Issue link: https://www.e-digitaleditions.com/i/1282913
56 / September 2020 powderbulk.com PBE SUPPLIERS' TIPS What factors in my plant can impact my scale or load cell's accuracy? H ere are some factors that I think can affect a system's weighing accuracy in a plant: • A machine in the immediate vicinity of the weigh- ing system could introduce vibration that could affect the system's accuracy. • If the plant environment is wet or has corrosive chemicals, then the load cell's internal and external electrical connections can corrode and affect system performance. In these environments, it's best to use stainless steel hermetically sealed cells. • Air currents or pressure differentials in upstream or downstream plant processes can cause unwanted movement of the scale, so ensure that connected equipment is properly vented or isolated. • It's also important to make sure that the load cell's voltage source is well grounded and protected from voltage surges and electrical noise. If the voltage to the load sensor varies, then the scale reading can too. • If welding is planned in the load cell's immediate vicinity, disconnect the load cell and put a spacer in its place. If this isn't possible, then make sure that the welding ground clamp is positioned so that the current doesn't pass through the cell. • Finally, make sure that it's not possible to overload the load cells during cleaning or service to the scale by stepping or standing on it. If this is a possibility, a warning sign should be placed at the equipment. Terry Stemler, president, APEC, 616-374-1000 T he biggest single factor affecting weighing accuracy is the amount of material versus equip- ment being weighed by one or more load cells. The greater the percentage of material relative to the equipment or vessel containing it, generally, the more accurate the material weight measurement can be. In the case of weighbatching for example, a gain-in- weight (GIW) system typically employs a central weigh hopper suspended on load cells, such that material fed into the hopper comprises a large percentage of the gross weight, allowing for the use of load cells with a lower weight capacity but higher accuracy. Conversely, a loss-in-weight (LIW) batching system, such as a bulk bag weighbatching discharger, typically requires high-capacity load cells that have lower accuracy to support the gross weight that includes the heavy equipment and full bulk bags. A LIW system has to measure the gross weight while also detecting small amounts of material being discharged. David Boger, executive vice president, Flexicon, 888-353-9426 T he most common factor affecting load cell accu- racy in today's industrial environments is load cell signal interference. Nearby electric fields, magnetic fields, or both have an effect on the load cells transmission lines and sens- ing circuitry. Any electrical device that generates, con- sumes, or transmits power is a potential source of elec- trical interference (noise) in load cell circuits. The higher the voltage or current draw of these devices along with their closeness to the load cell, the greater the induced signal interference (signal-to-noise ratio) will be. Inter- ference can lead to inaccurate measurement results and incorrect interpretation of the load cell signal at its receiving device. Some of the most common sources of electrical noise emitters are: AC power lines, motors, transform- ers, generators, motor starters, relays, arc welders, and radio transmitters. The best practices to follow when it comes to load cell wiring is to: 1) Adhere to the manu- facturer's installation and wiring instructions. 2) Keep load cell lead wires as short as possible and away from potential noise sources. 3) Use a dedicated conduit run or cable tray for load cell lead wires in high noise- susceptible environments. 4) Minimize the number of load cell lead wire junction points and solder all con- nections if a junction is needed. 5) Use only environ- mentally approved enclosures for load cell lead wire junction points. 6) Use individually shielded twisted pair cabling for each pair of load cell signal wires (exci- tation, signal, and sense) and terminate drain wire as directed by the manufacturer. Robert Villano, director of business development, Thayer Scale, 781-826-8101