Reflex level gauges allow the medium to be viewed through a reflex glass.The side of the glass which is exposed to the medium has a prismatic surface while the other side is smooth.The medium level inside the level gauge is indicated as the result of the light refraction principle since the liquid filled area completely absorbs the rays of light and appears dark whereas the rays of light are totally reflected in the gas/air filled area which appers bright.Reflex level gauges offer great advantages in terms of low purchase and maintenance costs as well as easy level reading.These are available in different sizes depending on customer request.
Prismatic face — level reads as sharp black/silver, even in poor light
Reflex gauge glass is the level gauge you fit when the reading has to be unambiguous. Its face is not flat: it carries machined prismatic grooves on the liquid side, and those prisms exploit the difference in how light refracts through liquid versus vapour. Where there is vapour, the light is totally internally reflected and the glass appears bright silver-white. Where there is liquid, the light passes through and the glass appears black. The result is a hard, high-contrast band that can be read instantly from across a boiler house, in poor light, without interpretation. That is why reflex glass is the standard for boiler drum level.
It is made from tempered borosilicate to DIN 7081, the international dimensional standard, and is rated to roughly 300 bar and 400 °C.
But that 400 °C figure needs a serious caveat, and it is the most important thing on this page. It applies to benign, non-aggressive service. In steam and hot water — which is precisely what a boiler gauge sees — the practical limit is far lower, around 245 °C. The reason is chemical, not thermal: hot water and steam slowly leach and etch the glass surface, which becomes progressively cloudy, then weakened, and eventually fails. This is why boiler gauge glasses are fitted with a mica shield — a thin, replaceable mica sheet between the glass and the water that takes the chemical attack instead of the glass. Fitting a reflex glass to a boiler without a mica shield, on the strength of the headline temperature rating, is a genuine and dangerous mistake.
Reflex glass only works with a clear, single-phase liquid. If you need to see through the glass — to judge colour, turbidity or an interface between two liquids — you need transparent (plain) gauge glass instead, which is viewed with a light behind it.
Specifications
Typical properties for DIN 7081 tempered borosilicate reflex gauge glass. Note carefully the difference between the maximum rating and the practical limit in steam service.
Reflex gauge glass is made from borosilicate glass — chosen because its low coefficient of thermal expansion gives it excellent resistance to thermal shock, which matters enormously on a boiler gauge that can see a sudden temperature change. The prismatic grooves are machined into one face, then the glass is tempered (toughened), which puts the surface into compression and dramatically raises its strength and its ability to survive pressure and thermal cycling.
Tempering has one important consequence for the user: a tempered glass cannot be cut, drilled or ground. Any attempt to modify it destroys the surface compression and the glass will shatter. Gauge glasses must therefore be ordered in the correct DIN 7081 length — they cannot be trimmed to fit on site.
Manufacture to DIN 7081 means the dimensions are standardised, so a gauge glass from one manufacturer is dimensionally interchangeable with another to the same size code. That is genuinely useful for maintenance stores — you are buying to a standard, not to a proprietary size.
DIN 7081 Standard Sizes
Standard section is 34 × 17 mm. Gauge glasses are ordered by their DIN size code, which fixes the length. They cannot be cut to length — tempered glass shatters if machined, so the correct size must be ordered.
DIN Size
Length
Section (W × T)
Typical Use
Size 1
115 mm
34 × 17 mm
Small vessels, compact gauges
Size 2
140 mm
34 × 17 mm
Small / medium vessels
Size 3
165 mm
34 × 17 mm
Medium vessels
Size 4
190 mm
34 × 17 mm
Medium vessels
Size 5
220 mm
34 × 17 mm
Boiler drums, common size
Size 6
250 mm
34 × 17 mm
Boiler drums, common size
Size 7
280 mm
34 × 17 mm
Larger boilers
Size 8
320 mm
34 × 17 mm
Larger boilers
Size 9
340 mm
34 × 17 mm
Large vessels
Custom
70 – 400 mm
25 – 35 × 15 – 25 mm
To order
Always replace the gaskets and the mica shield when you replace the glass. Re-using a compressed gasket or an etched mica shield is a common cause of a new gauge glass leaking or failing early. Tell us your gauge make and size and we will supply the glass, gaskets and mica together.
Chemical Resistance
Borosilicate is chemically resistant to most media — but it has one important weakness that directly affects its main application: hot water, steam and alkalis chemically attack (leach) the glass surface.
