Abstract: An optical probe apparatus (10) is for testing physical properties of a fluid (F) such as turbidity. The apparatus comprises a probe assembly (16) removably insertable into the fluid. A photometric light source (12) generates light and fiber optic cables (14) transmits light to and from the probe assembly. Light entering the assembly is directed into the fluid, the light being absorbed or scattered by particles in the fluid as it passes therethrough. The amount of remaining light is sensed and a light output from the probe is produced whose intensity is a function of the amount of remaining light. A light detector measures the light output of the probe assembly and produces an indication of the absorbance or turbidity of the fluid.