Radar Liquid Level Measurement Through a Sight Glass

radar level measurement installed on tank sight glass
Radar level control installed at tank sight glass
Courtesy VEGA
Level measurement, ubiquitous throughout processing operations, can be accomplished through the use of a number of different technologies. VEGA, a globally recognized innovator in level measurement, has authored a white paper outlining how radar level measurement instruments can be successfully employed when installed on tanks with sight glasses. The white paper is excerpted below, and you can access the full article and a wealth of application expertise by reaching out to an application specialist.

The balance of this article is excerpted from "Using radar sensors to measure liquid level through sight glasses", released by VEGA on 10/25/2016.

Vessels with sight glasses permit users to measure liquid level in a unique way: by mounting a radar sensor above the glass. Radar instruments emit microwaves that penetrate the glass, reach the product inside, and reflect through the glass back to the sensor. This eliminates two major expenses because users are spared from retrofitting a tank to accommodate a sensor and can continue running a process during installation. Functionally, nothing changes as users can simply move the sensor for a moment to look through the glass and see what’s happening inside a vessel.

Challenges to radar level measurement through sight glass

Any radar sensor can measure liquid level through a sight glass, but what happens after a signal penetrates glass varies depending on the sensor. Glasses are often welded, bolted or clamped directly onto a vessel wall or roof with a circular flange, while others are mounted on a nozzle. Radar sensors with a transmission frequency of 26 GHz release wide beams that contact the sides of the flange, the nozzle, and sometimes the roof of the vessel itself. This creates noise at the top of the output, especially on taller nozzles, forcing operators to leave empty space inside a tank to make a clear distinction between the signal received from the vessel and the signal received from the product.

Further complicating the use of 26 GHz sensors with sight glasses is the fact that most sight glasses are installed at a natural slope in the tank. Angled glasses narrow the path to the liquid, increasing the degree of difficulty in setting up a sensor so the beam is perpendicular to the product. Perpendicularity is important because it’s in direct relationship to the strength of the signal the sensor receives. However, to minimize the small signals that bounce from the glass to back the sensor, it’s recommended that users pair a 26 GHz radar sensor with a sight glass installed at a 45° angle. This forces users to choose between a strong signal from the product accompanied by reflections from the glass or a weak signal from the product and no reflections for the glass. Neither scenario is ideal.

Enhanced signal focusing makes all the difference

The problem of noise from fittings and narrow paths can be solved by installing a radar sensor that operates at a higher transmission frequency and produces a more focused signal. The VEGAPULS 64, for example, has a frequency of 80 GHz and can emit a beam angle of only 3°. 26 GHz sensors, on the other hand, emit beam angles of approximately 10°. A narrow beam angle misses the sides of the flange and the nozzle, silencing signal noise. That same focused beam can travel a tight path to the product without sacrificing signal strength. Finally, 80 GHz radar sensors don’t need sight glasses at extreme angles to minimize reflected signals, as a sight glass installed at a 5-10° angle will do.

Other benefits of external level instruments

All this is welcome news to processes where sight glasses already exist and is also noteworthy for those struggling with level measurement technology in traditional tanks. Users in the latter camp may find it more economical to install an external radar level sensor and a sight glass than a new internal instrument because removing a sensor from the interior of a vessel presents users with several benefits. In applications involving harsh, caustic liquids, there’s no risk of the product damaging the sensor with a quick splash or corroding it over time through buildup. This saves users in routine maintenance costs, and lack of exposure extends a sensor’s life. Users can mount a radar sensor above such tanks, and the emitted microwaves penetrate the glass and reliably measure the harsh liquid inside.

External access to a level measurement instrument is also useful for a quick repair or recalibration. With the sensor on the outside of the vessel, users can keep the plant’s process moving while they perform routine maintenance. If a problem arises with an instrument inside of a tank, that particular tank—or worse, an entire line—might have to be shut down, potentially leading to thousands of dollars in lost production. What company can afford that?


In conclusion, radar sensors of any transmission frequency can be mounted above sight glasses for accurate, non-contact level measurement. Separation from the product helps preserve sensors, and the instruments are easy to access when calibration and maintenance are necessary. When researching their options, users should consider 80 GHz sensors because they emit focused radar beams that take a narrow path to the liquid and fewer signals are reflected by flanges and mounting nozzle interiors. Given radar technology’s accuracy and reliability, and all that can go wrong if an internal level measurement fails, a radar sensor mounted above a sight glass offers nothing but advantages.