Sight Glasses vs. Viewports

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Viewports are a subset of Sight Glasses. In more general terms the name “sight glasses” is used when referring to a small window on process equipment or machinery of some sort. Viewports are specific sight windows, designed to transmit a laser beam for measuring purposes.

Round Flange Viewport
Square Flange Viewports

In semiconductor applications such as vacuum chambers viewports are made of sapphire or quartz, or often borosilicate, depending on the cost requirements. These viewports meet stringent SEMI standards for surface finishes and the choice of materials. For infrared applications viewports are made of Calcium Fluoride, or Magnesium Fluoride or Zinc Selenide. All of these viewports are sealed into the reactor vessels with a knife-edge of a stainless steel Conflat flange, or with O-rings. If an elastomer material is used for sealing, it’s typically Viton, or Kalrez (TM). For sterile applications metal O-rings are used made of Waspalloy for high temperature chambers, up to 700 degrees C, or Inconel for high pressure (up to 50,000 PSI). That’s 50kpsi, or 3,447 bar – very high pressure, way beyond what normal applications are. Diesel injection pressure is 20,000 psi, the pressure inside a handgun is abut 30,000 psi. Water jet-cutting pressures are up to 200,000 psi. Water actually compresses by 1 percent in volume at 304,000 psi. These are extreme pressures and there are sealing methods for that.

Viewports in pharmaceutical equipment also meet stringent standards for materials compatibility and surface finishes. In food processing applications the sight glasses have to meet the 3-A Sanitary Standards which dictate the choice for sight glass housing metals and window materials. Sometimes the process requires the use of narrow-wavelength windows, which may not be compatible with the 3-A Standards. This presents a design challenge where double window sight glasses have to be designed.

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