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However unlikely the possibility may seem, sight glasses (also called level glasses) do suffer physical damage, also referred to as mechanical damage, from time to time.

Fortunately, by using these four best practices, you can easily avoid scenarios where an external force or pressure could lead to a crack or break in the sight glass. 

But first, let’s talk about the likelihood of your sight glass undergoing physical damage, and why it’s important to always be prepared.


How Likely Is Physical Damage To A Sight Glass?

As you’ll soon see, physical damage to a sight glasses while it’s secure in the gauge is not very likely. That’s because sight glass, at least the kind we sell at Zight Glass, is manufactured to meet international industry standards of pressure resistance, tensile strength, chemical resistance, and other important factors of safety and function. 

For example, our tempered sight glass and borosilicate sight glass are manufactured in adherence to DIN 7080 glass, the industry standard for this type of product. DIN ensures high quality metrics in the areas we just mentioned.

Similarly, our reflex glass and transparent glass are manufactured according to DIN 7081, which guarantees excellent quality for these products, as well. 

As long as you perform proper maintenance and regular replacements of your sight glass and use products like Zight that adhere to industry standards, you shouldn’t experience any problems, barring a freak accident. 

Make no mistake, though. This is not an area to take lightly. Accidents do happen, and they always seem to happen when you’re not prepared for it. 

That’s why it’s important to implement the following best practices for all types of sight glass, whether it’s a transparent glass, a reflex glass, or a borosilicate sight glass. If you don’t, that’s when you’re more likely to see physical damage occur.


1.  Regular Replacement Of Sight Glass To Relieve Sealing Stress

One way that a sight glass or level glass can become damaged is through sealing stress. 

Sealing stress is the stress placed on the edge of a sight glass by the gasket seat that is clamped down on it. The amount of stress that a gasket seat can place on the edge of a sight glass is enormous. Over time, the glass is no longer able to withstand this immense pressure, at which point cracks begin to form. 

It takes a long time for the edge of the sight glass to reach breaking point, however, which gives us plenty of time to develop a strategy for dealing with this problem. 

The simplest way to avoid cracks or breaks that result from sealing stress is to schedule regular sight glass replacements. This is a smart idea in any case, as sight glass can erode over long periods of exposure to a liquid’s fluctuating temperatures and pH levels. 

With regular replacement, you will be several steps ahead of the glass cracking under sealing stress before this scenario even becomes a real risk.


2.  Install Thick Sight Glass To Prevent Pressure Driven Blowout 

Pressure-driven blowout has always been a point of concern among sight glass manufacturers. In this scenario, the level glass ruptures when the internal pressure of the gauge rises too high. 

This risk is easily dealt with using one simple practice: installing thick sight glass. The thicker the sight glass, the better equipped it is to handle large amounts of pressure. 

At Zight Glass, our borosilicate sight glass comes in various thicknesses to help you prevent pressure-driven blowout. But you may also prefer to fuse your glass plates together, which is another acceptable and effective method for increasing thickness.


3.  Shielding To Protect From Projectile Damage

Projectile damage is another possibility for which sight glass manufacturers and boilermakers should prepare. 

A projectile may be a tool, a rock, a piece of machinery, or any blunt or sharp object. Most of these objects won’t do much damage unless there is already a great deal of force behind it. However, it doesn’t hurt to prepare for these kinds of scenarios, considering that anything can and does happen. 

One method of protecting your sight glass from becoming damaged by a projectile is to shield it with another piece of glass. This is done by gluing one piece of glass to another, also known as laminating them together.

This “shield” should have the same pressure rating as the original, so that if a projectile hits it, the projectile won’t bore through it and damage the first one. That would defeat the purpose of the shield.


4.  Compression To Protect From Projectile Damage

The second method of protecting a level glass or sight glass from projectile damage is through compression. Compression increases the glass’s tensile strength, enabling it to withstand sudden and extreme pressure as of that from a projectile.

Compression can take place in several ways. One is by tempering, which is a process of heating up the glass to a high temperature and then quickly cooling it down with air pressure. We use this process at Zight for our borosilicate glass and soda lime to make tempered sight glass.

Another way to compress and strengthen glass is to treat it with chemicals. 

Lastly, you can fuse it into a ring of metal, which manually compresses the glass and makes it more resistant to impact cracks or breaks.



Avoiding physical damage to your sight glass or level glass is simple enough, providing you consistently perform basic maintenance and are smart with your choice of glass product. 

We recommend changing out your sight glass or level glass regularly and replacing it each time with the thickness that the gauge manufacturer recommends.