Reservoir and Inline Sight Glass for
Lubricant Management

by David Doyle, CLS, OMA I, OMA II
General Manager, Tribology

Sight glasses or sight gauges will vary in design and application. They are a useful and convenient inspection tool for gearboxes and circulating system reservoirs for visually monitoring of the fluid level, fluid clarity, color, as well as sediment and water contamination levels. Flow sight glasses can be installed inline to allow visual inspection of flow and clarity of the circulating oil.

Column sight glasses or sight gauges are designed to monitor fluid level without physically accessing the reservoir, thus reducing the chance of contamination and increasing the reliability of monitoring practices. The appropriate fluid level is often marked on the sight glass for easy monitoring. A sight glasses can also provide qualitative information in case the lubricant is degrading or contaminated if clarity or a severe color change occurs.

Column sight glasses need to be able to breathe, either though the reservoir or through the sight glass itself. There should be adequate headspace in the column sight glass to reflect the head space existing in the reservoir. Column sight glasses should not have flow restrictions coming from reservoirs that will create a discrepancy in the sight glass fluid level relative to the true fluid level in the reservoir.

Oil sight glasses or bull’s eye sight glasses provided a qualitative inspection of the fluid and reservoir condition. Oil sight glasses or bull’s eye sight glasses installed at the bottom of a reservoir provides easy inspection for bottom contaminants such as water and sediment. Some models allow you to easily remove water by draining out of the system through the sight glass. They also provide a way to monitor fluid foaming in reservoirs when a bull’s eye sight glass is situated at the top of the fluid level near the head space.

Most quality sight glasses on the market are made of stain-resistant acrylic. Some material will stain after a while, and depending on maintenance practices, the true color of the lubricant may not visually show through the sight glass after it has been in service for a time. Sight glasses should be inspected periodically for staining, surface cracks, or oil seepage. They can be cleaned with soap and warm water using a proper brush that will not scratch or deposit material.

The use of a properly installed sight glass in a reservoir or circulating system can provide a convenient and valuable compliment to an oil analysis program and asset reliability practices.