Sight Glass

A sight glass, also called a sight window or view port, provides a means of seeing inside a process vessel or tank. Operators use sight glass to visually verify stages of a process, inspect process media, and to observe liquid levels. It may seem low tech, but a sight glass is a highly engineered component, designed for particular levels of pressure, temperature, corrosion resistance, thermal stress and impact.

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Online Selection Guide

Sight Glass

There are many considerations for selecting ideal sight glasses. This online guide walks you through the steps.

Mounting Type

Mounting may be clamp-on, weld-on, bolt-on, or threaded. Sanitary clamp mounting is standard in bio-pharma applications.

Sight Glass Assembly vs. Fused Sight Glass Window

There are two types of vessel sight glass: a conventional glass disc sandwiched between two gasketed metal rings, and glass fused to a metal ring during manufacturing.
In addition, the metal ring takes the stress of mounting, which allows the sight glass window to be returned to service after removal for cleaning—a practice not advised for glass discs.

A conventional sight glass assembly is fine for low-pressure non-critical applications.

Fused sight glass windows are recommended in critical and high performance applications. Glass typically fails under tension. With fused sight glass windows, the compressive force of the metal ring exceeds the tensional force (i.e., pressure) and, as a result, the sight glass is extremely resistant to failure. Fused sight glass windows offer high pressure ratings and high safety margins.


The glass materials selected, the unsupported diameter, and the glass thickness all play a role in determining the pressure capabilities of a sight glass. The pressure ratings are indicated on the product datasheet.


Most of our sight glasses are available with either common soda lime glass (for temperatures below 300◦ F) or borosilicate glass (for temperatures up to 600◦ F.)
Borosilicate glass is more resistant to thermal shock than soda lime glass, and should be specified in areas where hot equipment may experience washdowns with cool water.
Quartz or sapphire glass is recommended for temperatures greater than 600◦ F, such as in high temperature steam applications. Mica shields are available for extreme applications.


Stainless steel offers corrosion resistance and is standard in pharmaceutical applications. 316 stainless steel is most common, with other steel grades often available by special order. Our popular Metaglas® brand sight windows are made of duplex stainless steel, a formula that is particularly corrosion resistant.

Product Contact

Corrosive and abrasive process media can cause glass to become hazy. In such cases a borosilicate glass will perform better than soda lime glass.


This doesn’t really affect selection, as all sight glasses are available with a wide variety of gaskets. Just be sure that the gasket material selected is compatible with your process media and temperature.

Want to know more?

chemical and pharmaceutical sight glass

L.J. Star’s 20-page handbook is a convenient reference for experienced engineers and a great primer for those new to sight glass selection. It offers a full overview, including

  • Types of glass
  • Sight glass construction
  • Lighting
  • Installation
  • Maintenance

Providing high-strength, quality sight glass products is the heart of L.J. Star’s business. We supply a variety of sight windows and ports with borosilicate or soda lime glass. Our product line includes Metaglas® fused sight glasses, the #1 selling fused sight glass, proven in thousands of installations around the world.

At L.J. Star, we understand the importance of sight glass products for your process equipment. They provide visual inspection of industrial and pharmaceutical processes, and are often used in conjunction with lights that illuminate tank and pipe interiors.

Glass is often the weakest link in any process, but because the observation of interiors of vessels and pipelines is necessary, the glass simply cannot be eliminated. Sight glasses are highly engineered products. Although brands look alike, differences in their specs have tremendous importance for worker safety, sterile processes, and maintenance costs.

Metaglas, the ultimate in sight window safety, is made by fusing pure borosilicate glass within stainless steel rings. The result are  sight glasses that have been described as having the transparency of glass with the strength of steel. Proven in thousands of installations, Metaglas is the #1 selling sight glasses.

L.J. Star will always provide third-party documentation of standards compliance and product performance claims.