Viewing ports and sight glasses · DIN 7080 disc size and pressure, soda-lime vs borosilicate vs quartz, toughened vs metal-fused glass, gaskets and bolting
A viewing port, or sight glass, is a glass window in a vessel, chamber door or pipeline that lets operators see the process without opening it. Ask three things first: the pressure and temperature at the window, since together with diameter they set the glass type and thickness; whether the glass is a gasketed disc or fused into a metal ring, since that changes how it fails and whether it can be removed for cleaning; and which gasket and body materials suit the process.
Viewing-port glass selection: round and obround sight-glass discs, DIN 7080 size/pressure combinations, borosilicate base-glass data and metal-fused versus gasketed glass. Mounting types are compared to explain the glass interface. Complete mounted windows with flange or clamp hardware, lights and cleaning accessories are covered on the Sight glass assembly page; weld-pad vessel ports are covered on the View port page. Tubular level-gauge glasses, camera inspection systems and complete vessels are outside this page.
01
Metaglas, a sales representative for MAXOS toughened borosilicate sight glasses, publishes the standard DIN 7080 round range with disc diameter and thickness, inspection aperture and permissible pressure: from 45 × 10 mm with a 32 mm aperture at 40 bar, through 100 × 15 mm with an 80 mm aperture at 16 bar, to 265 × 30 mm with a 240 mm aperture at 8 bar. Long sight glasses follow DIN 7081 in reflex or transparent form. It attributes the safety of the glass to a borosilicate with low thermal expansion that is thermally tempered for high thermal-shock resistance. [1]
L.J. Star says glass material, unsupported diameter and glass thickness all determine a sight glass's pressure capability. Its sight glasses come in soda-lime glass for temperatures below 300 °F or borosilicate up to 600 °F; borosilicate resists thermal shock better and should be specified where hot equipment may be washed down with cool water, and it holds up better against corrosive or abrasive media that make glass hazy. Quartz or sapphire is recommended above 600 °F, with mica shields for extreme applications. [2]
Editorial calculation: in the DIN 7080 table a 100 mm disc is rated 16 bar at 15 mm thick, 25 bar at 20 mm and 40 bar at 25 mm, while a 25 mm disc rated 40 bar at 100 mm diameter drops to 16 bar at 150 mm and 8 bar at 250 mm. [1] Editorial calculation: 300 °F is about 149 °C and 600 °F about 316 °C. [2]
Give the supplier the design pressure and vacuum, process and ambient temperature with any washdown or thermal shock, the process medium, the mounting (vessel nozzle, chamber door, flange, weld-on, clamp or pipeline), required viewing aperture, glass type and standard, gasket and body materials, lighting or wiper needs, sanitary or hazardous-area requirements, and quantity.
02
L.J. Star describes two vessel sight glass types: a glass disc sandwiched between two gasketed metal rings, and glass fused to a metal ring during manufacture. In the fused type the ring takes the mounting stress and its compressive force exceeds the pressure load, so it is recommended for critical, high-performance applications and can be returned to service after removal for cleaning, which is not advised for loose glass discs; a conventional assembly is fine for low-pressure, non-critical duty. [2]
Metaglas, which makes metal-fused sight glasses, says the different expansion of glass and steel leaves the glass in compression. In its comparison, toughened glass can fail catastrophically from surface damage, erosion, impact, over-torquing or uneven bolting, needs flat seating surfaces within 0.07 mm, and must be replaced unless cleaned in place; it claims fused glass stays leak-tight after surface damage, carries higher pressure ratings at similar thickness, and cannot be over-torqued because the bolting is metal on metal. [3] Editorial note: these are a fused-glass maker's comparisons, so ask any supplier for pressure ratings and test documentation for the exact window.
L.J. Star lists 316 stainless steel for sight glasses and duplex stainless for its METAGLAS windows. It says stainless steel is “standard in pharmaceutical applications” and offers gasket choices to be checked against process media and temperature. [2]
03
SCHOTT, which makes BOROFLOAT 33 borosilicate float glass, publishes the material data that sit behind a borosilicate viewing window. It gives a transformation temperature Tg of 525 °C (ISO 7884) and a maximum operating temperature of 500 °C for short-term use (under 10 h) and 450 °C for long-term use (10 h or more), and states that the thermal expansion of BOROFLOAT 33 is three times lower than that of soda-lime glass, which is why it handles a hot centre against a cold edge. Its mechanical and thermal values are in the table. [4]
Editorial calculation: L.J. Star's 600 °F limit for borosilicate sight glasses is about 316 °C, well below SCHOTT's 450 °C long-term limit for the base glass, so the rating of a finished window is set by the assembly (glass treatment, gasket and frame) and not by the glass alone. Use the sight-glass maker's rated temperature, not the base-glass figure, on the order. [4] [2]
SCHOTT lists BOROFLOAT 33 in thicknesses from 3.3 to 15.0 mm in its standard range, with thicker panels to 25.4 mm in its tolerance table; the DIN 7080 discs above run from 10 to 30 mm, so ask the window supplier which glass and thickness the pressure rating is based on. [4] [1]
| Property (SCHOTT BOROFLOAT 33) | Published value | Use in the order |
|---|---|---|
| Transformation temperature Tg | 525 °C [4] | Upper reference. |
| Maximum operating temperature | 500 °C (< 10 h); 450 °C (≥ 10 h) [4] | Base-glass limit only. |
| Density | 2.23 g/cm³ [4] | Window mass. |
| Young's modulus; Poisson's ratio | 64 kN/mm²; 0.2 [4] | Stress calculation. |
| Knoop hardness HK0.1/20 | 480 [4] | Scratch resistance. |
| Thermal conductivity (90 °C); specific heat (20–100 °C) | 1.2 W/(m·K); 0.83 kJ/(kg·K) [4] | Heat-up and shock. |
04
Listings include sanitary clamp and union sight glasses, flanged and four-way flow sight glasses, DIN 7080 borosilicate discs, sight glasses with wipers and lights, and manway covers with sight glasses; ask each for the glass type and standard, rated pressure and temperature, and gasket material.
