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Measure level or weight?

The correct answer depends on the objective

December 01, 2012
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Do you ask yourself whether it’s best to measure level or weight? You should, in regards to monitoring your inventory of bulk solids in bins and silos. The correct answer depends upon what your real objectives are and what the specific primary question is that you need to answer. Do you need to know how much material you have in storage at any given time? Or do you need to know if you have room to fit another load — by truck or rail car — in storage as a part of refilling a silo so you do not run out? Answering these questions will determine the best measurement strategy.

Inventory is an asset, from an accounting perspective, and appears on a company’s balance sheet. The company balance sheet is a record of its assets, tangible and intangible, as well as its liabilities and the equity that ownership retains in the business. The sum of liabilities and ownership equity will equal the value of the assets. And, of course, the two sides of the equation must be in “balance.” Keeping track of the inventory of the business falls to that department within the company charged with oversight by accounting — which can vary by company. Inventory accounting needs to be precise. This is relatively easy for physical parts and pieces. However, the case is somewhat different for materials in liquid or powder/granular form held in bulk storage such as a tank or silo.

Usually a powder or granular material is purchased by weight and a liquid by volume. Sometimes the material may be purchased and accounted for by units, such as containers like bottles. But when these items are purchased in bulk they are generally bought by the gallon or liter for liquids and by the kilo or pound or ton for a bulk solid. From an accounting perspective the inventory is maintained in gallons or pounds, as an example, and therefore needs to be measured accordingly.

However, often it is not. With the inventory measurement of powder and bulk solid material in storage silos and bins you have two measuring strategies: measure the level of the material in the storage vessel, or measure its weight. Since the natural choice is to measure the inventory by the purchased units, i.e., weight, why then is level measurement even a choice?

Choosing level measurement

Measuring the weight of the material in a storage silo for inventory management purposes seems like a simple strategy to choose. However, “measuring” the weight of the material is not always practical or affordable. Many storage silos or bins when initially purchased and installed either do not have inventory measuring capability or the manufacturing process measures the weight of the material as it is drawn out of the storage vessel.

In the first situation, where the storage vessel is not purchased or built to accommodate weight measurement systems, the cost to add such a weighing system can be very high and unaffordable. Many weigh-scale systems cost tens of thousands of dollars to add to a silo “after-the-fact,” given the silo would have to be physically lifted off its existing mount and refitted with mounting and load cell systems. Even the somewhat easier project, to install bolt-on or pressed-in weighing systems, will have an installed cost of $10,000+ per silo with a measurement accuracy of within 1.0-3.0%.

Level measurement sensors offer an alternative quantitative measurement on a continuous or periodic basis that seems far more affordable and cost effective, typically $1,300+ for a relatively simple installation. But before choosing this measurement strategy you need to remember that these sensors typically measure the distance of empty space between the material surface and the sensor.

In addition, the distance measurement is at a single point on the material surface. This is acceptable if you are measuring liquid material. In fact with reasonably accurate vessel dimensions the liquid’s flat surface allows you to easily calculate the volume of liquid in the vessel and have a volume measure that has fairly good precision and accuracy. The “level” measured is usually inferred — not directly measured — based on the empty space distance measurement and a dimension value that represents a full vessel. Some technologies do actually measure the level or height of the material but most measure distance.

Bulkier issues

Powder and bulk solids present a challenge that needs to be overcome to achieve high accuracy of the calculated volume and weight based on a level measurement, along with the vessel dimensions and material density. The challenge follows from the irregular, non-flat or angled profile of the material surface shape. The primary reason the challenge exists is the flow properties of a solid material versus a liquid. Liquid molecules move at a very fast rate and the molecules in solids essentially do not move at all. This means that liquids flow well and solids do not. Because liquid molecules move at a high rate of speed they are not attracted to one another where solid particles are. This is why, where liquids have a flat surface profile, solids in containment generally have an irregular non-flat surface profile or shape. Some materials, due to the vessel design and flow properties, may appear to flow like a liquid but they do not. Proper vessel design, however, can aid in reducing the effect of this solids challenge to calculating the volume and weight of a bulk solid from a level measurement with a reasonable degree of accuracy.

The irregular and angled material surface profile, or angle of repose, of a powder or bulk solid presents a challenge for the accuracy of the calculated volume and weight because the level measurement sensor measures only at a single point on the surface of the material. Any calculation of volume and weight based on the single-point distance/level measurement will assume — you remember the old saying about assume — a flat surface because nothing is known about the profile. The flatter, or more regular, the material surface actually is, the more accurate and precise the calculated volume can be. This also improves the accuracy of the calculated weight — volume x material bulk density — but that calculation also is impacted by bulk density accuracy.

Final words

I know we haven’t gone into great depth here about weighing silo contents and the wide variety of level measurement technologies. I refer you to the book titled Solids Level Measurement and Detection Handbook, copyright © 2012 by Momentum Press®, LLC all rights reserved for further and complete information. However, it is worthwhile to excerpt a table from Chapter 5 of the referenced book, which is reprinted, with permission, on page 18, to illustrate the differences between the two choices, measuring weight or level.

Be preemptive and cautious about the need for and choice of an inventory measurement strategy for powder and bulk solids kept in a silo or bin. Do you need inventory measurement for the purposes of your business accounting? If yes, then consider this up front and choose weight measurement over level measurement. However, if you are more concerned about whether you have enough room to replenish the material in your silos, or you just need to control the filling cycle, then the level measurement sensor with the lowest cost of ownership, or even simple level control (point level) devices, would be the best choice.



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