A method and apparatus to calculate the optical density of a fluid (110) traveling through a narrow gap (30) with a set width. Typically, a densitometer employs a light source (10) that is configured to transmit light across the gap and a detector, opposite the light source (10), configured to detect light transmitted from the light source, across the gap, for calculating the optical density of the fluid. The apparatus and method further include a transparent element (120), the transparent element part of a set of replaceable transparent elements, each replaceable transparent element having a particular width, the width less than the width of the gap. Typically, the transparent element is moved into the gap between the light source and the detector to narrow the effective sampling width of the gap for calculating the optical density of the fluid. In some examples, the transparent element is further configured to rotate in the gap to enhance the flow of the fluid through the gap.