A Comprehensive Guide to Oil Sight Glasses

Jonathan Trout, Noria Corporation

Oil sight glasses work by giving you a real-time snapshot of the oil inside your machinery without having to shut anything down. This live inside look plays a critical role in condition monitoring and early detection.

Oil sight glass  

What Is an Oil Sight Glass?

An oil sight glass is a transparent tube or bowl installed in a tank or reservoir allowing anyone to visually inspect what's happening with the liquid inside the container. An oil sight glass is an all-encompassing term for the different types of sight glasses that we'll discuss below. Common uses and applications for sight glasses prove especially useful for monitoring oil or lubricant quality and quantity inside pipes, tanks and reservoirs. You'll find them on equipment across industries such as:

  • Manufacturing plants
  • Pharmaceuticals
  • Food and beverage
  • Bio fuels
  • Wastewater treatment plants
  • Chemical plants

Sight glasses are commonly found in industries that use pressurized systems like boilers, steam generators, cement production and even breweries. Using a sight glass is a safe way to check the level and quality of the pressurized liquid without needing to open the tank or reservoir. Most modern oil sight glasses offer 360-degree views and are made of acrylic, polyamide and even a new high-performance copolyester that we'll discuss later.


Types of Oil Sight Glasses

Oil sight glasses typically come in some variation of two forms: the bullseye and the columnar. However, with modern technology and ingenuity, companies are starting to manufacture higher-quality sight glasses and level monitors, often with multifunctional components or combining multiple components. We're going to look at five types of common oil sight glasses on the market today, take a look at some state-of-the-art oil sight glass products and see what they can do for you.

    bullseye sight glass
  • Bullseye sight glasses are made of transparent material like glass, acrylic or polyamide. They are threaded into a port sitting just above the oil level inside the reservoir. Bullseye sight glasses are great for checking the oil level because there is no piping which could give you a false level reading. Traditional bullseye sight glasses offer a flat, transparent viewing window.
  • Columnar sight glass
  • Columnar sight glasses attach to an oil-containing component near the drain port. They're typically made of acrylic or glass and allow visualization of the oil level fluctuating up and down. A columnar sight glass must be able to "breathe" as oil levels fluctuate, so you'll need to connect it to the headspace using a length of pipe or flexible tubing, which in turn is connected to a desiccant breather. It's also important to mark the idle and running oil levels as a reference for when the levels are off. One disadvantage of a columnar sight glass is that it doesn't clearly show the presence of foam, since the oil entering the columnar is coming from the bottom of the sump, giving air a chance to escape.
  • 3D sight glass
  • The 3D sight glasses are new and improved bullseye sight glasses. Luneta's 3D sight glass (shown right) is a great example of a modern 3D sight glass. The most obvious difference is that the 3D sight glass offers a 360-degree visual of the oil with a flat front and crosshairs to check the oil level. Most 3D sight glasses are made with glass, acrylic or polyamide. Luneta's sight glass uses a clear copolyester material called Tritan, which gives the 3D sight glass optimal visualization and the highest durability rating of any sight glass on the market.
  • Bowl sight glass
  • Bottom sediment and water (BS&W) bowls are 2-in-1 devices that act as a sight glass and a BS&W bowl, so you can visually inspect the oil and see potential debris and harmful contaminants like water and sediment. Our example shows Luneta's Bowl - a BS&W/sight glass combination made of the same impact-resistant Tritan copolyester as the company's 3D sight glass. The Bowl uses a rare-earth magnetic drain plug to attract wear debris while the machine is running, drawing wear particles to the bottom and letting you easily drain contaminants.
  • CMP sight glass
  • Condition Monitoring Pods (CMPs) are kind of like an all-in-one oil sight glass thanks to their multi-use functions. Displayed to the right is Luneta's CMP. It has a 3D sight glass offering a 360-degree view so you can check the oil level, color and clarity as well as if there's any foaming, corrosion or wear debris. It also includes a visible magnetic plug inside the glass to capture wear debris and make it easily visible. The sight glass rotates, allowing you to remove the magnetic plug and corrosion indicator for further inspection. In addition, Luneta's CMP includes a built-in sampling valve with an attached pilot tube for live-zone oil sampling. Like the company's other products, the CMP is made of Tritan copolyester.

