An optical sensor may include a sensor head that has an optical window for directing light into a flow of fluid and/or receiving optical energy from the fluid. The optical sensor may also include a flow chamber that includes a housing defining a cavity into which the sensor head can be inserted. In some examples, the flow chamber includes an inlet port defining a flow nozzle that is configured to direct fluid entering the flow chamber against the optical window of the sensor head. In operation, the force of the incoming fluid impacting the optical window may prevent fouling materials from accumulating on the optical window.