A high pressure optical flow cell system suitable for use in a real time optical particle monitoring system. The system is modular, with at least two housings joined together with removable mechanical attachment devices. Inlet and outlet passageways introduce and remove high pressure fluid into a flow cavity located between adjacent housing faces. An o-ring or other compliant member seal is provided between the faces to prevent leaks of the high pressure fluid. At least one optical window is provided with a substantially planar face flush with the flow cavity surface. An optical assembly maintains the face of the optical window flush with the flow cavity surface over a wide range of temperatures and pressures. A system and method for maintaining the face of the optical window flush with the flow cavity surface over a wide range of temperatures and pressures.

Optical window assemblies are provided. An example apparatus includes a first fixture defining a fluid flow passageway. The example apparatus also includes a second fixture defining an aperture. The second fixture is coupled to the first fixture. A first optical window is disposed in the aperture. The first optical window has a first end and a second end. The first end is to be in contact with fluid in the fluid flow passageway, and a cross-sectional size of the first optical window decreases from the first end toward the second end along a portion of the first optical window.