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Understanding Accuracy

Understanding Accuracy
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Understanding Accuracy

More than 70 percent of all products containing ingredients have bulk solids at some point, according to The term, “Inventory Record Accuracy,” was coined to describe the discrepancy between recorded quantity and actual inventory quantity physically stored. In the retail world, inventory record accuracy can mean an average of 1.75% loss of revenue.

Industry 4.0 and blockchain practices call for accurate data and a discrepancy with inventory measurements undermines inventory control, make ordering decisions ineffective, and may result in poor delivery and higher inventory costs.

When it comes to monitoring inventory in bins, tanks, and silos, one of the first questions often asked is “How accurate is it?” BinMaster will tackle the question in regard to level monitoring systems.

Accuracy of a Single Point Inventory Measuring System
A single-point sensor system measures one point in a bin/silo. Several types of single-point systems are available for different environments and materials. They include SmartBobs, guided wave radar, open-air radar, and ultrasonic level sensors. When installed properly, these sensors perform well delivering “measurement accuracy” at one point within the storage vessel. Two or more of these sensors installed in a single vessel can create a more accurate average.

What does ‘measurement accuracy’ mean? Where do I find it?
Measurement accuracy is published in product literature at One example of a measurement is the distance between a sensor and a material in a tank. This distance is also referred to as, “headroom.” BinMaster says that headroom measured by a SmartBob is highly accurate within ± 0.25% of the total distance measured.

However, every measurement must be considered on its own. Volume and mass of material in a bin, for example, would not fall within the accuracy rating of headroom distance.

So where do I get a measurement of volume?
Volume is the amount of three-dimensional space the material takes up. Headroom (distance between the sensor to the material) may help estimate volume. The calculation depends on internal vessel dimensions and headroom. Inaccurate vessel dimensions will increase overall error in a volume calculation. Volume is tricky because material flow, buildup, or bridging can affect calculations. Sensor placement and the location of filling and discharge points also have an impact on overall volume measurement accuracy.

How about measuring mass?
Not all bulk materials are the same. You may have cement that has a different bulk density from the competitor cement miles down the road. Bulk materials all have unique bulk densities. BinMaster publishes charts to determine density, and they are based on an average. Expect variations.

That brings us to mass. Mass=density times volume.

Density changes according to particle shape, size, and moisture. Material density even varies in a bin where compaction tends to happen towards the bottom of a silo/bin. While charts are great, to boost accuracy on density, a facility might consider measuring bulk material before and after a “known-weight” load is put into a bin. Then, adjust density according to pounds/foot3 to estimate weight.

Converting volume to mass will always include inherent inaccuracies due to variations in bulk density, regardless of whether you use a single- or multiple-point measuring device. Remember, volume accuracy also impacts mass accuracy.

Monitoring bulk solids' behavior may reduce problems with the pneumatic conveying of bulk materials
Measurement can get trickier as the product moves through pneumatic lines. In most cases, pneumatic measurement is a second priority as compared to continuous operation and flow. These systems can experience overflow, wear, plugged lines and chutes, and more costly problems. BinMaster sensors and flow detection products can key success especially since pneumatic systems include line chargers (aka feeders), pipeline layouts, elbows, and bends that can prevent the proper movement of material through a system.

Okay teach, homework’s done. Now what?
With accurate vessel geometry, strategic placement of the sensor, and a good average bulk density, the accuracy of the mass may be around 8 to 15%.

A vessel that’s center fill, center discharge with material that flows freely and symmetrically, tends to give the best results when using a single-point measuring device

What if that’s just not exact enough?
Single-point sensors measure a single distance. Bulk materials usually sit in hills and valleys within a vessel. Did it measure a hill or valley? While some plants increase accuracy using multiple single-point sensors, others are looking for a more comprehensive measurement.

A 3DLevelscanner measures multiple points. The 3DLevelScanner doesn’t average point heights, rather it relies on a complex algorithm to provide a more accurate volume of material in a bin. This scanner provides an accurate 3D profile of the top surface within a storage vessel. This is beneficial for uneven material surfaces, multiple fill and discharge points, or materials such as powders that do not fill/discharge symmetrically.

Like all sensors, accurate numbers depend on good vessel dimensions and bulk density, but improved accuracy comes from the 3D volume numbers.

Given correct vessel geometry and proper sensor placement, you can expect volume accuracy of 3% to 5% for 3DLevelScanners. When combined with a good average bulk density, the accuracy of the mass may be around 5-10%

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