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Sight glass

Sight glass – For inspection of industrial processes

Sight glass – also known as viewing glass, peep hole glass or inspection glass – are used to inspect or monitor a process. It could be a liquid in a container. With sight glass you can look in from the outside and ensure that gases, quantity, temperature, colour or process are acting as desired, while keeping the substance inside the container.

Mirit Glas is a supplier of custom-made glass throughout the Nordics. We can offer same-day delivery if it is very urgent.

Want to hear more about how Sight glas can be the right solution for you, please contact us.

Properties and function

Sight glass is typically used to keep instruments or measuring devices separate from industrial processes and to monitor:
  • As a level indicator in liquid substances as oil (controlled with an oil level glass), water (controlled with a water gauge), chemicals or liquid foods (flow gauge) . These can be checked with inspection glass, either as flat glass or sight glass in tubes.
  • Solid masses and materials such as grain in a grain silo or cement in a tank.
  • Combustion processes with high temperatures, where an inspection glass is needed
  • Freezing processes
  • As Food Contact Material (FCM). For example in a flow indicator or a level gauge
  • Air, gases or heat – possibly under pressure / vacuum (eg a vacuum chamber)
  • Many sectors use sight glass. For example the food industry, process industry, pharmaceutical, offshore, marine and transport (cars, boats, ships and aircraft). We supply sight glass to be used in  ATEX areas (to monitor explosion safe facilities).
Mirit Glas has supplied sight glasses to companies throughout the Nordic countries since 1952, and contributed to streamline a large number of processes. For example our customer Swire Blue Ocean has saved 75 maintenance days a year by replacing steel plugs with glass from Mirit Glas. The sight glass was mounted in a housing of stainless steel (a flange) that makes it possible to inspect oil levels visually.


Fabricating and special processing is possible at Mirit Glas. For example with an anti-dirt coating, preventing oil and other substances to stick to the glass keeping the process fully visible at all times.

We can also print on the glass if, for example, if an indication of measuring steps is desired. We can bend the glass making the sigth glass curved. The glass can also be processed and cut into shapes or provided with a special edge.

Complies with standards

Regardless of which substance is to be inspected via inspection glass, the sight glass is adapted to specific needs and requirements, and is of course manufactured in accordance with approved standards.

  • Round sight glass: complies with DIN 8902 or DIN 7080
  • Sigth glass with curves or rectangular sigth glass: complies with DIN 8903 or DIN 7081
  • Sight glass with special processing: Eg special cut-outs or drilling
  • Sight glass in other glass materials: Eg quartz glass, sapphire glass or ceramic glass such as Robax or Neoceram

Sight glass from Mirit Glas can be ordered with various certificates, eg Certificate EN 10204 Type 2.2 and Certificate EN 10204 Type 3.1 as well as certificate for food contact material (FCM) according to regulation (EC) No 1935/2004.

We can to produce gaskets in custom sizes and pack these together with the sight glass.

Important questions in choosing the right sight glass

1. Temperatures
DIN 8902 and DIN 8903 can be used at temperatures up to 150 °C and 100 °C. DIN 7080 and DIN 7081 can withstand higher temperatures – up to 280° C. High pressure affects the temperature maximum the glass can withstand

2. Pressure
Mirit Glas has many different glass thicknesses in stock. If we know the diameter and clearance of the glass (the part of the glass that is not covered by the flange), we can find the right thickness so that the glass can withstand the desired pressure.

3. The substance behind the glass
Are there chemicals, oils or anything else in the container? What process is done behind the glass? Glass is generally resistant to most chemicals, but for example steam under pressure is a very aggressive process to ordinary glass, so special precautions must be taken.

4. The application
Why a sight glass? Just to able to look in visually, or does the application require that electronic sensors are able to measure through the glass? In that case, we need to pay attention to the transmission at certain wavelengths.


A sight glass is a common term for a glass that is used to get an overview of a process. It can be a boiler glass where you can see into a boiler. It can also be a glass tube that gives an overview of oil flow or a glass that makes it possible to see into a grain drying plant. Sight glasses are available in many varieties. What they have in common is that they make it possible to see otherwise hidden parts of a production.


Sight glass is a broad term for a tube or glass that is used to keep an eye on, among other things, a process, a liquid or a combustion.


A sight glass is made from the type of glass that makes sense. Many spectacle glasses, for example, are made of borosilicate glass. Others are made as heated windows to prevent fogging. Some sight glasses are made of Robax, which is very heat resistant because the purpose is to monitor a burn.


A sight glass can be square, round or a glass tube. Sight glasses are manufactured to suit the situation. Many of the spectacle glasses that Mirit Glas manufactures are not round, oblong, shaped or square.

We look forward to hearing from you!

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