Application Note

What is Sight Glass?

Sight Glass Application Note

Sight glass, also known as a sight window or view port window, is a visual observation window made from a robust glass substrates such as SCHOTT Borofloat® and is used to verify conditions in piping, vessels, chemical reactors or other industrial equipment. Sight glass is generally subjected to elevated temperatures, chemical attack and high pressure. So selecting the appropriate glass substrate is critical in order to ensure safety even in the harshest of environmental conditions.

The sight glass is a disc held between two metal frames which are bolted, clamped, or fused to the frame. The glass itself can be strengthened (heat tempered) for additional durability, and may be coated for enhanced visibility and corrosion resistance. Fused sight glass is also called mechanically pre-stressed glass, because the glass is strengthened by compression of the metal ring. Heat is applied to a glass disc and its surrounding steel ring, causing a fusion of the materials. As the steel cools, it contracts, compressing the glass and making it resistant to tension. Because glass typically breaks under tension, mechanically pre-stressed glass is unlikely to break and endanger workers.

When selecting a sight glass substrate the following characteristics of the glass should be considered:

  • Transparency
  • Thermal Shock
  • Corrosion
  • Abrasion
  • Pressure
  • Impact
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