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May 31, 2016

Sample testing plays a big part in quality control of substrates and soil mix manufacturing, and improved process control techniques represent the newest development in this industry. These new developments are in the use of sophisticated devices to measure volume, moisture, mass and density. When incorporated into the most up-to-date management systems, these improved techniques ensure high-quality, uniform end products. 

Whether you’re contemplating improving an existing quality control system or purchasing a new one, it’s helpful to better understand what these devices do.

Management System

The main component of quality control is its management system. Without a management system most of the devices used along a processing line are not optimally efficient and cannot be monitored properly. The standard management system is a graphic touch screen, while high-level management is provided by a Supervisory Control and Data Acquisition system (SCADA) on a desktop computer. A good SCADA will:

- Provide a comprehensive and accurate picture of the operation

- Regroup production data from different equipment and control devices

- Display production data quickly with simple graphs

- Acquire and facilitate analysis of production data

- Monitor and store faults and alarms for analysis 

The most technologically advanced SCADA versions allow you to calculate overall equipment effectiveness (OEE), using 3D images to provide comprehensive and accurate real-time assessment of the quality control process, and color indicators that show the status of individual pieces of equipment along the line.

Gravimetric Dosing

The control device that has been used the longest is probably the gravimetric (i.e., weight- measuring) dosing unit for fertilizers and additives. Common in other industries for several decades, it has become widely used in the production of substrates and soil mixes during the past 10 to 15 years. This device comes in different configurations: vibratory, screw and belt type, with the belt-type unit best-suited for this industry. The most technologically advanced units have the capacity to be fed directly from a bulk bag during processing, like the widely used volumetric dosing unit. 

However, a gravimetric dosing unit has the advantage of being more precise than its volumetric counterpart. This precision comes from the former’s closed-loop control, which constantly adjusts output to match the desired quantity; when equipped with an adjustable gate, it has the widest possible flow range. Consequently, the amount of material it delivers is more precise than a volumetric dosing unit delivers, and improves stock management of fertilizers and additives.

Volume Measurement

Another control device is the belt scale, used in the aggregate industry to measure the amount of bulk material passing by on conveyors, usually expressed in tons per hour. Because material used in substrate and soil mixes is of lower density than aggregate, belt scales are not very accurate for substrate/soil mix production. That’s because belt tension and belt tracking variation have little influence in measuring dense material like aggregate, but introduce a large margin of error into the measurement of lightweight material. If a belt scale is to be used, the entire conveyor should be mounted on a scale to eliminate unwanted forces in order to obtain accurate measurements. 

The most accurate way to measure the flow of lightweight material is by using laser volume sensors. Instead of measuring weight, these sensors measure the profile of material as it passes by on a conveyor belt. Since the substrate/soil mix industry is accustomed to measuring in volume, laser volume sensors are easily integrated and understood by equipment operators. Laser volume sensing also can be used to control the set point of a gravimetric dosing unit by measuring the flow rate of incoming bulk material. Flow measurement is used in many applications, from controlling feeding hoppers, to ensuring the even flow of raw material, to measuring the amount of produced material for inventory.

Moisture Control

Moisture control is another area that’s advancing through the use of specialized components. The traditional ball valve and drilled pipe setups are being replaced by sophisticated servo valves and flow meters coupled to sensors, which achieve an accurate and repeatable level of moisture in material being processed.

Density Measurement

Last but not least, density is an important characteristic of substrates and mixes, especially in Europe. It can refer to moisture level and be used to establish volume of material sold in bulk. There are several ways to determine it. One is by pairing a laser volume sensor with a conveyor on a weight system, as described above under “Volume Measurement.”

When a laser volume sensor is properly positioned as an in-line device, it has the advantage of being able to handle multiple tasks. For example, it can simultaneously control moisture level and input of material while assessing density. The second type of density measurement is via a sampling unit, which is composed of a container of known weight on load cells, plus a sampling device (usually a pick-up conveyor or moving plate). Once material that needs its density determined has been added to this container, the sampling unit indicates the density of the added material. Unlike the multitasking laser volume sensor, a sampling unit can be used only for density measurement. Another method of determining density uses a radioactive sensor. When material passing in front of this sensor is uniform in quantity and composition, the sensor is able to indicate the material’s density. However, since substrates and mixes typically include different base materials (peat, compost, soil, etc.), density readings from a radioactive sensor are somewhat variable. In addition, radioactive sensors require special governmental approval and are expensive to own and dispose of, all of which makes them less than ideal for this industry.

Author : Julien Dubé - Programming Automation Engineer


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