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Sight glass typically comes in two types of glass: tempered soda lime or tempered borosilicate. What’s the difference between them? And which one is better for observing liquids within a vessel or the interior of a vessel? We answer these questions below.


What Is Soda Lime?

Soda lime glass is a common material that you’ll find in many of your household goods, including windows, bottles, plates, and food containers.

Soda lime has a low thermal shock resistance, which means it will break easily when it’s exposed to extreme temperature changes. It also doesn’t stand up well to a lot of pressure or high temperatures.

After the tempering process, where the glass is heated up to a high temperature and then quickly cooled down again using cooling air jets, soda lime is much stronger. At this point, it is considered suitable for sight glass, but it’s not the best option out there. It’s still only resistant to temperatures up to 150 degrees Celsius, which is too low for many sight glass applications.


What Is Borosilicate?

Borosilicate is a type of glass that has an inherently low thermal expansion coefficient. This allows it to handle swings of extreme temperature without breaking. Tempered borosilicate is even stronger and more durable. It is also more chemical resistant than soda lime, a quality which will prevent or slow corrosion throughout the life of the sight glass.

Another advantage of tempered borosilicate sight glass is it can withstand much higher temperatures. This, along with its increased tensile strength after tempering and chemical resistance, makes borosilicate an excellent, durable choice for industrial uses.



Is Soda Lime Or Borosilicate Better For Sight Glass?

While both types of glass are serviceable in the form of a sight glass, we prefer borosilicate over soda lime. The advantages of the tempered borosilicate sight glass are obvious: greater tensile strength, greater pressure resistance and chemical resistance, and a high tolerance for thermal shock. Soda lime scores less in all these metrics, even after tempering. 

However, we would also like to note that, providing that the application fits within the limits of tempered soda lime in terms of pressure resistance and temperature, soda lime is not a bad option for sight glass. The key is to make sure that it meets the industry standards of quality and safety. 

At Zight, we manufacture our tempered soda lime glass in accordance with DIN 8902. This norm ensures that the soda lime is made with high enough quality to be suitable for certain applications, such as sight glasses.

Similarly, our tempered borosilicate sight glass follows the DIN 7080 norm. This way, no matter which type of sight glass you choose, you know it has been manufactured and produced with the utmost care and designed to hold up against high pressure resistance, chemical resistance, and tensile strength ratings.



In the end, while tempered borosilicate sight glass is the smarter choice to meet industrial standards of quality, strength, and resiliency, soda lime can also be serviceable in the right conditions.