It is vital to the performance of a weighing system to select load cells of the correct capacity. Here are some guidelines on how to choose the correct system.
• All load cells selected must be of the same capacity.
• Estimate the vessel dead weight, including all piping, pumps, agitators, insulation and vessel heating fluids.
• Add the maximum live weight of product to be weighed to the dead weight. This is the gross weight of the vessel and contents.
• Divide the gross weight by the number of legs or support points. This is the nominal weight which will be carried by each load cell.
• Select a load cell with a capacity somewhat greater than the nominal weight.
The following should be considered when determining how much greater the load cell capacity should be:
• Is the dead weight accurate?
• Will the load be evenly distributed on all cells?
• Is the vessel fitted with an agitator or subjected to shock loading?
• Is it possible the vessel will be overfilled, exceeding the live weight value?
A good rule is to select a load cell with a capacity 25-50% in excess of the calculated nominal load per cell.
High accuracy systems are generally considered to have system errors of ±0.25% or less; lower accuracy systems will have system errors of ±0.50% or greater. Most weight indicators typically have an error of ±0.01%, hence, the main source of error will be the load cells and, more importantly, the mechanical arrangement of the scale itself. In vessel weighing, each installation is unique in terms of the mechanical arrangement, site conditions and environmental factors.
Therefore, it is impossible to be specific about the system accuracy that can be achieved. The first requirement is to determine what your accuracy expectations are, then design the system accordingly. We strongly suggest that you have these load cells for tanks installed and calibrated by a reputable company in your area.
