Coating Solutions

Glass Products


Glass Fabrication


What is Sight Glass


Download this information as a pdf adobe acrobat logo


Sight glass, also known as a sight window or sight port window, is a visual observation window made from a robust glass substrates such as SCHOTT Borofloat®and is usef 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 ti 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.


Typically, a sight glass window is made from a thick borosilicate glass such as SCHOTT® Borofloat®, an optically clear, very versatile glass that has excellent light transmission, thermal and chemical properties. For less critical applications, soda lime glass may be used. For the most extreme applications, one should choose quartz or sapphire.


She sum of SCHOTT Borofloat® properties makes it unique for sight glass applications as it has outstanding thermal resistance, it is chemically durable, has excellent mechanical strength and exceptionally high transparency all of which are selection criteria.


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


  • Thermal Shock
  • Corrosion
  • Abrasion
  • Pressure
  • Impact

For operating conditions of up to 500°F, borosilicate glass should be selected. For temperatures over 500°F, such as high temperature steam, quartz or sapphire would be the recommended substrate. Applications requiring a temperature range of 300°F or less may use soda lime glass.


The chart below provides comparative information for sight glass selection:




Sight glass windows can be found in the following industries:


  • Chemical & Petrochemical
  • Pharmaceutical & Biotech
  • Food & Beverage
  • Oil & Gas
  • Utility Industry
  • Biogas & Biofuels
  • Wastewater Treatment

Sight glass windows are typically made from stepped, flanged, or beveled rounds or rectangles. They may or may not be tempered.


Typical sizes are up to 8” in diameter and ¾” thick, but can be provided as large as 25” in diameter and up to 1” thick if required. For more information contact us at: or call: 877-622-7472.


Home | About Us | Markets & Applications | Products & Services | News & Events | Contact Us
Terms of Use | Privacy Policy | Site Map | Terms of Sale | Send Comments About this Site to: Abrisa Technologies Webmaster © 2010 Abrisa Technologies All Rights Reserved