Benefits of Using an Oil Sight Glass

Today, oil sight glasses are engineered to be stronger than ever and serve as multi-functional tools, so you not only can check and monitor oil levels and detect wear debris but can even draw an oil sample from the same device. The benefits you'll see from using a modern sight glass include:

  • Clear and concise oil level monitoring. For optimal oil level monitoring, look for a 3D oil sight glass with crosshairs on a flat front or a columnar sight glass with the proper oil level marked on the tube.
  • Clean and concise oil condition monitoring. Products like Luneta's Bowl and CMP help you attract, see and even extract wear debris and other contaminants that may be in your oil for testing.

For example, say you installed an oil sight glass on a tank. Then, during a routine inspection, you noticed foam in the sight glass. Foam in your oil or lubricant results in the reduction of the oil's lubricating properties, eventually leading to system wear and failure. Foam is a symptom of many issues, such as water or solid contamination, mechanical issues causing too much aeration, or lubricant cross-contamination. Without a sight glass, you may not have detected the presence of foam for a while, since you'd have to manually open the tank to look, which would expose the lubricant to more potential contamination.


Oil Sight Glass Installation

Installing a modern oil sight glass should be a relatively simple process revolving around hand-threading the sight glass and tightening with a crescent wrench. If you don't properly install the sight glass, however, your oil readings will be wrong, and depending on whether your machinery is pressurized, the wrong sight glass could spell trouble. So, what do you need to know before installing your oil sight glass?

  • Determine whether you're installing a variation of a bullseye or columnar sight glass. The installation location depends on the sight glass type. Bullseye sight glasses are installed in a port right at the oil level. Columnar and BS&W bowls are installed in the drain port.
  • If you're using a length of piping to connect the sight glass to the headspace, make sure there are no "goosenecks" or traps in the piping that would prevent oil from moving freely.
  • Note and match the recommended temperature ranges and maximum operating pressure requirements with a sight glass that can satisfy those requirements.
Oil sight glass installation

Let's look at what the installation of a sight glass might entail using Luneta's Sight Glass as an example. As mentioned earlier, modern sight glasses can be installed fairly quickly, with this one requiring just five steps.

  1. Apply thread sealant to the threads.
  2. Hand-thread the sight glass into the port.
  3. Use the appropriately sized crescent wrench or socket wrench to tighten until the hex nut comes into contact with the port face. If using an O-ring, tighten until the O-ring is compressed.
  4. Check for leaks.
  5. Don't overtighten.
Luneta Sight Glass
Luneta Sight Glass Size 1-inch 0.75-inch 0.5-inch 0.375-inch 0.25-inch
Wrench Size 1-5/16 1-1/18 7/8 3/4 5/8
Temperature Range
-40°F to 200°F / -40°C to 93°C
Max. Operating Pressure 65 psi/4.48 bar

Oil Sight Glass Comparison

Even though all oil sight glasses serve to accomplish the same goal (monitoring oil level and condition), it's important to compare not only which sight glasses are best for certain applications but also their makeup and manufacturing properties as well. For example, a poor-quality sight glass installed at the bottom of a reservoir has a high risk of incurring impact damage, which could lead to a major oil leak. Conversely, a sight glass that doesn't meet mechanical pressure ratings can spell disaster for high-pressure machines.

Look at the makeup of various oil sight glasses and compare each one's quality. Specifically, consider things like durability, how resistant it is to impact, its chemical resistance and its thermal ratings. Below is a comparison of the most common oil sight glass materials in relation to these factors.

Good better best chart

So, how do you know which type of oil sight glass is best for your specific need or application? Machinery Lubrication compares bullseye and columnar sight glasses side by side and rates them from poor to good based on six forms of detection.

