Calculating the optical density of a fluid traveling through a gap

A method to calculate an optical density of a fluid traveling through a gap, comprising:

• transmitting light across the gap using a light source;
• detecting the light transmitted from the light source, across the gap by a detector located opposite the light source;
• rotating a transparent disk configured to occupy a portion of the gap to reduce available width of the gap for the flow traveling through the gap, and configured to enhance a flow of the flow of the fluid through the gap to compensate for a reduction of the available width of the gap limiting the flow of the fluid through the gap;
• and
• calculating the optical density of the fluid as it passes through the gap.

An apparatus to calculate an optical density of a fluid traveling through a narrow gap having a width, comprising:

• a light source configured to transmit light across the gap;
• a detector, opposite the light source, configured to detect light transmitted from the light source, across the gap for calculating the optical density of the fluid;
• and,
• at least one transparent member, the transparent member configured to be moved into and out of the gap, between the light source and the detector so as to modify an effective sampling width of the gap,
• wherein the transparent member is rotatable within the gap to enhance a flow of the fluid through the gap to compensate for a reduction of the effective sampling width of the gap limiting the flow of the fluid through the gap.