Every acrylic plane disc viewport needs to rest in a seat support that penetrates the submersible hull and is sized appropriately to bear the stresses that will be placed upon the viewport when the vehicle is submerged. The seat support itself is a hull penetration and must be fabricated according to ABS and/or ASME PVHO standards. For plane disc viewports, the bearing support (the flange that the viewport rests upon) must provide a specific ratio between the viewable segment inside the submersible and the overall diameter of the viewport. In the diagram below, you can see that the overall diameter of the viewport is marked as Do.   This is the high pressure side of the viewport that is exposed to water when submerged.   You can also see that the viewable diameter on the inside of the hull is smaller and marked as Di.   This is the low pressure side of the viewport that is exposed to the 1-ATM interior of the submersible. ASME PVHO standards require that the ratio of Do/Di must be 1.25 or greater. Do/Di = 1.25 (or greater) To calculate these dimensions, first decide what size viewing diameter you want to provide on the inside of the hull.   Next, multiply that dimension by 1.25 and you will have the required overall diameter of the viewport. Example: (inches) Di = 8 Do = 8 * 1.25 = 10 In this example, we want a viewport that will provide 8 inches of visible diameter as seen from inside the submersible cabin.   We multiplied 8 * 1.25 and calculated that the outside diameter must be 10 inches to comply with ASME PVHO standards.