The weigh forward
October 1st 2003

Rod Morgan from Applied Weighing International looks at the critical elements of any industrial weighing system accuracy, reliability and durability The weighing industry has seen a number of significant changes over the past decade. The developments in information technology, the importance of manufacturing traceability, the legislative considerations of health and safety and the adoption of much stricter hygiene standards have resulted in weighing companies having to readdress existing practices and come up with innovative solutions that suit the demands of a highly competitive industry. However, the critical elements that are required for any static weighing system used for storage remain unchanged. The three determining factors for anyone specifying such a system are still - accuracy, reliability and durability. Let us look at each of these factors in turn. Accuracy Accuracy is often the first consideration but sometimes the term is confused with similar terms such as 'displayed weight' and 'display readability'. These terms are sometimes used when referring to the accuracy of a weighing system, but are not necessarily the same thing. Stand-alone weighing systems, even small platform scales are now accurate to 1g, or even better. However, the factory environment and other practical considerations mean that for process applications, a platform scale with a capacity of, for example 30kg, will usually have weight increments of 2g, or a scale with a capacity 3000kg by 0.5kg. A weight controller or terminal can be linked to a number of different capacity and sized platform scales. The controller will guide the operator to make sure the correct article is being weighed and that the most accurate scale is being used. Manufacturing schedules and other data can also be down loaded from other higher-level systems to the weight controller. Systems such as this can provide detailed reports and are an invaluable aid to product traceability. Controllers, such as the Applied Weighing Model 920i, are capable of taking inputs from up to 28 separate weighing systems or from devices such as flow meters. Each system can work independently or for process control in sequence. Outputs available include Profibus, Devicenet, Ethernet, Modbus and 4-20mA. The accuracy of vessel weighing systems is mostly determined by mechanical considerations, such as pipe-work, motors and other less obvious considerations such as the ratio of the vessels dead weight to its contents weight. A good installation, using flexible connections on pipe-work and incorporating appropriate loading assemblies and load cells should achieve at least 0.1% system accuracy, for example, 5000kg x 5kg. Many applications achieve much better accuracies than this, typically achieving 0.01% or more. However, frequently, the main objective for a vessel weighing application is to achieve good repeatability of results and although accuracy is important it is not necessarily the main consideration for manufacturing processes using vessels or tanks. Reliability is a key factor in the storage industry and equipment used should be fit for the purpose intended. Clearly, as with other capital equipment, weighing equipment is no different, you usually pay for what you get. Using lightweight components and platform scales for applications that call for robust designs will always mean that reliability is jeopardised. Sometimes though there has to be a trade-off between robustness and accuracy. For example, very high accuracy equipment, designed for laboratory conditions, may not have the robustness required for the factory floor. The critical component for most modern weighing systems are the load cells and it is the one electronic device which is most exposed to its environment whether incorporated within a platform scale or under the leg of a vessel. Fortunately, they are so robustly constructed and reliable these days that companies such as Applied Weighing are confidently able to give them a five-year warranty. Loss of revenue An engineers worst nightmare is for a failure to occur in his weighing equipment, resulting in incorrect amounts added to a vessel/bin resulting in the loss of revenue and even a client. For vessel weighing applications this can be prevented by using equipment such as the Intelligent Junction Box this clever device can detect an imbalance or damage to a load cell and warns the engineer via an integral display or output to an alarm before any harm is done. Durability Durability plays a big part when it comes to the storage industry. Sometimes, there are strict hygiene regulations. To comply with these regulations, systems need to be designed to prevent chemicals from gathering in screw heads, between crevices, etc. This applies to platform scales and components used for vessel weighing systems. Critical components such as load cells and weight instrumentation are usually manufactured in stainless steel and constructed in such a way as to minimise the ingress of material. These components should also be hermetically sealed to IP68 so as to withstand the frequent wash down, often at high pressure, which is mandatory in some industries. Pit-installed platform scales are unobtrusive and allow trolleys to be easily pushed on but in recent years they have been a less popular choice because pits can be difficult to clean and be kept clean. However, new platform scale designs such as those using gas-lift tops make it much easier for the pit and the scale to be washed down. For those applications where pit installations are not desirable very low profile platform scales are available instead. Some designs are only 50mm off the ground and using ramps even tall trolleys can be pushed on or off without too much effort. Further developments Although process weighing for industrial storage has come along way, the fundamental elements remain the same. Our rapidly growing need for more warehousing and storage introduces further competition into the market place. Consequently, the need to keep cost to a minimum is more important than ever. As technology changes and the way in which items are stored continues to develop we will see further advancements in weighing systems at competitive prices. Increasingly, for example, digital load cells and communication bus systems are being used for applications where there is no need for local weight instrumentation or operator intervention.

More articles from Applied Weighing International Ltd: