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Process reliability with load cells and controllers

Right combination of coordinated components guarantee high system availability
Process reliability with load cells and controllers

Today, weighing controllers are integrated into many industrial processes as an important part of weight measurement equipment. The ideal combination and coordination of individual components, easily integrated into an overall automation concept, are essential for dependable process flow.

Manfred Kophal

When designing a hopper, tank or reactor with built-in load cells, a number of aspects must be taken into account to attain the desired measurement results. In addition to key factors such as conduit layout and deflection of substructures, other aspects should be covered by the load cell mounting elements, so that the special requirements of high-precision load cells are all met to enable the design engineer to easily integrate the mounting elements into his overall design. These requirements include:
  • Correct introduction of force, allowing for top and base plates, and material compatibility
  • Lift-off protection
  • Resistance to destructive lateral forces
  • Device for changing load cells
Sartorius mounting kits, specially designed for use in tank and hopper weighing, integrate all of these features. Moreover, the model PR6001 kit has the additional advantage that the load disc remains in place even when the load cell is not mounted. This mounting kit was recently tested and certified by Germanischen Lloyd. This ensures simple and consistent documentation of equipment testing and certification, saving time for the design engineer.
Junction box and connecting cables
At first glance, junction boxes and connecting cables just look like conventional equipment, nothing to attract particular notice. On closer inspection, however, it becomes clear that this equipment actually has to transmit signals in the mV range for thousand-fold resolution and processing. The slightest signal distortion can affect measurement results. The PR6130 cable junction box from Sartorius is developed specifically to prevent signal distortion. Available in both plastic and stainless steel versions, the PR 6130 cable junction box provides stable signal transmission even under the most adverse conditions. Double shielding on cables and connectors, together with an ingenious layout inside the cable junction box, ensure reliable results even in the presence of potential disturbance factors as humidity and vibration. Uniform color coding on terminals and cables minimizes the risk of error when installing and commissioning equipment.
Weighing controllers
As the final link in the measuring equipment chain, the weighing controller re- ceives, converts and displays the measurement signal and processes it for the application being used. It also transmits the signal to any downstream or post-connected peripheral systems. One of the most impor-tant steps in this procedure is the calibration of the measurement signal. The weighing controller must determine the zero point and set the end scale value of the weighing instrument. In the world of industrial processes, the weighing instruments are usually hopper, tank, container or reactor scales. It is not unusual for this type of equipment to weigh up to several hundred tons.
This is why the calibration points are not only defined with an actual load; it is particularly important to specify a calculated point of measurement in mV/V for the end scale value. This can be done by performing calculations based on the nominal values for the load cells, or simply by inputting the certified load cell specifications (sensitivity, resistance, hysteresis) and activating the SmartCal function built into the Sartorius controller. SmartCal even takes the gravitational acceleration at the place of installation into account. Sartorius offers the option of creating certificates for the load cells installed, listing all load cell data.
When the various components are all brought together in a single chain of measuring equipment, there are risks of error at every device interface. As long as all of the components are coordinated to work together, however, the result is a calibrated measurement device with a pre-defined accuracy and the highest possible reliability, just depending on the individual factors such as the accuracy class of the load cells or the weighing controller. With coordinated components, it is no longer necessary to perform elaborate testing on every piece of equipment in the weighing system. On the contrary, the outermost interfaces supply all the required information – a further gain in process reliability.
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