Lumiglas 81-type sintered sight glass DIN 11851 fused-to-metal borosilicate disc in duplex stainless steel ring for pharmaceutical hygienic process vessels

81-Type Sintered Sight Glass for DIN 11851 Fittings — Hygienic Borosilicate Fused-to-Metal Design

Lumiglas® 81-Type sintered sight glass for DIN 11851 hygienic fittings.
Fused-to-metal borosilicate, CIP/SIP rated to 300°C, DN32–DN125, 40 bar.
METAGLAS® Type 81 compatible. EN 10204-3.1. Request quote →

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Description



✓ EN 10204-3.1 Certified
✓ PED 2014/68/EU Compliant
✓ CIP / SIP Compatible
🏭 ISO 9001:2015 Bureau Veritas
🔥 100% Inspected

The Lumiglas® 81-Type Sintered Sight Glass is a safety-grade hygienic sight glass for screwed process fittings conforming to DIN 11851 (EN 10357, dairy thread standard). Indeed, the borosilicate glass disc is permanently fused — sintered — into a precision-machined metallic ring. Consequently, this creates a hermetic fused-to-metal seal that eliminates the gasket degradation and contamination risks inherent in conventional loose-disc designs.

Furthermore, this DIN 11851 sight glass is the direct Lumiglas® equivalent of METAGLAS® Type 81 and other sintered sight glass disc designs for dairy-thread screwed fittings. Specifically, it fits directly into existing DIN 11851 fittings across DN32 to DN125 without modification, making it a drop-in replacement for pharmaceutical, food processing, and fine-chemical vessel installations worldwide.

Rated to 40 bar (DN32–40) and 300°C, it withstands the full CIP/SIP sterilisation cycle with no seal relaxation, no glass fragment contamination risk, and no loss of visual clarity over repeated thermal cycling. Therefore, it is the preferred choice wherever GMP compliance, zero contamination, and long-term seal integrity are mandatory.

Why Fused-to-Metal Construction Outperforms Conventional Hygienic Sight Glasses

Conventional DIN 11851 sight glass assemblies rely on compressive O-ring or gasket loading to seal the glass to the fitting. However, this approach has a fundamental weakness: under repeated CIP/SIP thermal cycling — typically 121°C steam sterilisation followed by cool water flush — the gasket material compresses, relaxes, and eventually loses sealing force. As a result, this leads to micro-gap formation and potential contamination of the process fluid.

Fortunately, the sintered fused-to-metal design eliminates this failure mode entirely. Since the borosilicate glass bonds permanently to the metallic ring during the sintering process, there is no gasket to relax, no seal to degrade, and no gap to form — regardless of how many sterilisation cycles the assembly undergoes.

Design Factor Conventional Loose-Disc (O-ring sealed) Sintered Fused-to-Metal (81-Type)
Seal type O-ring or gasket — degradable Glass permanently fused to metal — no degradation
CIP/SIP cycles Seal relaxes with repeated cycling → replacement needed No seal change — hermetic integrity maintained indefinitely
Glass fracture risk Fragments can enter process stream Fragments retained in fused ring — fail-safe design
GMP compliance Risk of contamination from degrading elastomers Zero elastomer contact with process — fully GMP compliant
Maintenance Periodic gasket inspection and replacement required No scheduled seal maintenance
METAGLAS® Compatibility Note: The Lumiglas® 81-Type sintered sight glass is dimensionally compatible with METAGLAS® Type 81 and equivalent sintered disc designs from other manufacturers. Therefore, it fits directly into existing DIN 11851 screwed fittings as a drop-in replacement — no fitting modification or re-welding required.

Technical Specifications

Parameter Value / Standard
Fitting Standard DIN 11851 / EN 10357 screwed hygienic fittings (dairy thread)
Construction Type Borosilicate glass disc permanently sintered (fused) into metallic ring — no process-side gasket
Nominal Sizes DN 32 · DN 40 · DN 50 · DN 65 · DN 80 · DN 90 · DN 100 · DN 125
Max. Pressure 40 bar (DN32–DN40) · 25 bar (DN50–DN65) · 16 bar (DN80–DN125) · Higher ratings on request
Operating Temperature −60°C to +300°C depending on ring material selection
CIP / SIP Compatibility Yes — compatible with clean-in-place and steam-in-place sterilisation to 134°C / 3 bar
Ring Material (Standard) Duplex Stainless Steel 1.4462 — −30°C to +280°C
Ring Material (Options) Hastelloy C-22 (2.4602) · Hastelloy C-4 (2.4610) · Other alloys on request
Sealing Principle Hermetic atmospheric-side fused seal — no process-side elastomer or gasket required
Material Certificate EN 10204-3.1 material certificate — supplied as standard
Pressure Equipment PED 2014/68/EU · AD 2000 Rules
Factory QC ISO 9001:2015 (Bureau Veritas) · 100% visual and dimensional inspection per batch

All values at ambient temperature. Derate pressure for elevated temperature service per P-T curve. Higher pressure ratings and custom alloys available on request.

