An optical oil monitor that measures particle contamination in oil by passing light through an oil sample and picking up the light that is scattered at 90 DEG by the particle contaminaton and measures chemical breakdown by the attenuation of the light passing substantially straight through the oil with a second sensor. Alternately a sample and a reference are passed between the light responsive sensors for error correction and calibration so that each sensor will have an output signal alternating between a sample signal and a reference signal. A control signal of the same frequency is provided in combination with a plurality of gates to separate the sample and reference signals. The attenuation reference signal is compared to a standard signal to produce an error output for regulating the intensity of the light source to compensate for power source fluctuations, lamp characteristic changes, window soiling and the like. The scattering reference signal is used to adjust the gain on the scattering sample signal amplifier to obtain the desired output from the sample circuit peak detector. The sample and reference are housed within a rotor provided with vanes so that it may be driven as a pump by a motor or be driven by fluid flow as a turbine. When the rotor acts as a turbine, the frequency of the light responsive sensors will be correlated to the fluid flow so that an appropriate frequency responsive gauge is provided in circuit to monitor the fluid flow. Also, a malfunction light will signal the absence of a reference attenuation signal.
