Most solid mechanics textbook calculations of sight windows consider only the unsupported diameter, thickness, and mechanical properties of the glass window. In reality, the greatest stresses on the glass do not arise at the center of the unsupported surface; they are usually posed by the sealing gaskets. The gaskets have to undergo some degree of plastic deformation in order to fill in the microscopic crevices in the glass and flange surfaces in order to create an effective seal. In conventional sight glasses that are sandwiched between two flanges, this stress is provided by the bolts of the flanges directed through the planar surfaces of the glass, which usually results in far higher deviatoric (Von Mises) stresses around the sealing gaskets than at the unsupported center of the glass. 

With such designs, insufficient torque on the bolts will result in leaks, but excessive torque will cause glass failure. Furthermore, the inherently plastic nature of gaskets will cause them to "cold flow," resulting in leaks that have to be re-sealed by additional torque on the flange bolts. Eventually, it will become impossible to seal the sight window without destroying the glass. 

PresSure Products' higher performance sight windows make use of a radial sealing scheme that de-couples the sealing stress on the glass from both the bolt thrust and process pressure. The bolts push down on the packing material place around the side walls of the glass, and the bolt thrust is not directed at any surface of the glass. This scheme reduces the peak deviatoric stress in the glass, and greatly reduces the chances of causing glass damage while sealing the sight window. This unique sealing paradigm also allows the softer high temperature glass to be used in high pressure applications, allowing PresSure Products to offer industry leading combinations of high temperature and high pressure ratings.