Medium
Resistance
Notes
Steam & Hot Water
Fair
Leaches/etches the glass — a mica shield is required
Acids (most)
Excellent
Borosilicate is highly acid-resistant
Hydrocarbons / Oils
Excellent
Unaffected
Solvents
Excellent
Unaffected
Alkalis / Caustic
Poor
Attacks glass — mica shield essential, or use a different gauge type
Hydrofluoric Acid
Poor
Dissolves glass — never use
Phosphoric Acid (hot)
Poor
Attacks glass
Water (cold)
Excellent
No issue at ambient temperature
General guidance for the material family at normal industrial concentrations. See the full Chemical Compatibility Chart, and confirm your exact chemical, concentration and temperature with us before final selection.
Applications
The default gauge for direct level reading where the liquid is clear and the reading must be instant.
Boiler Drum LevelThe classic application. The black/silver contrast is readable instantly across a boiler house in poor light — which is exactly what a safety-critical level reading needs. Always fit with a mica shield.
Steam Drums & SeparatorsSteam and condensate vessels where a clear, unambiguous liquid level is needed and the media is clean.
Pressure Vessels & ReceiversHigh-pressure vessels up to around 300 bar, where the reflex glass’s pressure rating and thermal shock resistance are both required.
Chemical & Process TanksStorage and process tanks holding clear liquids. Borosilicate resists most acids and solvents well — but not alkalis or hydrofluoric acid.
Oil & Fuel TanksHydrocarbon service, where borosilicate is completely unaffected by the media and the reading stays clear.
Condensate & Feedwater VesselsDeaerators, feedwater tanks and condensate receivers — hot water service, so the mica shield caveat applies here too.
Industries
Why Reflex Gauge Glass is specified in each of the industries we serve.
Power Plants
Boiler drum level is a safety-critical reading, and reflex gauge glass is the standard for it precisely because the reading is unambiguous: liquid shows as hard black, vapour as bright silver, and an operator can take that in at a glance from across a hot, poorly lit boiler house. Getting it wrong has consequences, which is why the mica shield matters so much here. Boiler water is hot and often chemically dosed, and it slowly leaches and etches the glass surface — a gauge fitted without a mica shield goes cloudy, then weak, and can eventually fail under pressure. The mica sits between the water and the glass and is sacrificed instead. Replace the glass, the gaskets and the mica together as a set at every changeout; re-using an old mica shield or a compressed gasket is the usual reason a freshly fitted gauge glass leaks or fails early.
Chemical Plants
Borosilicate’s chemical profile makes it a natural fit for a good deal of chemical plant service: it resists most acids and solvents completely, and it handles thermal shock, which matters on vessels that see rapid temperature swings. Where it must not be used is against alkalis and hydrofluoric acid, both of which attack glass directly — HF dissolves it outright. That is a hard material limit, not a rating to be pushed. The other selection question in chemical service is whether reflex is the right type at all: reflex glass reads a level by reflection and cannot show you colour, turbidity, or an interface between two liquids. If what you need to see is the condition of the fluid rather than simply where its surface is, transparent gauge glass or a sight glass is the correct choice.
Sugar Mills & Process Industries
Mills run their own boiler houses, and reflex gauge glass on the boiler drums is standard for the same reasons it is in a power station — instant readability and a high pressure and temperature rating. The mica shield requirement applies identically. The characteristic mistake in a process plant is fitting reflex glass to a vessel containing something that is not a clear, single-phase liquid. Reflex works by the optical difference between liquid and vapour; put a dark, viscous or particulate-laden fluid like juice or massecuite behind it and the contrast that makes it readable simply is not there. For those services, a transparent gauge with a backlight, or a different level technology altogether, will actually be readable — a reflex gauge will not.
Oil & Gas
Hydrocarbon service suits borosilicate very well: oils, fuels and solvents leave the glass completely unaffected, so the reading stays clear over a long service life without the chemical attack that makes steam service so demanding. Reflex gauges are used on separators, receivers, knock-out drums and storage vessels, and the ~300 bar pressure rating covers most of what a refinery or wellhead installation requires. The selection caveat is again about what you actually need to see: on a separator, where an interface between two liquids (oil and water) is the thing that matters, a reflex gauge cannot show it — it only distinguishes liquid from vapour. An interface reading requires transparent gauge glass with illumination behind it.
