When you look at the tempered borosilicate sight glasses offered on Zight, you’ll notice that the glass has been manufactured according to DIN 7080. You may be wondering what this term means, why we adhere to it for our sight glasses and circular glass replacements, and what makes DIN 7080 glass superior to other types.

We answer all your questions in the following guide to DIN 7080.


What Is DIN 7080?

DIN 7080 is a standard, also called a norm, to ensure the quality of a sight glass. DIN stands for Deutsches Institut für Normung, which in English means, “German Institute for Standardization.”

The German Institute for Standardization’s mission is to produce standards of quality, safety, environmental protection, and other important criteria for all industries. Anyone can submit a proposal for a standard, and if the proposal is accepted, it is developed by a relevant committee within DIN.

All standards are reviewed every five years to determine if they’re still needed or if they need to be revised according to the latest technological advancements.

Some standards, like the DIN 7080, are applied internationally, which is why at Zight, we adhere to it for our tempered borosilicate sight glass and other products.


Why Is It Important For Sight Glasses To Follow A Norm?

Sight glasses are an important component of industrial processes. They allow you to visually monitor the inside of tanks; weather its for level, flow, or process view.

Due to the nature of the environment where a sight glass is located, it needs to be strong and resilient enough to stand up to high amounts of heat and pressure, as well as resistant to chemical attacks. Otherwise, it will do one of two things (or both):

    1. The surface of the sight glass will become cloudy, obscuring your view of the water level inside.
    2. It will shatter, thereby endangering anyone positioned near the boiler.


Both of these scenarios are extremely dangerous. For example, f you’re not able to watch the boiler’s water levels, you won’t notice when it gets too high or too low. Without this knowledge, you won’t be able to take the necessary steps to shut down the boiler and prevent it from leaking – or even exploding. Your safety and the safety of everyone else in the building is at risk whenever the boiler sight glass is nonfunctional.

For this reason, it’s crucial that as sight glass manufacturers, we use processes that are guided by an internationally recognized norm, like DIN 7080, to produce sight glass that is strong, safe, and functional.

Adhering to DIN 7080 ensures that our circular borosilicate sight glass does not shatter under pressure, that it doesn’t cloud over, and that it lasts a long time without the need for a replacement.


Why Does Zight Adhere To DIN 7080?

Why use DIN 7080 as a norm for our sight glass and circular sight glass replacements, especially considering that it was established by a standardization institute in Germany? Isn’t Zight a US-based company?

To answer the second question first: yes, Zight products are manufactured in North America. But we use DIN 7080 for two reasons: 1) it’s the standard for our industry, and 2) it guarantees high quality for sight glass in these 7 key areas.


1. Material

DIN 7080 requires that sight glasses be made of borosilicate glass. Borosilicate glass is transparent and perfect for sight glass material, as it is well known to have a low thermal coefficient. The term “low thermal coefficient” simply means that it can undergo major temperature changes and not crack.

Another material commonly used for sight glasses is soda lime, but this type is weaker and inferior to borosilicate. To learn more on the differences between soda lime and borosilicate, click here.


2. Tempering And Pressure Resistance

The borosilicate glass should also be tempered rather than annealed to strengthen it and increase its resistance to pressure (to learn why tempering is superior to annealing, click here).

Tempering is the process of heating glass up to a temperature of over 600 degrees C and then rapidly cooling it down with a high-pressure air-cooling system. Under the pressure of the air, the center of the glass tries to move away from the outer surfaces, resulting in a tension in the center and compression on the outer surfaces.

This tension and compression are responsible for the glass’s increased strength and pressure resistance.

Under DIN 7080, the required pressure resistance of the borosilicate sight glass is defined according to the diameter of the glass. Meeting this standard ensures that a specific size of sight glass can withstand a maximum amount of pressure.

Circular Sight Glasses

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3. Mechanical Strength

DIN 7080 requires that the borosilicate glass must be thermally prestressed (in other words, tempered) until its strength is 100 N/mm2. This measurement indicates that the glass can handle an enormous amount of stress from outside forces, which is critical for a sight glass. It prevents it from breaking against internal stress from the process itself, as well as against external stress, such as a projectile hitting it.

Mechanical strength is the reason that tempering the sight glass is so important. Another treatment process for glass, called annealing, renders the material softer and not as strong. Annealed glass does not stand up as well to the stresses and pressures of a system as required by DIN.


4.   Thermal Properties

The thermal shock resistance of borosilicate for sight glass, as laid out by the DIN 7080, should be 230 degrees Celsius. As we mentioned earlier, this quality means that the sight glass can stand up to wild swings in temperature, which may occur when cold water is inadvertently allowed to enter the boiler.

In addition, the borosilicate glass should be able to withstand working temperatures of up to 280 degrees C. This is well within the typical temperature range of a boiler and even allows for overheating.


5.   Chemical Properties

The glass must also meet certain chemical resistance requirements. It should comply with the DIN norms for hydrolytic resistance (which measures the glass’s level of resistance to a chemical attack), acid resistance, and alkali resistance. These standards ensure that the sight glass is not compromised by exposure to toxins.

Chemical resistance is a key quality of a sight glass because it protects it from shattering or releasing toxins in the face of a chemical attack. Chemical attacks can occur over time as the sight glass is continuously exposed to the water in the boiler. Depending on the pH and temperature of the water, the sight glass can corrode and eventually shatter.

But with a good chemical resistance, which Zight circular sight glass replacements have, this process is prolonged, and your sight glass will last for a long time.


6.   Defects

DIN 7080 defines which defects are non-permissible in borosilicate sight glass, and which are acceptable. For example, bubbles in the glass are allowed, but only bubbles of certain numbers and diameters. Only three crystalline inclusions are allowed, and they must not exceed a certain diameter or be less than a certain distance apart.

Three defects that are not allowed in sight glasses (according to DIN) are viscous knots, sharp striations, or bands of striations visible to the naked eye. These could render the sight glass completely useless because they could obscure the water levels of the boiler.


7.   Tolerances

Tolerances are deviations from the norm in the diameter, thickness, flatness, parallelism, and edge of the sight glass which DIN allows. Defining tolerances lets the manufacturer know that if the sight glass does not meet DIN 7080’s exact requirements in these areas, but they still land somewhere within the tolerances, then it can still be certified with this standard.

Tolerances ensure a certain standard of quality for manufacturers, while at the same time allowing for minute deviations from the norm which are inevitable in a factory.



In the end, we adhere to DIN because it is the industry standard and we believe it results in a higher quality, stronger, longer lasting, and most importantly, safer sight glass. It ensures that we produce the best work in the most important areas: material, tempering and pressure, mechanical strength, thermal properties, chemical properties, defects, and tolerances.

So, when you see the DIN 7080 norm listed on Zight’s tempered borosilicate sight glass and circular glass replacement products, you know that we manufactured these products to provide you with the utmost safety, longevity, and functionality. These qualities will bring you and your boilermakers peace of mind.