The magnetic level indicator is currently widely used in process industries as an effective visual indicator.

A magnetic level indicator is often used in applications where a sight glass (or glass gauge) is both an inadequate basis for process variables and poor performance for plant requirements. These may include increased security for employees; environmental risk situations including media leakage or fugitive emissions; need for reduced maintenance; or need for high visibility from a distance. Typical shortcomings of glass displays include:

High pressures, extreme temperatures, deteriorated seals/rings/gaskets, and toxic or corrosive materials can cause the risk of fugitive emissions of hazardous substances.

The glass in a sight glass can quickly become discolored, thereby decreasing the level’s visibility, or it can acquire microfractures, which can become a personal safety issue if left undetected.

Liquid/liquid interfaces can be very difficult to read on a display, especially if the liquids are similar in color. Clear liquids can also be difficult to see on a viewfinder.

Liquids that tend to coat or accumulate on surfaces can make visibility difficult, forming an opaque film on the glass.

To cover a large measuring range, display sets must typically be staggered using multiple sections.


The main reasons for selecting a magnetic level indicator over a sight glass are:

  • Greater safety due to the absence of fragile glass and a substantially reduced number of potential leak points.
  • Greatly increased visibility.
  • Reduced maintenance.
  • Easier initial installation and addition of transmitters and switches without interrupting the process.
  • Dual technology redundancy, with the addition of transmitters or switches, for greater security.
  • Lower long-term cost of ownership and legitimate return on investment benefits.
  • Single chamber measurement over 6 m (20 ft) without baffling cameras.


The obvious safety benefit of MLI over a display is the reduced chance of breakage. If the process fluid is under extreme pressure or temperature, the likelihood of glass breakage increases. The pressure limit of an MLI is made of robust metal, often the same as the vessel piping, making MLIs as safe as the surrounding piping system itself. Indicators, transmitters and switches are all externally mounted and therefore are not affected by toxicity, corrosivity or other characteristics of the process fluid.

Another safety benefit is that chemical compatibility with the fluid in an MLI is restricted to just three components, the metal chamber, gaskets and float. With glass sight glasses, the process fluid may have chemical compatibility issues with any of the wetted materials—glass, metal, or sealants.


MLIs are virtually maintenance-free once installed because the indicator never touches the process fluid. With sight glasses, gauges should be checked periodically for leaks and cleaned regularly. Scaling, etching and build-up on the process fluid glass can cause the display to become unreadable.


Fluid level visibility over long distances is another important reason to select an MLI over a sight gauge. The sight gauge level indicators should be seen from maximum distances of about 3 meters (10 feet). However, the bright contrasting colors of flags or a fluorescent bus on an MLI allow visible level indication at distances of up to 30 meters (100 feet) or more. Newer, more advanced visual indicators, such as those from Orion Instruments®, have viewing ranges of up to 60 m (200 ft).