Dimensions & Pressure Ratings — DIN 11851 Series

All dimensions in mm. d1 = outer diameter · d2 = ring OD · d3 = viewing diameter · h = total height · g = glass thickness

DN d1 (mm) d2 (mm) d3 Viewing Ø (mm) h (mm) g Glass (mm) Max. Pressure (bar)
32 50 41 23 13 10 40
40 56 48 25 13 10 40
50 68.5 61 35 14 11 25
65 86 79 45 16 12 25
80 100 93 50 16 12 16
90 111 103 55 20 15 16
100 121 114 60 20 15 16
125 150 137 70 23 17 16

Ring Material Selection Guide

The ring material determines the chemical compatibility and temperature range of the complete hygienic sight glass assembly. Generally, for most pharmaceutical and food applications, Duplex 1.4462 is the standard choice. Conversely, for aggressive media or extreme temperature cycles, Hastelloy alloys provide the necessary corrosion resistance.

Material Grade Temperature Range Recommended Service
Duplex Stainless Steel 1.4462 −30°C to +280°C Pharmaceutical, food, beverage, cosmetic — standard GMP applications
Hastelloy C-22 2.4602 −60°C to +300°C Aggressive oxidising acids, mixed acid systems, high-temperature CIP
Hastelloy C-4 2.4610 −60°C to +300°C High-purity chemical and fine chemical processes — superior intergranular corrosion resistance at elevated temperature

Industry Applications

The 81-Type DIN 11851 sight glass is specified wherever a screwed hygienic fitting requires reliable visual observation under sterile process conditions. Specifically, the following industries and vessel types represent the primary application areas:

💊 Pharmaceutical & Biotech

GMP bioreactors, autoclaves, fermenters, WFI storage vessels. CIP/SIP compatible — sterile visual inspection without system breach. EN 10204-3.1 certs meet FDA and EMA documentation requirements.

🍺 Food & Beverage

Dairy, brewery, juice, syrup, and sugar processing lines. DIN 11851 dairy-thread connection — no dead zones, cavity-free design, easy inline CIP cleaning without disassembly.

🧴 Cosmetic & Personal Care

Emulsifiers, mixing vessels, reaction kettles. Hygienic design prevents product contamination from glass fragments or seal degradation throughout the production campaign.

🔬 Fine Chemical

Crystallisers, dryers, centrifuges, and reactors handling aggressive media at high temperature. Hastelloy ring options provide corrosion resistance where 316L stainless is insufficient.

⚗️ Process Vessels & Columns

Evaporators, distillation columns, separators, and stirred vessels. Direct screwed installation into DIN 11851 nozzles — no welded flange modification needed for retrofit.

What Is DIN 11851? The Dairy Thread Standard Explained

DIN 11851 (also known as EN 10357 in its updated form) is the German and European standard defining the dimensions and design requirements for screwed hygienic unions used in food, dairy, pharmaceutical, and biotechnology processing. Specifically, the connection uses a coarse round thread — informally called the “dairy thread” — with rounded thread edges specifically designed to eliminate crevices where bacteria could accumulate.

Since the DIN 11851 standard demands cavity-free, fully drainable connections, it requires the sight glass disc to sit flush with the vessel inner wall with no protruding edges. Consequently, the sintered fused-to-metal construction of the 81-Type satisfies this requirement — the glass disc surface is flush with the ring face, eliminating any crevice or lip that could harbour contamination.

Selection Note: DIN 11851 and SMS 3008 (the Scandinavian dairy standard) use different thread geometries and are not interchangeable. If your installation uses SMS fittings, contact Lumiglas® for SMS-compatible sintered sight glass options.

