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Weighing over Level Sensors and Flowmeters

June 9, 2017
Eugen Schibli, Mettler Toledo
Eugen Schibli, Mettler Toledo

Accurate inventory control inside tanks and silos greatly depends on material characteristics and environmental conditions. Gravimetric measuring technology typically provides higher accuracy compared to volumetric or level-detection methods. This article explains the advantages and disadvantages of each technology.
    
The characteristics of the stored material dictate the design that should be used for tanks or silos. For tanks or silos that store solids, usually a combination of cylinder and funnel shape is chosen. The slope of the funnel is determined by the material property. Liquids are stored in nearly cylindrical tanks. A small funnel at the bottom of the tank makes it easier to ensure complete draining of the liquid. In contrast to level sensors, weighing provides accurate measurement for all shapes. Flowmeters are also independent from tank shape, but they can't control content inside the tank and worse they can't detect leaks.

Challenges Resulting from Material Characteristics
Tanks can host aggressive, flammable, out-gazing, cold or hot material. This requires use of a dedicated material or even a coating for the tank wall. The same coating and protection quality has to be applied for level sensors that are partly inside the tank. While weighing technology is always safely outside the tank.
    
Accurate inventory control is mostly impossible for volume or level measurement, if volume of stored material is temperature-dependent or if it has density variations. The same if solids build caves or bridges at the tank exit. Particular shapes or surfaces of pellets or powder might require tests to determine if level measuring provides accurate results. But weighing is the ideal technology because it is independent from material characterization.

Maintenance and Cleaning
Efficiency and quality of maintenance, including cleaning, depend partly on accessibility of the sensor. Level sensors must be removed from the opening into the tank. Flowmeters have to be dismounted from the pipes, depending on what has to be inspected or cleaned. Such activities are expensive if the material is hot, aggressive or under elevated pressure. The tank or pipe may have to be cooled down, emptied and rinsed. In extreme cases, that must be done with a neutral material. Open pipe ends have to be closed during inspection to ensure no material or gas comes out. After inspection or replacement, the openings must be carefully sealed again. Weighing is easily accessible at any time and doesn't require protection from material.

Accuracy and Calibration
Weighing technology enables inventory control for tanks of up to three tons with a reproducible readability according to International Organization of Legal Metrology (OIML) guidelines of 30,000 divisions or 0.0033% in ideal situations. Inventory inside tanks from three to several thousand tons can be determined with accuracy between 0.01 and 0.5%. These values are independent of flow behavior, temperature, density, viscosity or any other material characteristics.
    
A calibration of a tank can be executed with traceable weights and a certificate can be obtained by documenting the calibration values to prove performance and traceability to the standard for mass. The calibration and adjustment compensates for deviations due to rigid pipe connections or deflection of the tank or silo's support structure at full load.
    
A flowmeter or level sensor usually has to be removed for calibration at the factory or inside mobile calibration stands. Indirect calibration via a weighing device or the installation of a calibration flow meter at a designated location can be an alternative. But this method cannot detect measurement errors due to differences in density and temperature influences.

Conclusion
Weighing for inventory control leaves no restrictions with respect to the tank or silo shape or material characteristics. The maintenance and calibration over the entire lifecycle are easier or at maximum as difficult as with alternative technologies. The expenses related to constructive adjustments for the installation of weighing, as well as high-quality level sensors or flowmeters are similar in most cases, but weighing is typically more accurate.

Eugen Schibli has a degree in electronic engineering. For 10 years, he has worked for Mettler Toledo in business development for machine and instrument builders. Prior to that, he held various positions in sales, service and marketing for automation technology.

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