Liquid level sight glass are industrial gauge glasses designed to make the liquid surface visible and measurable in boilers, tanks and process vessels. This B2B product page explains the three common types (tubular, reflex, transparent), materials, pressure & temperature ratings, inspection & installation best practices, technical specs, testing and ordering options (OEM, samples, certifications). Use this guide to pick the right sight glass for your plant and to speed up procurement.
A liquid level sight glass (also called gauge glass, boiler sight glass, level gauge glass) is a visual indicator that allows operators to see the fluid level inside a vessel. Good sight glass design emphasizes two things: visibility (material and clarity) and readout accuracy (structural type). We supply industrial-grade borosilicate and soda-lime sight glasses optimized for process, boiler and tank service.
Tubular (straight) sight glass
Description: circular tube connects vessel top and bottom; simplest, widely used.
Materials: soda-lime (low pressure), borosilicate (higher pressure/temperature).
Typical uses: low-pressure boilers, diesel tanks, fire water reservoirs, sight-only applications.
Reflex (reflective) sight glass
Description: glass with internal 60° V-grooves or prism profile — liquid appears dark, vapor/steam appears bright for a sharp contrast line.
Materials: borosilicate only.
Typical uses: medium-pressure boilers, black liquor, dark oils (where color contrast matters).
Transparent (flat-plate) sight glass
Description: two flat plates clamped with gasket (often with mica protection), backlit for precise readings and camera transmission.
Materials: thick borosilicate + mica or protective films.
Typical uses: high-pressure boilers, high-temperature reactors, applications requiring mm-level reading or remote camera readout.
Soda-lime glass — pressure ≤ 2.5 MPa; economical; avoid high thermal shock and low temperatures.
Borosilicate glass (e.g., borosilicate-3.3) — pressure ≤ 6.3 MPa; thermal shock resistance and up to ≈450 °C depending on structure; preferred for reflex/transparent types.
Low-temperature borosilicate — for ammonia and cryogenic service (safe to −60 °C).
Coatings / treatments — PTFE coating for HF or strong alkali resistance; mica sheets for alkali protection in high-pH environments.
| Type | Typical sizes | Representative pressure rating | Typical temp range |
|---|---|---|---|
| Tubular | DN15–DN50; custom lengths | ≤2.5 MPa (soda-lime) / ≤6.3 MPa (boro) | −60 ℃ to 200 ℃ (depends on glass) |
| Reflex | 100×100 mm to 650×400 mm windows | ≤4.0 MPa (boro) | up to 450 ℃ |
| Transparent (flat) | up to 650×400 mm plates, thickness per ASME | ≥6.3 MPa (boro + mica) | up to 280–350 ℃ (assembly dependent) |
Note: final thickness and safety margin must follow ASME/EN pressure vessel rules — do not undersize glass thickness.
Define service conditions: maximum pressure, operating temperature, fluid chemistry, presence of vapor or foam.
Choose type by readout need: tubular = simple visual; reflex = strong black/white contrast; transparent = high-precision or camera feed.
Specify material: borosilicate for higher T/P and aggressive media; soda-lime for low-risk, low-cost jobs.
Ask for test certificates: thermal shock, hydrostatic test, light transmission, and batch traceability.
Confirm installation needs: protective shields, backlighting, gland torque sequence, and spare mica or PTFE film spares.
Protective shield: always use a guard cage or metal shield for tubular glass to prevent forklift or impact damage.
Gasket & torque: use recommended composite or graphite gaskets; follow torque sequence and values — overtightening can cause star cracks. Retorque fasteners after initial heat cycle for transparent types.
Groove cleaning for reflex glass: schedule routine cleaning (e.g., weekly 5% citric acid rinse) to avoid scaling that blurs the black/white interface.
Cold-weather: drain tubular gauges before freeze periods; use low-temp glass for ammonia.
Lighting for remote readout: prefer white incandescent or warm LED backlight for best contrast on reflex glass; industrial cameras with AI OCR can be set up for remote monitoring.
We can supply:
Material certificates (raw glass batch)
Hydrostatic and thermal shock test reports
Light transmission / clarity test (for tubular & transparent)
ISO 9001 factory quality assurance and per-batch QR code traceability
Compliance support: ASME/EN design references, FDA statements for food-contact components (where applicable)
Samples: standard tubular samples available; reflex grooves demo pieces.
MOQ: small runs for parts (negotiable for prototypes); OEM batches for standard sizes.
Lead time: stock tubular cut-to-order same day; reflex groove turnaround typically 24–48 h; transparent plate machining (holes/rounded corners/PTFE) 3 days for urgent orders.
Packing: padded pallets, protective sleeves, clear labeling and per-roll QR tracking for audit.
Support: installation drawings, torque specs, and on-site/remote commissioning support available.
Q: Can I use LED lighting for reflex glass?
A: LED blue-rich light can reduce contrast — use warm white LEDs or incandescent lamps for the sharpest boundary.
Q: How to read a sight glass remotely?
A: Apply an external barcode/scale strip and use an industrial camera + AI reader; typical accuracy ±0.5 mm with proper backlight.
Q: What if glass is accidentally over-torqued?
A: Stop installation, inspect for radial/star cracks, and replace if any micro-cracks are visible.
“Tubular liquid level sight glass on low pressure boiler”
“Reflex sight glass showing sharp black-white level line”
“Transparent flat-plate sight glass with mica protection for high pressure boiler”
“Close-up of borosilicate sight glass reflex V-groove”
Industrial-grade materials & in-house machining for custom sizes.
Fast sample/urgent order turnaround to meet shutdown windows.
Batch traceability (QR per piece) and full test reports for audit.
OEM experience for global boiler makers, oil & gas, chemical and power plants.
Send your basic service data: maximum working pressure, operating temperature, fluid type, required visible length and mounting flange size. We’ll return a recommended type, glass spec and quote (including test certificates) within 24 hours. For urgent site failures, request “priority” and we can ship a tubular replacement same-day (stock permitting).
Closing / final note:
This page has shown how to choose and specify a liquid level sight glass for industrial applications — from tubular to reflex and transparent solutions. Specify pressure, temperature, medium and accuracy first, then choose the correct sight glass type to avoid downtime and safety risks. Contact us for CAD drawings, TDS and sample shipments for your next project involving liquid level sight glass.
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