What Are the Different Types of Sight Glass?

15 August 2020

AUTHORED BY: Reliability360

A sight glass is an object that is added to a reservoir tank or machine to see the oil inside. They come in two basic forms, bullseye and columnar, and can be used to monitor oil levels as well as contaminants. Sight glasses are an alternative to using a dipstick or simply opening a tank to look inside, because both of these options allow air, condensation and other contaminants to enter closed systems.

It’s also worth considering the risks associated with installing them. Columnar sight glasses are mounted outside tanks, which can leave them vulnerable to damage. Bullseye sight glasses don’t have this drawback, but you must be able to place them precisely or they’ll give inaccurate readings.

In this post, you’ll learn how to identify the different types of sight glass and understand why it’s important to use one.


Types of Sight Glass

Almost every sight glass can be classified as either bullseye or columnar, but there are some variations you should be aware of before deciding which is best for your operations.

A bullseye sight glass is installed by drilling a hole in a tank at the top of the optimum oil level. The sight glass is then threaded into the hole and looks like a bolt with a glass head. Some have a line in the glass that shows where the oil level should be when the equipment is running. One of the benefits to using a bullseye sight glass is it will help you detect foaming in your system. This usually happens when water and other contaminants are present, so you can catch problems before serious damage occurs.

These are clear, vertical tubes mounted outside tanks that allow you to see a larger sample of oil than bullseye sight glasses. However, they tend to be more difficult to install and they are typically mounted near the bottom of tanks where they are susceptible to damage. Another issue is they tend to draw from the bottom of tanks, so they don’t usually show problems with foaming when it is present. A benefit of columnar sight glasses versus bullseye is that you can mark the correct oil levels for a machine when it is running and idled. Bullseye sight glasses only show one or the other.

BS&W Bowls
Bottom sediment and water (BS&W) bowls are mounted to the bottom of tanks and often incorporate a magnet that draws metallic contaminants to them. Many also feature a release valve that can be used to collect oil analyses. Since these are mounted to the bottom of tanks, it is possible for them to get damaged, but most are much smaller than columnar options.

This is a variation of a bullseye sight glass, but instead of being flat, it extends outward from a tank. This allows you to view the oil from multiple angles, as well as permitting light to pass through the oil so you can see contaminants easier. Most of these are made from glass, acrylic or polyamide and are rated for extreme pressures and temperatures.

These devices have features that combine those found in the previously listed options, but also add abilities such as sight glass removal for cleaning and inspection.


Why You Should Use a Sight Glass

Oil life depends on a number of factors, but it’s possible to spot most of them with a simple visual inspection. As mentioned above, a sight glass doesn’t require opening a closed system, so no contaminants like moisture or debris get added when you need to do an inspection.

Beyond looking at the color of the oil, it’s important to know the amount of contaminants present, as well as their size. If a sight glass suddenly fills with dirt, you know you have a problem. But it’s also worthwhile to scan for metal fragments that come from wear. Some sight glass options include a magnet that draws these fragments to them, which will help you determine if the wear you’re seeing is normal or needs to be addressed.

At the very least, it’s good to monitor oil levels in your equipment. A sight glass makes it very easy to determine with a glance if the proper amount is in a tank or machine.


Contact Us to Learn More About Using A Sight Glass

Let the Reliability 360 lubrication specialists show you how adding a sight glass to your oil tanks and other equipment will help you keep everything running properly.