Abstract: Apparatus and method for measuring the absorbance of fluid samples. The length of the light path through the sample is adjusted to optimize the amount of light absorbed by the sample, and the absorbance of the sample is calculated from the length of the path and the amount of light absorbed. Accurate measurement of the absorbance of the fluid may be made by taking measurements at two different path lengths, since light level changes due to fouling of the optical windows or other slow changes in the optical train are independent of the path length, while that of the fluid varies proportionally with the path length. A preferred form of probe includes a cylindrical cell having an exposed variable path length sensing portion at one end and an actuator for varying the spacing at the other end, and a cylindrical cell insertion tube having seals and a large valve mechanism to permit removal of the probe without loss or escape of the fluid being measured.