More than 70% of all products containing ingredients were bulk solids at some point, according to Bulk-Online.com.
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 loss of revenue of 1.75%.
Industry 4.0 and blockchain practices call for accurate data. A discrepancy with inventory measurements undermines inventory control, makes ordering decisions ineffective, and may result in 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?” We'll tackle this 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 or silo. They include SmartBobs®, guided wave radar, open-air radar, and ultrasonic level sensors, chosen based on the type of material being measured.
When installed properly, these sensors deliver 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 "Measurement Accuracy" Means
A measurement is defined as the distance between a sensor and the material surface in a tank. This distance is also referred to as “headroom.”
For example, 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. Additionally, volume and mass of material in a bin would not fall within the accuracy rating of headroom distance.
How to Get Volume Measurements
Volume is the amount of three-dimensional space that the material occupies. Headroom may help estimate volume, depending on internal vessel dimensions. 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.
What About Measuring Mass?
Not all bulk materials are the same. Your cement may have a different bulk density from the competitor's cement down the road. Bulk materials all have unique bulk densities.
That brings us to mass. Mass = Density x 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 the vessel. While charts are great to boost the accuracy of 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 lb./ft.³ to estimate the 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' Behaviors May Reduce Pneumatic Conveying Problems
Measurement can get trickier as 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 flow detection sensors provide a solution, especially since pneumatic systems include line chargers (or feeders), pipeline layouts, elbows, and bends that can prevent the proper movement of material through a system.
Next Steps to Accuracy
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 It's 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 seek more comprehensive measurements.
The 3DLevelScanner measures multiple points. This sensor doesn’t average point heights, but rather relies on a complex algorithm to provide a more accurate volume of material in a bin. It 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 symmetrically fill or discharge.
Like all sensors, accurate numbers depend on good vessel dimensions and bulk density, but improved accuracy comes from 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%.