05
| Element | Role | Fix in the order |
|---|---|---|
| Lens or pane | Glass disc or fused window. [1] [2] | Glass type, size, thickness. |
| Sealing gasket | Seals disc to rings. [2] | Material for media and temperature. |
| Mounting frame | Rings, flange or weld-on body. [2] [3] | Material, connection. |
| Protective cover | Optional guard over the glass. | Cover type. |
06
| Field | What it means and published values | Ask the supplier for |
|---|---|---|
| Viewing diameter (mm) | DIN 7080 apertures 32–240 mm. [1] | Aperture and disc size. |
| Operating temperature (°C) | Soda-lime <300 °F; borosilicate to 600 °F. [2] | Rated range and shock. |
| Glass thickness (mm) | DIN 7080 range 10–30 mm. [1] | Thickness for your pressure. |
| Light transmittance (%) | Depends on glass and coatings. | Value if critical. |
| Seal material | Match media and temperature. [2] | Gasket material. |
| Body material | L.J. Star lists 316 stainless; duplex for METAGLAS. [2] | Ring and body grade. |
| IP rating | Relevant to lights and electrical parts. | Rating if lit. |
| Weight (kg) | Depends on size and body. | Weight. |
07
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Duty: [design pressure; vacuum; temperature; thermal shock] Medium: [process fluid; abrasion; cleaning] Glass: [soda-lime, borosilicate, quartz; DIN 7080 or fused] Size: [aperture; disc diameter; thickness] Mounting: [door, nozzle, flange, weld-on, clamp] Materials: [gasket; body] Extras: [light; wiper; cover; certificates]
16 companies list this product among what they make. Company figures are quoted from each company's own website; every card states where the relationship came from.
Lists straight, clamp-union and four-way sanitary sight glasses, some with protective covers.
Product scope checked 2026-10-11
Lists tri-clamp and union-type sanitary sight glasses.
Product scope checked 2026-10-11
Lists a flange sight glass with wiper.
Product scope checked 2026-10-11
Lists sanitary sight glasses and a round sight-glass manway cover.
Product scope checked 2026-10-11
Lists borosilicate and DIN 7080 sight glasses and tubular sight glasses.
Product scope checked 2026-10-11
Lists sanitary sight glasses.
Product scope checked 2026-10-11
Lists a sight glass series.
Product scope checked 2026-10-11
Lists sight glasses among sanitary fittings.
Product scope checked 2026-10-11
Lists clamp and flange sight glasses, including tank windows with optional wipers and lights.
Product scope checked 2026-10-16
Lists tank sight glasses with a body, glass and seals.
Product scope checked 2026-10-16
Lists a round scraper sight glass with window-material options.
Product scope checked 2026-10-16
Lists union and flanged sanitary sight glasses.
Product scope checked 2026-10-16
Lists flat-plate, flanged and union sanitary sight glasses.
Product scope checked 2026-10-16
Lists union sight glasses for sanitary tanks alongside pipeline designs.
Product scope checked 2026-10-16
Lists tank-fitting union and clamp sight glasses, including an integrated borosilicate window.
Product scope checked 2026-10-16
Lists flanged and union sight glasses with lighting options.
Product scope checked 2026-10-16
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
In the published range a 45 × 10 mm disc is rated 40 bar and a 265 × 30 mm disc 8 bar; thicker discs of the same diameter carry more. [1]
L.J. Star gives soda-lime below 300 °F, borosilicate up to 600 °F, and quartz or sapphire above 600 °F. [2]
L.J. Star says a fused window can be returned to service after removal for cleaning, which is not advised for loose glass discs. [2]
Sources checked 2026-10-16. L.J. Star and Metaglas sell sight glasses, and Metaglas makes the metal-fused type it compares favourably with toughened glass. The DIN 7080 table is published by Metaglas as a MAXOS sales representative; the standard itself should govern a design. SCHOTT's BOROFLOAT 33 page gives base-glass properties, not ratings for a finished window.
Ask for use case, specification boundaries, supplier type, and RFQ preparation information for this product.
Compare manufacturer profiles with relevant product and process capability.