Contamination Control Lubrication Oil Analysis

All Sight-Level Gauges Aren’t Created Equal

Ken Bannister | March 13, 2017

A customized dual-point-entry gearbox column sight-level gauge with combined fill line and external Hi-Lo level limit markers.

Your gauge of choice can have a significant impact on your PM efforts.

Webster’s dictionary defines a gauge as, “a [telltale] device attached to a container to show the height of its contents.” Google expands on that definition by describing a gauge as, “an instrument or device for measuring the magnitude, amount, or contents of something, typically with a visual display of such information.”

Listen above to the latest in a series of monthly lubrication-related podcasts with Ken Bannister. The March podcast focuses on sight-level gauges and applications.

Today’s industrial plants use countless oil reservoirs and gearboxes in their lubrication and hydraulic systems. Typically attached to these components are sight-level gauges, in various types and configurations, to help personnel monitor lubricant levels and, depending on the instrument, lubricant condition.

Sight-level gauges are inexpensive and passive human/machine interface (HMI) visual indicators that are inadequately exploited in most preventive-maintenance (PM) programs. The most efficient type of sight-level gauge is one that does not require the maintainer to physically open a gearbox to atmosphere, and possible contamination, to check the lubricant.

The majority of passive sight-level gauges in today’s marketplace fall into one of three design categories: planar sight glass, column sight gauge, and three-dimensional sight glass (the most recently introduced design). All can be upgraded.

Planar sight glass

The planar sight glass is a two-dimensional glass porthole screwed or press-fitted into the reservoir wall with its centerline positioned approximately at the recommended reservoir-fill level.


• Very inexpensive
• Available in a variety of materials
• Available in a variety of diameters.


• Minimalistic

If the device is used in a re-circulating lubrication or hydraulic system, the maintainer needs to know if the center level line is correct for when the machine is in operation or when it is at idle and all lubricant has returned to reservoir. This should be clearly noted on the reservoir.

Because planar sight glasses are installed in the reservoir inner wall and, thus, in direct contact with the lubricant, the glass can be susceptible to staining/varnishing if the lubricant is not regularly changed. This can cause a false-level indication or make it difficult to see the lubricant level. 

Because there is no backlighting, it’s difficult to discern the oil’s color and/or determine its condition.


• A larger-diameter sight glass is preferable.
An upper and lower fill-level line can be printed on the glass if the porthole is large enough.

A single-entry vented column sight-level gauge.

A single-entry vented column sight-level gauge.

Column sight-level gauge

Column site-level gauges, arguably the most commonly used of the three categories, are available in two-point- or single-point-entry versions. Dual-point-entry models are solidly affixed to the reservoir, allowing very long column tubes. As the top entry is open and vented to the reservoir, there is no columnar pressure buildup as in a single-point-entry unit.

Single-point-entry gauges are usually much shorter than dual-point designs and are susceptible to movement and damage. Because they can allow hydraulic-pressure buildup in the capped column tube that might lead to false level readings, they incorporate a 2- to 5-micron vent cap that equalizes tube pressure.

The tube itself can be customized with calibration marks depicting fill levels and fitted with fixed or movable upper- and lower-limit level indicators. In addition, the device’s lower entry point, which is typically fitted into a reservoir drain port, can be equipped with an external drain port that doubles as an oil-analysis sample port.


• Inexpensive
• Available in a variety of materials
• Can be fitted anywhere on the reservoir
• Can accommodate very large reservoirs
• Delivers an almost 360-deg. sightline
• Can be calibrated to show reservoir volume
• Can be calibrated to show running level and idle level
• Best for showing oil condition/color
• Can show emulsified water
• Can show aerated oil
• Can double as a reservoir drain
Certain single-entry designs can double as a filler port
Can be set up with a live oil-analysis sampling port
Less susceptible to glass staining.


• Requires fittings and gaskets that can leak if not installed correctly
Difficult to see oil foaming
Single-point-entry units easily damaged.
Single-point-entry unit will require breather maintenance.


• Multiple level and zone markers
Calibrated columns
Drain ports
Sampling ports
Filler port
Color-coded column backdrops.

3-D sight glass

The 3-D sight glass is positioned in the reservoir in the same manner as a planar sight glass. The major difference is that the 3-D design protrudes from the reservoir wall (like a glass jam jar) and allows the maintainer to see exactly what the oil is doing inside the reservoir.


• This device is a true physical extension of the reservoir
Uninterrupted sightline to the lube level
Can be set up with a live oil-analysis sampling port
Can show oil condition
Best design for showing oil foaming
Best design for showing oil aeration
Best design for showing emulsified water.


• Does not show running or idling level very well.


• Built-in probe-style oil-analysis sampling port.

Regardless of type, passive sight-level gauges are important first-line PM tools that let us actually see what’s going on with oil in the reservoir as a machine runs.  

Contributing editor Ken Bannister is co-author, with Heinz Bloch, of the recently released book Practical Lubrication for Industrial Facilities, 3rd Edition (The Fairmont Press, Lilburn, GA). He is managing partner and principal consultant for Engtech Industries Inc., Innerkip, Ontario. Contact him directly at, or telephone 519-469-9173.

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Ken Bannister

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