Reflex vs Transparent vs Sight Glass
These are three different instruments, not three price points. Choose on what you need to see.
Property
Reflex Gauge Glass
Plain / Transparent
Sight Glass
Shows Level
Yes — sharpest contrast
Yes
Roughly
Shows Colour / Turbidity
No
Yes
Yes
Shows Liquid-Liquid Interface
No
Yes
Yes
How It Reads
Black = liquid, silver = vapour
See through it
See through it
Needs Backlight
No
Yes
Usually
Readability in Poor Light
Excellent
Fair
Fair
Suits Dark / Dirty Fluids
No
Yes
Yes
Typical Use
Boiler drum level
Interfaces, coloured fluids
Flow / condition viewing
Advantages & Limitations
Advantages
The clearest, most unambiguous level reading of any gauge glass — hard black against bright silver.
Readable instantly from a distance and in poor lighting, with no backlight required.
High pressure rating (~300 bar) and high temperature capability.
Excellent thermal shock resistance, thanks to low-expansion borosilicate.
Excellent chemical resistance to acids, oils, fuels and solvents.
Manufactured to DIN 7081, so sizes are standardised and interchangeable between makers.
Tempered for strength and pressure-cycling durability.
Limitations
The 400°C headline rating does not apply to steam. In steam and hot water the practical limit is around 245°C, because the water chemically leaches the glass. A mica shield is required for boiler service.
Attacked by alkalis and hydrofluoric acid. HF dissolves glass outright. This is a hard material limit.
Cannot show colour, turbidity, or a liquid-liquid interface — it only distinguishes liquid from vapour. Use transparent gauge glass for those.
Does not work with dark, dirty, viscous or particulate-laden fluids — the optical contrast it relies on is not there.
Cannot be cut, drilled or ground. Tempered glass shatters if machined — it must be ordered in the correct DIN length.
Glass is brittle: it will fail if mechanically shocked, over-torqued or unevenly clamped.
Installation, Storage & Maintenance
Installation
Never over-tighten a gauge glass. Uneven or excessive clamping load is the most common cause of a glass cracking, and it usually happens on installation rather than in service. Tighten the cover bolts evenly, in a criss-cross pattern, in stages, to the gauge manufacturer’s torque — not by feel. Always fit new gaskets and a new mica shield with a new glass; re-using a compressed gasket or an etched mica shield is the usual reason a freshly fitted gauge leaks or fails. Never attempt to cut, grind or drill the glass. Allow the assembly to warm up gradually on commissioning rather than admitting hot steam suddenly to a cold glass.
Storage
Store gauge glasses in their original protective packaging, laid flat, where they cannot be knocked, dropped or have anything placed on top of them. Tempered glass is strong in service but a sharp edge impact will chip it, and a chipped gauge glass is a failure waiting to happen — do not fit one. Keep mica shields flat, dry and clean.
Maintenance
Inspect for clouding or etching of the glass surface — in steam service this is the visible sign that the glass is being chemically attacked and is progressively weakening. A cloudy boiler gauge glass should be treated as a component approaching failure, not merely a cosmetic problem: replace it, and check that a mica shield is fitted and being renewed. Replace glass, gaskets and mica as a set. Never re-tighten a leaking gauge under pressure.
Buying Guide
What to tell us so we quote the right grade first time.
Is it steam or hot water service? If yes, you need a mica shield — and the practical temperature limit is around 245°C, not the 400°C headline figure. Tell us and we will supply the mica with the glass.
What do you need to SEE? Just the level, clearly? Reflex. Colour, turbidity, or an oil-water interface? You need transparent gauge glass instead — reflex physically cannot show those.
Gauge make and DIN size. Gauge glasses are ordered to DIN 7081 sizes and cannot be cut to length. Give us the size code, or the make and model of the gauge, and we will match it.
Operating pressure and temperature. Standard reflex handles ~300 bar. Confirm your duty, especially if it is high-pressure steam.
What is the media? Critical if there is any alkali or hydrofluoric acid — both attack glass, and HF dissolves it. Tell us, because glass may be the wrong technology entirely.
Do you need gaskets and mica too? Almost always yes. Always replace them with the glass — ordering the set together saves a second failure a month later.
Standards & Compliance
DIN 7081 — the dimensional standard for level gauge glasses. Sizes are standardised and interchangeable between manufacturers.
