Recently we added budget sight glasses to our catalog. We started with super strong, fused sight glasses made of exotic stainless steels and borosilicate glass. Our customers like these for demanding applications where the high price is justified. You do not want a weak link in your system, and sight glasses potentially create a weak link in a system. A sight glass completes a pressure boundary of a process vessel. You definitely cannot have this component fail, especially while it’s in use for observing the process.
Many times however, we have been asked for cheap sight glasses which do not need to perform in corrosive chemistry and under high pressure. As a result we
offer now low cost solutions
for observing oil level, colors of the process, or simply to admit light into the vessel. Prices start at $6. We prepared all the technical drawings along with CAD models of these products. Made of plain carbon steel, nickel plated for added corrosion resistance, these sight glasses have fused soda lime glass. The principle is the same between expensive and less expensive fused sight glasses. Basically, the glass is melted and subsequently cooled while contained inside a metal housing. This process fuses the glass to the metal and forms a single-piece part. The process is similar to water drops sticking to a wall by molecular tension, where there is a bond formed between the boundary layer of the water and the surface of the wall. Similar forces are in effect between the fused glass and the metal housing.
During the fusing process both the glass and the metal are at the same temperature, above the melting point of the glass. During cooling the glass transforms into a solid phase and annealing begins. The glass relaxes giving away its residual stress. Meanwhile the metal housing is still hot, expanded around the glass. Cooling continues, metal contracts collapsing onto the solid disk of glass in the center of the part. This contraction of the metal compresses the glass in the radial direction toward the center; the metal develops a permanent hoop stress which does not drift as the metal already went through its annealing and remains in the elastic region of the Hooke’s curve.
If all of this sounds too technical, just read this: Fused sight glasses are strong. They also do not break catastrophically as a simple glass window. They also require no additional tools for clamping the glass into its housing – they are single-piece parts.
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