Lasers come to level measurement

Laser-level technology is expanding options in sensor applications, and CONTROL’s favorite sensor expert, David Spitzer, writes about the good and bad points of this new measurement technique.

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In general, pneumatically filled vessels pose more problems than mechanically filled vessels because pneumatically filled vessels tend to disperse particles in the vapor space where they can hinder a laser level measurement system. Van Rems cites a rule of thumb: “If you can see the level despite these influences, the laser level technology will work.”

Ray Charron, product sales specialist with Optech adds, “Particle size and color can have an influence on the ability of the laser energy to penetrate dust. Vapors with coarser particles are usually more easily penetrated than vapors with fine particles. Darker particles are typically less reflective, and are easier to penetrate than lighter particles that tend to reflect the laser energy.”

Also, the intensity of the reflected light and its color can affect the level measurement, so compensation for color is often necessary to accurately measure level. This is especially important for high-temperature applications, such as molten steel, where the color and appearance of the metal can change in a matter of minutes.

Laser energy typically doesn’t penetrate foam to measure the level of the liquid. This may be a disadvantage when liquid level is desired, but in some applications require measuring of the level of the foam. Charron adds that laser measurements generally benefit from diffusion on the material surface, and that surfaces that reflect too much laser energy can saturate the receiver.

Ivo Radanov, laser level product manager at K-Tek points out, “Laser light energy is scattered from the material surface in all directions. This means that laser level measurements are independent of the angle of the material encountered by the laser beam because part of the scattered reflection will return to the sensor.”

In some applications, this scattering will completely defeat the measurement method. Radanov adds, “This is particularly important in solids applications where the material surface usually isn’t perpendicular to the energy beam and provides an advantage over other non-contact technologies that use energy that can be reflected away from the sensor measuring the reflected energy.”

Laser-level transmitters can be sensitive to the reflectivity of the material. This concept is closely related to the material color because different color materials reflect different amounts and different frequencies of light. Radanov mentions that some laser level transmitters are designed to compensate for these differences. 

Evert Pol, preventive predictive maintenance and reliability engineer, Dow Chemical Company (Temeuzen, Netherlands) uses several laser-measurement systems for granular materials. “These instruments work well, after start-up problems, and are versatile in that they can measure from the top to the bottom of the silo.” Pol warns other end-users, “Be careful to consider a dirt protection tube that addresses the presence of dust that can hamper laser level measurements.”

Pat White, director of training at Weyerhaeuser (Simsboro, La.), gives laser-level measurement high marks, but not without some of the pain associated with applying new technology. One of White’s applications involved flakes in horizontal bins that are raked away from the fill nozzles, while the bottom belt moves the flakes towards the exit, which is located below the fill nozzles. The net effect is to create a horizontal level where the material tends to form a pile towards the front of the bin. The laser-level transmitter is mounted on the far side of the bin, and measures the horizontal level in the bin.

“When first installed, fines in the vapor space near the transmitter caused the level measurement to indicate false levels at times,” White says. White solved that problem by installing a hood in the bin over the level transmitter to keep the fines from getting in front of the transmitter. Though this method was successful, the hood could become bent if the bin was too full. In the end, the manufacturer developed software that could ignore the fines, and so measure the level accurately and reliably without the hood.

Laser-level measurements have become viable recently enough that there are relatively few suppliers. The prospects for this technology look promising, especially for applications where other level measurement technologies have difficulty. Time will tell whether laser level technology will become viable for more applications, and give end-users another tool in the battle for productivity. 


  About the Author
David SpitzerDavid W. Spitzer is a principal in Spitzer and Boyes LLC, which offers consulting services for the process industries. He can be reached at 845/623-1830 or at spitzerandboyes.com.
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