Its face carries machined prismatic grooves. Where there is vapour, light is totally internally reflected and the glass looks bright silver-white. Where there is liquid, light passes through and it looks black. The result is a hard, high-contrast band that shows the level instantly.
Why is my boiler gauge glass going cloudy?
Chemical attack. Hot water and steam slowly leach and etch the glass surface. A cloudy glass is not just cosmetic — it is progressively weakening and should be treated as approaching failure. Replace it, and make sure a mica shield is fitted and being renewed.
What is a mica shield and do I need one?
A thin, replaceable mica sheet fitted between the water and the glass. It takes the chemical attack from hot water and steam instead of the glass. For boiler and steam service, yes — it is required. Fitting a gauge glass to a boiler without one is a genuine mistake.
The glass is rated to 400°C. Can I use it at 400°C in steam?
No, and this is the most important caveat about this product. The 400°C figure is for benign, non-aggressive service. In steam and hot water the practical limit is around 245°C, because the water chemically attacks the glass. Design to that, and fit a mica shield.
Can gauge glass be cut to length?
No. It is tempered, which puts the surface in compression. Cutting, grinding or drilling it destroys that compression and the glass will shatter. You must order the correct DIN 7081 length.
What is DIN 7081?
The international dimensional standard for level gauge glasses. It means sizes are standardised, so a glass from one manufacturer is dimensionally interchangeable with another to the same size code — genuinely useful for maintenance stores.
What is the standard size?
The most widely used section is 34 mm wide × 17 mm thick. Lengths run from 70 mm to 400 mm in standard DIN increments.
Reflex or transparent — which do I need?
Ask what you need to see. If you just need the level, sharply and instantly, choose reflex. If you need to see through the glass — colour, turbidity, or an interface between two liquids — you need transparent, which is read with a light behind it. Reflex physically cannot show you those things.
Can I use reflex glass on a liquid-liquid interface?
No. Reflex only distinguishes liquid from vapour. It cannot show you where oil meets water. Use transparent gauge glass with a backlight for interface reading.
Can I use it with dark or dirty fluids?
No — it will not read. Reflex works by the optical contrast between liquid and vapour, and a dark, viscous or particulate-laden fluid destroys that contrast. Use a transparent gauge or a different level technology.
Which chemicals attack gauge glass?
Alkalis and caustic attack glass, and hydrofluoric acid dissolves it outright — never use glass with HF. Hot water and steam also chemically leach the surface (hence the mica shield). Acids, oils, fuels and solvents are all fine.
Why did my new gauge glass crack on installation?
Almost always over-tightening or uneven clamping. Tighten the cover bolts evenly, in a criss-cross pattern, in stages, to the manufacturer’s torque — never by feel. A chipped glass, or one warmed too suddenly, will also fail.
Should I replace the gaskets when I change the glass?
Always — and the mica shield too. Re-using a compressed gasket or an etched mica shield is the most common reason a freshly fitted gauge glass leaks or fails soon after. Replace all three as a set.
What pressure can reflex gauge glass handle?
Around 300 bar for standard DIN 7081 tempered borosilicate, which covers the great majority of boiler and pressure-vessel applications.
Is borosilicate resistant to thermal shock?
Yes, very — that is one of the main reasons it is used. Its low coefficient of thermal expansion means it tolerates rapid temperature change far better than ordinary glass, which is exactly what a boiler gauge needs.
Can I fit a chipped gauge glass?
No. A chip is a stress concentration in a tempered, pressurised component — it is a failure waiting to happen. Discard it.
Can I re-tighten a leaking gauge under pressure?
No. Isolate and depressurise the gauge first. Tightening a glass under pressure risks cracking it, with obvious consequences on a hot, pressurised vessel.
How do I know what size to order?
Give us the make and model of your gauge, or the DIN size code of the existing glass. Since it cannot be cut, ordering the correct length matters.
How long should a gauge glass last?
In benign service, a long time. In steam service, life is determined by chemical attack — which is exactly what the mica shield exists to slow down. Inspect for clouding and replace before it becomes a weakness.
Do you supply the gaskets and mica as well?
Yes — tell us the gauge make and size and we will supply the glass, gaskets and mica shield together, which is how they should be replaced.
Need a quote or a spec check?
Send us your operating conditions and we will confirm the right grade for the job.
