Sight Glass Oil Analysis

Jim Fitch, Noria Corporation
Tags: oil analysis

Several years ago, when my son was a Cub Scout, his den went on a night hike at our local wildlife sanctuary. He and I went together on this outing. We were led by a professional nature guide who worked full-time at the sanctuary. I wasn’t expecting much; after all, what can one see in the forest when it’s pitch dark?

Luneta Oil Sight Glass

It wasn’t long into the hike before I realized that I was nothing more than a babe in the woods. While I was concentrating on my footing along the trail, our guide was pointing out everything from faint insect smells to the luminescent trailers of caterpillars on tree branches. To her, the forest was an orchestra of sights, sounds and smells. Using only her senses, she named the species of countless birds, mammals and reptiles we encountered along the way. She was in her element … a true professional in tune with her environment. I was impressed.

How does this experience relate to oil? Let me ask the obvious question: Using only your senses, how in tune are you with the properties of your lubricants? For instance, can you pinpoint certain contaminants present in a hydraulic fluid? Can you recognize a wrong grease by its color, texture or consistency? Are you alert to the modulate sounds made by operating machinery with distressed lubricants? In past issues of Practicing Oil Analysis, we have tackled many subjects relating to sensory inspections of lubricants. We’ll continue to publish these articles because these inspections are vital to machine reliability.

You might be thinking this is inconsequential to you because your company does routine laboratory oil analysis… right? Before you turn the page, let me ask you how many times a day you sample the oil on critical equipment? That’s right, how many times per day? Did you know that each time you visually inspect the oil in a sight glass, you are both sampling and analyzing at the same time? If your operators do this once per shift, they are sampling and analyzing three times per day. The practice is called sight glass oil analysis.

However, it works only if you are a lubrication nature guide. Fortunately, most of the skills are fairly easy to acquire. Actually, the 80-20 rule probably applies; in other words, you need only about 20 percent of the knowledge to yield 80 percent of the available benefit. Many training programs in oil analysis, including those seminars offered by Noria Corporation, include sensory inspection topics. Field engineers working for lubricant suppliers are often a treasure trove of information about in-service lubricant properties.

Some things to search out relating to sight glass oil analysis include:

  1. The significance of a bathtub ring, foam, varnish or turbidity in the sight glass level gauge
  2. The cause of sediment, sludge line, phase line and fouling in BS&W bowl
  3. How to use a laser pointer to identify suspended solids and oil/water emulsions
  4. Easy ways to assess slight oil color changes and what the changes could mean

After all, if your compressor sight glass oil analysis reveals a clear, bright, lemon-colored fluid, you know that while there is a long list of bad things that could be happening to your oil, at least for now, they are not. Sight glass oil analysis is by no means a replacement for routine laboratory analysis, but it is nonetheless still a powerful tool.


About the Author

Jim Fitch, a founder and CEO of Noria Corporation, has a wealth of experience in lubrication, oil analysis, and machinery failure investigations. He has advised hundreds of companies on developing their lubrication and oil analysis programs. Contact Jim at

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