January 1, 2010
by Nicole Armbruster and Fernando Tobler
Electronic scales have become an important part of today’s grain processing industry. They are an integral component in a wide variety of process operations ranging from raw materials reception to packing of the finished products. Their reading accuracy has a direct impact on the producer’s profit, which is reason enough to take a closer look at the different aspects of scale maintenance.
Regular scale maintenance is arguably the most important maintenance activity in a grain processing operation. Even the best weighing system — one minimizing trouble proneness while at the same time offering top measuring accuracy — will only produce consistently reliable readings if it undergoes regular professional maintenance.
What makes matters more difficult is that — apart from the wear and tear caused by continuous regular service — other influencing factors also contribute to the wear of scales. They include dust, moisture and humidity, incorrectly set valves, worn-down bearings and bushes, temperature fluctuations and vibrations. Thus, careful checking, cleaning and adjustment of the sensitive points are essential.
Regular monitoring and calibration are also necessary to fulfill the ISO certification requirements according to ISO 9001-9004. Today, virtually no company can afford to disregard this certification.
What must be addressed is how such scale maintenance is to be organized and what points must be taken into account.
First of all, it is advisable to check the static mechanical components of the scale system. In fully electronic scales, these include the load cell suspensions which prevent transverse forces from acting upon the load cells. In mechanical or hybrid scales, it is essential to minimize the friction of knife edges, blocks and hinges of the lever system.
In a second step, the flexible transitions between the scale hopper and the fixed inlet and outlet hoppers must be checked for their proper performance. The resilience of the rubber sleeves will deteriorate over time. In addition, oil-containing substances exuded by the product weighed — especially plant seeds — will chemically attack the rubber sleeves and cause them to become brittle.
Then, a closer inspection must be carried out of the mechanical inlet and outlet elements. If they are worn, they must be replaced. Slide-gate-type feeders, in particular, suffer from the elevated abrasiveness of granular materials.
Another important point is to check the bearings and hinges of all moving elements. If they run smoothly and quietly, they are okay.
An additional item on the scale maintenance checklist must always be an inspection of the pneumatic components. It is important to ensure that they are fully serviceable and do not have any leaks. It is also necessary to visually inspect the operating controls for advanced wear or even damage.
What is also important in scale maintenance is to verify the weight readings by means of calibrated weights. This is to validate the total weight and the corner load. If necessary, a final correction of the alignment must be performed.
In professional scale maintenance services, defective elements are replaced on
the spot if wear parts or small spare parts are involved which the service technician has taken along. A separate date is fixed in case of major repairs, with an order being placed for the required spare parts. If possible, the scale is provisionally repaired to allow operation to continue until the parts have arrived.
In addition to the regular scale inspections described above, official calibration is another aspect which might cause the owner high costs if it is neglected. The term “official calibration” refers to the legally required testing of a measurement instrument for compliance with calibration laws, in particular with regard to calibration error tolerances.
The best-known international institution in charge of standardizing metrology laws (official calibration regulations of individual countries) is the “Organisation Internationale de Métrologie Légale” (OIML), which is headquartered in Paris, France. In most countries, such a validation is required once annually and must be carried out by officially approved testing agencies. In order to make scales fit for official calibration, every scale design must pass an initial approval procedure. The manufacturer is responsible for having the scale design and characteristics tested. Official calibration is required for scales at product receiving and shipping points of a plant. All other scales require regular unofficial calibration.
Unlike official calibration, unofficial calibration involves testing of scales which do not require official calibration by law. The term also refers to scale measurement tests which do not comply with the legal requirements. Unofficial in-plant calibrations are based on the standards of a particular plant or organization. Unofficial reference calibrations are carried out using national standards. The advantage of such reference measurements is that they allow measurement results to be compared, since national guidelines are typically also internationally valid as standards.
Buhler, as a leading supplier of scales to the grain processing industry, also offers customers officially approved scales. Furthermore, the company employs officially certified service technicians for approving official calibrations that comply with the legal requirements. The appropriate documents are issued to customers for official as well as unofficial calibrations. They certify the proper performance of the tests for both their own customers and for the authorities. All the jobs carried out during a scale maintenance assignment are documented in an easy-to-understand manner. This information allows quick identification of the condition of the scales tested and ensures a high level of traceability. Also, complete documentation provides the basis for subsequent audits according to ISO 9000.
Meticulous and regular scale maintenance, including a comprehensive performance check as well as official or unofficial calibration, has an enormous impact on the product yield and quality. An increasing number of grain processors are recognizing the importance of this activity and thus ensuring the long-term success of their business.
Fernando Tobler, service engineer, can be reached at firstname.lastname@example.org.