Weighing
Weigh more accurately
Jeff Buxton of Precia-Molen UK offers a practical guide to avoid the
pitfalls associated with dynamic weighing
Dynamic weighing covers a wide range of applications, associated where products, goods or vehicles are weighed whilst in motion. Most dynamic weighing applications are considered to be either continuous or discontinuous machines. Typical associated types of weighing systems available to industry today include static types (weighbridges and vessel weighing); continuous types (belt weighing and belt weigh feeders) and discontinuous (check weighers and batching systems). What are the problems associated with the design of belt weighing and what is the best practice approach to achieve accurate, reliable and cost effective weighing solutions that are suitable for legal and non legal trade use, i.e. ‘any products or service sold by weight, the weighing equipment used is controlled by legal metrology legislation and such legal for trade equipment comes under the scrutiny of Weights and Measures Authorities around the world’? First of all let’s start with some definitions.
Static weighing
This is where a static load is placed on a weigh scale and the individual weight is recorded, for example a weighbridge.
Dynamic weighing
Dynamic weighing can be either continuous or discontinuous by a weight passing over a weigh scale within a given time and at a known speed for the gross weight to be calculated, for example a ship loading system.
Weight and mass
The mass of any object represents the quantity of the material it contains; it is independent of physical changes such as gravity, temperature and altitude. The standards of mass are referred to as weight. The international unit of mass is the kilogram, defined as the mass of the international prototype kilogram, which is held at the International Bureau of Weights and Measures. The term weight is ambiguous and is used to describe both mass and force, although the SI system clearly differentiates between mass and force, with no reference to the term weight. Scales that work by measuring the gravitational force on an object rather than comparing one to the mass with another are, therefore, only technically correct if calibrated and used in the same location.
Weighbridge
The weighbridge uses a static platform and is essentially formed from a single element: a ‘scale structure’ fitted with load cell technology to measure.
A weighbridge is the simplest method of weighing material passing over a weigh scale, used to measure the weight of a product, i.e., ‘the vehicle’ passing over a scale. The vehicle drives on, remains static, weight is recorded, the vehicle drives
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March 2010 • Solids & Bulk Handling 19
off making the use of the weighbridge a discontinuous form of weight measurement and is used extensively in all types of industries. Making the weighbridge suitable for legal trade use by using approved calibration practices and verification using certified test weights, thus achieving a guaranteed accuracy of 0.25% by weight.
Belt scales
Belt scales are used to measure the weight of material on a belt conveyor and are used extensively in the mining, quarrying and material handling industries.
A typical belt scale consists of one or more roller sections mounted onto one or more load cells through a structure or a flexure with a single load cell. The structure is mounted into the belt system to replace one or more of the existing rollers. Belt speed typically measured by means of a pulse generator receiving the signal from the rotating source. Multiplying belt loading by belt speed gives flow rate then by the integration within the control electronics; the total weight of material conveyed in a given time can be calculated. Flow Rate = m/s x s/t where m/s= weight of material per unit length of belt and s/t = belt displacement per time. The accuracy of any belt scale is dependent on a number of influencing properties, such as the material to be conveyed; the belt’s angle of incline, tension and speed; roller efficiency, the alignment and uniformity of the belt and uniformity of the belt loading.
A belt weigher comprises three elements: a scale structure with load cells to measure the product weight (W); a speed transmitter with a proximity switch to measure the belt speed (S) and a controller to integrate weight variation through time to calculate the flow rate (T). Throughput T=W x S. It is therefore
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