A measuring device for measuring the fineness and maturity of cotton fibers is provided. The measuring device includes a measuring chamber; a supply conduit; a flow regulator arranged along the supply conduit; a first pressure sensor upstream of the flow regulator; a second pressure sensor downstream of the flow regulator; an electronic proportional pressure regulator for regulating the air pressure in the supply conduit; and an electronic processing and control unit programmed to control the electronic proportional pressure regulator as a function of the detections of the first sensor and of the second sensor or of the second sensor alternatively and respectively to keep the difference between the air pressure upstream and downstream of the flow regulator or the air pressure entering the measuring chamber substantially constant and equal to a predeterminable value.