There are many different types of glass, such as borosilicate, soda lime, quartz, and sapphire. Unlike glass, in which the molecules are randomly arranged, quartz and sapphire are, technically speaking, crystals, in which the atoms are rigidly aligned. This is the secret to their high strength. Quartz is easily the most pressure-resistant material but is quite costly and brittle. Sapphire is generally not used in process observation equipment because it is expensive and cannot be molded and fused to metal.

Since specially treated glasses are sometimes used in industrial applications, this handbook includes a section providing essential information on a variety of types. However, borosilicate glass is the type best suited for use in most chemical and pharmaceutical applications, and therefore this handbook devotes primary attention to its use. Nonetheless, because soda lime glass is frequently used – often unknowingly – this section compares borosilicate glass to soda lime glass.

Choosing the Right Glass Type for an Application

Selection of an industrial sight glass based on the type of glass depends entirely on its application – in particular, its temperature, the reaction of its chemicals with glass, and its pressure.

Standard DIN 7080 rates borosilicate glass to 280°C (536°F) and Standard DIN 8902 rates soda lime sight glass to 150°C (302°F). Very high temperature equipment such as ovens and furnaces may require windows made of quartz. The following chart compares the maximum operating temperatures of commonly known glass in sight windows.

Common Optic Materials | General Temperature Ranges

Borosilicate Glass vs. Soda Lime Glass

It used to be that all sight glasses were made of soda lime glass. Then Corning developed borosilicate glass (brand name Pyrex®) and today this is a popular choice for sight glass construction. The key characteristics of borosilicate glass include strength, high temperature capabilities, great corrosion resistance, low thermal expansion (high thermal shock resistance), and of course, the main reason for its use – the ability to see through it.

Light Transmission for Borosilicate Glass

Soda Lime Glass

Soda lime glass owes its popularity to the fact that it is the least expensive and most common type of glass. Soda lime glass is used in the manufacture of bottles, light bulbs and windows. It is comprised of silica, soda, lime, magnesia and alumina.

Borosilicate Glass

Borosilicate glass is similar but it is made by replacing some of the silica with boric oxide. It is, then, comprised of silica, boric oxide and alumina. Borosilicate glass is used in the manufacturer of cookeries, laboratory equipment and glass pipe, in addition to sight glass windows. It has high chemical resistance, high strength, and a low coefficient of thermal expansion (high resistance to thermal shock).

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