What Is a Borosilicate Round Sight Glass?
Borosilicate round sight glasses (also known as borosilicate viewport glass or borosilicate level gauge glass) are precision-engineered components installed in pressure vessels and pipelines to provide a clear, durable window for direct visual inspection. The round configuration offers uniform pressure distribution, making it ideal for circular sight glass fittings and weld-on sight glass ports in high-pressure systems.
A borosilicate round sight glass is a transparent circular viewing component designed for visual monitoring of liquid levels, flow conditions, and process media within closed pressure vessels, pipelines, reactors, and storage tanks. Manufactured from thermally toughened borosilicate glass, it withstands demanding industrial environments with high thermal shock resistance and excellent chemical stability. This round sight glass window is also commonly referred to as a borosilicate sight glass disc, high pressure sight glass, or circular sight glass plate.
The glass is thermally prestressed (tempered), combining low thermal expansion with enhanced mechanical strength to deliver reliable performance where safety and durability are critical.
Key Applications
Borosilicate round sight glasses are widely deployed across industrial sectors where process visibility is essential:
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Boilers & Pressure Vessels: Used as boiler sight glasses and tank sight glasses to monitor water and liquid levels in steam boilers and storage vessels.
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Chemical Processing: Installed in chemical sight glasses for pipelines and reactors, offering superior resistance to aggressive acids, alkalis, and solvents.
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Food & Beverage / Pharmaceutical: Employed as sanitary sight glasses in hygienic processing lines where FDA-compliant materials are required.
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Oil & Gas / Petrochemical: Function as high pressure sight glass windows in hydrocarbon processing and hydraulic systems.
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Water Treatment & HVAC: Utilized for flow monitoring and fluid inspection in industrial sight glass assemblies.
Each application leverages the glass‘s ability to maintain optical clarity under thermal cycling, pressure fluctuation, and chemical exposure.
Material & Construction
| Component | Material / Specification |
|---|---|
| Material | Borosilicate 3.3, thermally toughened (tempered) |
| Glass Standard | ISO 3585, ASTM E438 Type I Class A |
| Thermal Expansion | 3.3 × 10⁻⁶/°C |
| Optical Clarity | ≥ 92% light transmission |
| Thickness | ≥ 6.5 mm (per DIN 7080 requirement) |
| Fracture Behavior | Shatters into non-sharp fragments (safety glass pattern) |
The borosilicate 3.3 glass (also called boro 3.3 glass) contains approximately 80% silica with a low alkali composition. This borosilicate sight glass material delivers exceptional hydrolytic resistance and long-term dimensional stability. Compared to standard soda-lime glass, borosilicate offers superior performance in high-temperature and chemically aggressive environments.
Standards & Certifications
Our borosilicate round sight glasses are manufactured in compliance with the following international standards:
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DIN 7080: Pressure resistant circular sight glasses of thermally toughened borosilicate glass, suitable for temperatures up to 280°C without low-temperature limitation.
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DIN 7081: Rectangular sight glass plates (applicable to reflex gauge designs).
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ISO 3585 / ASTM E438 Type I Class A: Borosilicate glass material specification.
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FDA 21 CFR 177.2600: Elastomeric gasket components for food contact compliance.
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USP Class VI: Biocompatibility for pharmaceutical applications.
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Pressure Equipment Directive (PED) 2014/68/EU: Component qualification for pressure equipment.
When used within the scope of the Pressure Equipment Directive, these sight glass panes are considered components of the pressure equipment and are suitable for use with hydrogen in all aggregate states.
Technical Specifications & Common Sizes
Key Performance Parameters
| Parameter | Value |
|---|---|
| Glass Material | Borosilicate 3.3 (thermally toughened) |
| Max. Operating Temperature | 280°C (per DIN 7080) |
| Thermal Shock Resistance | ΔT ≥ 230°C (ISO 718) |
| Strain Point | ~520°C |
| Annealing Point | ~560°C |
| Softening Point | ~820°C |
| Light Transmission | ≥ 92% (visible spectrum) |
| Refractive Index | ~1.473 (at 546.1 nm) |
| Density | 2.2 – 2.3 g/cm³ |
| Chemical Resistance | Excellent resistance to acids, alkalis, and organic solvents |
Standard Round Sight Glass Dimensions
| Viewing Diameter (d1) | Disc Diameter (d2) | Thickness | Typical Pressure Rating |
|---|---|---|---|
| 80 mm (3.15 in) | 100 mm | 15 mm | Up to 25 bar |
| 100 mm (3.94 in) | 125 mm | 15 mm | Up to 25 bar |
| 125 mm (4.92 in) | 150 mm | 20 mm | Up to 25 bar |
| 150 mm (5.91 in) | 175 mm | 20 mm | Up to 25 bar |
| 175 mm (6.89 in) | 200 mm | 25 mm | Up to 20 bar |
| 225 mm (8.86 in) | 250 mm | 30 mm | Up to 16 bar |
Note: Pressure ratings depend on glass thickness, mounting configuration, and operating temperature. Custom sizes and thicknesses are available upon request.
Advantages of Borosilicate Round Sight Glass
Superior Thermal Shock Resistance: The low coefficient of thermal expansion (~3.3 × 10⁻⁶/°C) allows the glass to withstand rapid temperature changes without cracking, making it ideal for steam boiler and high-temperature process applications.
High Pressure Tolerance: Thermally toughened borosilicate glass discs achieve pressure ratings up to 25 bar and higher, depending on thickness and mounting configuration.
Excellent Chemical Durability: High silica content (~80%) and low alkali composition provide exceptional resistance to hydrolysis, acids (except hydrofluoric acid and hot concentrated phosphoric acid), alkalis, and organic media.
Superior Optical Clarity: Low iron content ensures ≥ 92% light transmission and minimal optical distortion, enabling accurate visual inspection of process media.
Safe Fracture Behavior: When subjected to extreme overload, thermally prestressed borosilicate glass shatters into small, non-sharp fragments rather than dangerous shards.
Wide Operating Temperature Range: Suitable for continuous operation up to 280°C per DIN 7080, with no limitation in the low-temperature range.
Evidence & Testing
Chemical Durability Testing
Abrasion test results per DIN ISO 720 confirm hydrolytic resistance values below 0.1 mm/month in aqueous phases across a wide pH range at various temperatures. MAXOS-type borosilicate sight glasses demonstrate abrasion rates as low as 0.050 mm/month with acid resistance of <60 μg Na₂O/dm² and alkali resistance Class A2.
Dimensional Tolerances
All round sight glass plates are manufactured to strict dimensional tolerances per DIN 7080. Permissible material defects: round bubbles enclosed in glass have a maximum diameter of 2 mm; oval bubbles may not exceed (length + width)/2 = 2 mm.
Industrial Validation
These borosilicate round sight glasses are widely deployed in chemical processing plants, pharmaceutical manufacturing facilities, and power generation boilers globally for reliable liquid level monitoring and flow observation.
Why Choose This Product
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Manufactured to DIN 7080 and ISO 3585 standards
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Thermally toughened for enhanced strength and safety
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Custom sizes available — contact us with your requirements
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Fast delivery on standard dimensions
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Technical support for selection and installation guidance