Related Lumiglas® Sintered Sight Glass Types

Lumiglas® manufactures a complete range of fused-to-metal sintered sight glass types for different vessel connection standards. Therefore, if the 81-Type DIN 11851 screwed design does not match your fitting configuration, the following alternatives cover the most common flanged and welded vessel standards:

Type Connection Sizes Best For
81-Type (this product) DIN 11851 screwed DN32–DN125 Pharma, food, biotech — existing DIN 11851 nozzles
82-Type Sintered Sight Glass DIN 28120 flanged weld-on DN50–DN200 Pressure vessels, reactors — weld-on flange connection
73-Type Sintered Sight Glass Flanged, bolted DN50–DN150 General industrial and chemical vessel applications
73-SW Scraper Type Flanged with wiper DN50–DN150 Viscous, coating, or crystallising media — self-cleaning glass surface

Frequently Asked Questions

What is a DIN 11851 sight glass?

Fundamentally, a DIN 11851 sight glass is a transparent observation window designed to fit directly into a DIN 11851 screwed hygienic fitting — also called a dairy-thread union. The fitting standard uses a coarse, rounded thread profile that creates a cavity-free, fully drainable connection suitable for pharmaceutical, food, and biotechnology applications. Ultimately, the sight glass disc is mounted inside the screwed nut and allows visual inspection of the vessel contents without breaking process containment.

What is the difference between a sintered sight glass and a standard sight glass?

Typically, a standard sight glass uses a loose glass disc held in place by gasket compression. In contrast, a sintered sight glass — also called a fused-to-metal sight glass — has the borosilicate disc permanently bonded to a metallic ring through a high-temperature sintering process. Consequently, this eliminates the gasket entirely on the atmospheric side, resulting in a hermetic seal that does not degrade under repeated CIP/SIP sterilisation cycles and retains glass fragments if the disc fractures, preventing glass contamination of the process fluid.

Is the Lumiglas® 81-Type compatible with METAGLAS® Type 81?

Indeed, the Lumiglas® 81-Type sintered sight glass is dimensionally manufactured to DIN 11851 specifications and is a direct drop-in replacement for METAGLAS® Type 81 and other sintered disc designs following the same standard. Furthermore, it fits into existing DIN 11851 screwed fittings across DN32 to DN125 without any modification to the vessel fitting or nozzle.

What pressure and temperature can the 81-Type withstand?

The 81-Type handles up to 40 bar for DN32 and DN40, 25 bar for DN50 and DN65, and 16 bar for DN80 to DN125 at ambient temperature. Additionally, the operating temperature range is −60°C to +300°C depending on ring material selection. Moreover, for SIP sterilisation at 134°C and 3 bar, all standard sizes are fully compatible. Higher pressure ratings for larger DN sizes are available on request.

Is the 81-Type sight glass CIP and SIP compatible?

Yes, the 81-Type sintered sight glass is fully CIP and SIP compatible. Because the seal is a permanent glass-to-metal fusion rather than an elastomer gasket, it does not relax or degrade under repeated sterilisation cycles. Ultimately, there is no elastomer material in contact with the process side, eliminating the risk of gasket-related contamination or extractables affecting the process fluid — a critical requirement for GMP-regulated pharmaceutical and food manufacturing.

Which ring material should I choose for my process?

Generally, for standard pharmaceutical, food processing, and cosmetic applications, Duplex Stainless Steel 1.4462 is the correct choice — it covers temperatures from −30°C to +280°C and provides excellent corrosion resistance for most hygienic process media. Alternatively, for aggressive oxidising acids, mixed acid systems, or services requiring corrosion resistance beyond the capability of standard stainless steel, Hastelloy C-22 (2.4602) or Hastelloy C-4 (2.4610) are available. Ultimately, both Hastelloy grades extend the operating range to −60°C to +300°C.

Do Lumiglas® sintered sight glasses come with EN 10204-3.1 material certificates?

Yes. EN 10204-3.1 material certificates are supplied as standard with every Lumiglas® sintered sight glass order. Specifically, these certificates confirm the chemical composition and mechanical properties of both the metallic ring and the borosilicate glass disc, meeting the documentation requirements of PED 2014/68/EU, FDA process equipment regulations, and most pharmaceutical GMP audit requirements. Additionally, EN 10204-3.2 third-party witness certificates are available on request for critical GMP installations.

Need an 81-Type Sintered Sight Glass for Your DIN 11851 Fittings?

Send us your DN size, process media, and operating pressure/temperature.
Lumiglas® will confirm the correct ring material and provide a quote within 24 hours.

sophia@js-lumiglas.com | +86 189 1303 5768

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