Bullseye vs. Columnar Sight Glasses
Best For Columnar 3D Bullseye
Oil Level Confirmation Excellent Good Fair
Oil Color Detection Excellent Excellent Fair
Water Emulsion Detection Good Excellent Fair
Aerated Oil Detection Poor Excellent Fair
Foam Detection Poor Good Fair

What to Know Before Buying an Oil Sight Glass

It's critical to understand the environment and operating conditions your sight glasses must be able to withstand. Each situation may have multiple safety parameters to bear in mind, and you don't want your sight glass to be a contributing factor to these risks. Consider the following factors when choosing an oil sight glass:

  • Port availability: Take a look at where your ports are located on your tank or reservoir. Do you have a port at or above the oil level? If so, you can use a bullseye- or CMP-style sight glass. If your only access port is the drain port, you'll need a BS&W bowl or columnar sight glass.
  • Pressure: Understand the pressure requirements your sight glass will need to prevent high-pressure bursting.
  • Impact damage risk: Anything sticking out from a piece of equipment has a greater risk of impact damage. Review each sight glass's impact resistance, durability and wall thickness specifications.
  • Process fluid: Make sure you understand what materials the fluid in your reservoir or tank can corrode. Look for a sight glass with an "excellent" chemical resistance rating.
  • Normal service temperature: Knowing the maximum temperature at which your machine can operate and how long it can operate at that temperature is important, especially for high-pressure gas equipment. Spikes in pressure usually only happen when the temperature spikes.
  • Maintenance and handling: Some brands of sight glasses are easier to install, maintain and handle than others. With modern ingenuity, you should be able to find a sight glass that is simple to install and easy to clean. If the sight glass port is often used for other tasks like sampling or draining, consider a multifunctional sight glass like Luneta's CMP or Hub.
  • Oil level fluctuations: If the oil in your tank or reservoir fluctuates significantly, you should factor this into your decision. Bullseye sight glasses are good for gauging the correct oil level if the level stays within the crosshair marking on the glass. However, if the oil level reads high on a bullseye sight glass, there's no way of knowing just how high the level is inside. In this case, a columnar sight glass might prove useful once you've marked the idle and running oil levels on the glass. If you're seeing oil levels above (or below) these marks, you can get a snapshot of just how much oil might be in the tank.
  • Past problems: Consider whether there have been past problems you need to monitor. Have you previously had issues with wear debris contaminating your lubricant? If so, make sure you choose a sight glass with a magnetic plug to help attract that debris for testing.
  • Hardware requirements: Typically, columnar sight glasses require you to install more hardware because they're externally mounted. Unless there is a breather attached directly to the columnar sight glass, you'll need flexible tubing or a curved piece of pipe to connect it to the headspace. Other hardware requirements might involve creating a port for the sight glass installation if you don't already have one.

Oil Sight Glass FAQs

What are the components of an oil sight glass?
Oil sight glasses generally are made up of a transparent medium (acrylic, polyamide or Tritan copolyester), a unit body and other features. The "glass" or transparent part of the sight glass varies by shape, thickness and size depending on the type of sight glass. The body of the sight glass not only holds the "glass" in place, but it's also designed to manage stress passed onto the "glass." Other features on a sight glass might include an interior wiper to wipe away condensation or a sampling port to draw an oil sample.

What contaminants are captured by a magnetic plug in a sight glass?
Any metal filings and wear debris can be picked up by a magnetic plug. Even iron particles, which normally tend to sink rather than float, have been proven to be captured by magnetic plugs thanks to turbulence and the oil's viscosity suspending the particles.

How do I clean an oil sight glass?
Use warm water and soap. Cleaning products containing ammonia or alcohol can cause cracking, which leads to staining and weakening.

Are there any rules or regulations involving sight glasses?
Plants operating in the food and services industry cannot use glass products due to federal regulations on devices that could produce shards of glass upon failure.

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