Social media discussion: Selecting a level sensor for dry material

A user of the Automation & Control Engineering group on LinkedIn wants some help choosing a level sensor.

08/17/2010


Last week Idris Olawanle posed a question to the Automation & Control Engineering group on LinkedIn, asking, “What sort of sensor is best to sense the top level of a silo filled with sawdust like material?

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The material is known as expanded polystyrene pellets, the sort of material used to make bean-bag chairs. The silo is a vertical cylinder about 18 ft. tall, fed from a 6-in. diameter duct. In filling the silo, the pellets form a cone shape, intermittently blocking the line of sight of infrared photo sensors. What is the your opinion concerning a proximity sensor with a nominal distance of about 8 mm installed horizontally on the side of the silo?”

Olawanle didn’t say if he is concerned with a continuous level reading, or if the situation using point level sensors is adequate to provide the necessary information, so readers offer both solutions. And since there are no stated budget limitations, the whole cost spectrum is included.

Marshall Tsien offered an idea based on his experience: “I installed a silo system with two tanks about 30-40 ft. tall, with flour making Chinese pasta almost 20 years ago. Each silo had 3 level sensors, high, medium, and low, with a slow speed small motor turning a paddle inside the silo. When flour hits each level, the paddle is obstructed and the motor stalls. Something along these lines should work for your application with sawdust.”

Thinking more of a continuous sensing solution, Fabio Giannerini suggests, “I think an ultrasonic sensor is right for that purpose. I installed some of them for revealing the top level of a tank of liquid solution, with sawdust it should be the same.”

Sanoj James agrees with Giannerini: “Ultrasonic would be a good option unless you are looking for a fast response and measurement repeatability of <100 microns, in which case you should look at photo sensors.”

Sticking with the point level idea, Howard Thomas McClure suggests, “Bindicator makes a device that works on the principle described by Marshall Tsien. The paddle stops rotating when in contact with the material in the silo. The motor mount torques to activate a limit switch. Simple, cost effective level measurement and easy to maintain, they worked on similar systems for resin pellets and flour silos. A multi-bolt, sealed, flange mount provides easy access to the entire sensor and paddle assembly.”

Taking a more comprehensive approach, Sean Conboy considers a range of options: “There are three different technologies. 1) Capacitive - reliable, non contact, can be mounted on sight glass. Perfect for materials with a dielectric higher than air such as sawdust, plastic pellets, and cardboard. 2) Photoelectric - this is less reliable in dusty environments but choosing a sensor with enough power will get the job done. You could use diffuse type with foreground suppression if it is dusty. 3) Ultrasonic - might work, however they work with sound and a pile of sawdust will reflect and absorb the sound, and not provide enough energy back to the transducer. I would recommend the CFAK30 from Baumer.”

Anton Schädler says that you have to be concerned with the operating conditions to make the best choice. “Capacitive switches will be reliable if the dielectric stays high, but will require frequent adjustment if the saw dust is dry. Ultrasonic is not a reliable solution due to the high dust content (usually pneumatic filling), irregular surface and weak reflection from the dust. As an overflow switch I recommend a vibrating probe switch or an electromechanical paddle switch with cable extension. Both can be mounted through a flange or manhole above the silo and give a relay dry-contact output. Have a look at Hycontrol. They have a variety of products that might work for you.

Paul Sinclair hasn’t had good experiences with some of the solutions offered so far. “I'm very surprised no one has suggested radar level indicators. I've been using them very successfully for several years in some pretty nasty, dusty conditions without any failures or significant measurement errors. My favorite manufacturer is Ohmart Vega followed by Siemens. I have used the Vega 60 series extensively and really haven't found a substance they can't sense accurately.”

Going off in a completely different direction, Michael Thompson suggests rethinking the application. He asks, “Is the density of the pellets and packing characteristics amenable to a weigh-scale solution? For example, a load cell at the bottom of the silo could measure the weight of the product that rests upon it. A calibration could be performed so that the weight could be a reasonable estimator of column height. Any nonlinearities caused by packing could be addressed by a mapping function. An important aspect of this solution is that it eliminates the concerns of blocked signal paths, conical topping, sensitivity of the sensor to material properties, etc. However, one important point about this sort of solution is that it depends on the density of the column of material which in turn depends on the material's density and packing characteristics. Another concern is whether the material is hygroscopic. If it absorbs moisture in any substantial quantity, then a weight-based solution may be impractical.”

This discussion isn’t intended to be the final word on any particular application, but it can help you think through the process of how you should approach your next problem. Seeing how users combine product-related performance trade-offs with personal experiences offers pointers on the way to make appropriate choices. Discussion groups on social media sites like this can help add different opinions and suggest new solutions to your questions.

Peter Welander, pwelander(at)cfemedia.com
Control Engineering

Visit the Control Engineering Process Control Channel.



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