The design of the initial weighing systems was fully based on the work of Archimedes and Leonardo Da Vinci. Here they used positioning of calibrated counterweights on a mechanical lever so as to balance and determine the mass of unknown weights. Variation in this apparatus uses multiple levers, each of unlike length and balanced with a sole standard weight. Later on, calibrated springs replaced these weights, and development in fabrication and resources made these scales precise and reliable. However the introduction of electronic (usually strain gage-based) load cells was the first key design change in weighing technology. Today's processing plants, are preferring electronic load cells in most applications, though mechanical scales are used if the operation is manual and the working and safeguarding personnel prefer their ease.

Mechanical lever scales are used for number of applications like motor truck scales, hopper scales, tank scales, railroad track scales, platform scales and crane scales. The span shifts they experience owing to gradual temperature change can be corrected just by manual adjustment or by applying correction factors. Recompense for rapid or irregular temperature changes is more difficult, and often they cannot be corrected. Since the accuracy and consistency of good maintained and calibrated mechanical scales, it is used as standards for trade and is satisfactory to government authorities. Spring balances also are simple, and, if it is made up of high-grade alloys (having a modulus of elasticity unaltered by temperature variation), can be pretty accurate if properly maintained. They are reasonably priced and are best suitable for light loads.

The purpose of any weighing system is to acquire information on gross, net, or mass weight, or combination of these. To obtain the net weight of a vessel's contents it needs two measurements: total weight and the weight of unloaded container. Net weight can be obtained by subtracting one from other. Whereas bulk weighing involve weighing of large quantities. The total weight is often obtained by making incremental measurement and adding up these weights to attain the total. This allows a decrease in the size of the weighing system, thus reducing the cost and increasing accuracy. Belts also can be used for bulk weighing. But this is a less precise method, where the total bulk weight is attained by integrating the product of the belt speed and belt loading over a time phase. Batch weighing systems satisfy the necessities of industrial recipes by correctly dispensing a number of materials into general receiving vessel for reaction.

Performance of a weighing system is affected by many factors like temperature, vibration, environment, structural movement, and maintenance. Temperature compensation is generally provided for nearly all systems and its range should exceed the expected range of operating temperature variation. If the process vessel is hot or cold, tank-to-cell heat isolation pads could be provided. Temperature adjustment for zero and span are built into a good number of high quality strain gage load cell circuits. Pulsation influences can be minimized by separating the weighing system support from structures or solid foundations that support motors or other such vibrating tools or are affected by nearby vehicular traffic

Article Source: http://www.ArticleBlast.com