservoir, and for passing the liquid to the outlet. The displaceable element moves in transverse direction to the liquid flow, and has lifting means. The element may be substantially flat, or tubular with an elliptical cross section. A kit of parts and a method of assembly are also claimed.
