A: Keith Melton, Coperion K-Tron, says:
Load cells are critical components in dry bulk solids weighing systems, but there are many sources of interference for this process measurement equipment.
Environmental disturbance. Wherever there’s mass movement, there’s inevitably an impact on the common surfaces supporting the various pieces of equipment. For example, an extruder sitting on a frame together with a loss-in-weight (LIW) feeder is a potential source for disturbance (vibration) to the feeder’s weighing system. As the extruder may run at different fill levels and potentially different speeds, the vibration’s amplitude and frequency will vary greatly. In this case, the best solution would be to measure the disturbance so that it could then be eliminated from the weight signal, leaving only the LIW feeder’s actual weight. The big problem is that in order to measure the disturbance, you have to resolve a weight that’s cycling at frequencies of 1,200 rpm (20 cycles per second) or more. Using a technology with a fast sampling frequency, this disturbance can not only be measured, but, in conjunction with specialized control algorithms, it can be effectively eliminated in a range from only a few hertz — a frequency equal to one cycle per second — to near the limit of the measuring frequency. Combining this with a high-resolution load cell means accurate weighing is still possible, even under extreme conditions.
Pressure. A weighing system often needs to be enclosed, particularly when harmful or dangerous materials are being handled or when the natural environment may have an effect on the bulk material. This means that if there’s any pressure within the vessel, it will act as a force against any coupled nonweighed elements. As a result, pressure above the weighing equipment will press down on the scale, increasing the weight, while pressure at the outlet will press up, reducing the weight measurement. A typical example is a refill valve for a weighing tank above the LIW feeder. Even where these weight deviations are considered as very small, they can have a big impact on a continuous metering system. With advanced weighing systems coupled with pressure compensation, either electronic or mechanical, it’s possible to eliminate these influences and provide a clean weight signal.
Temperature. Whatever the weighing technology, changes in temperature will affect the weighing system’s mechanical properties so if temperature changes are unavoidable, compensation is required. Some sensors have a pretested temperature profile already programmed, meaning the sensor can adjust its response based on ambient conditions. As these changes are generally nonlinear outside of a certain range, it’s ideal to have a programmable profile of actual measurements for this compensation.
Coperion K-Tron is an international market and technology supplier that designs, develops, manufactures, and maintains systems in feeding and weighing technology, bulk materials handling systems and services, and compounding and extrusion.