Load cells play a central role in many powder processes since they allow to measure the weight of solids processed. Load cells are used for Dosing purpose or mass balance recollection.
The proper specification, calibration and then maintenance is key to make sure the process will performed as designed, will produce within quality specifications and at controlled cost.
There are different technology principles for load cells. The different weighing principles are summarized below :
Table 1 : Different types of load cells
|Strain Gage Load cells|
|Bending Load Cells|
|Shear Load Cells|
|Compression Load Cells|
|Ring torsion Load Cells|
In order to have a good and reliable measurement, it is 1st necessary to select the load cells regarding the load cells measuring range and the overload that may be applied to the load cells during their operation.
The process operator must determine what is the measuring range that will constitute the normal operating conditions of the load cells and for which he requires a certain accuracy. This range will define the load cells measuring range. From this range, different safety factors need to be taken to prevent damage of the load cells by overload.
The values given below constitute a general approach, an assessment must be done to check if the load cells will not encounter higher loads after installation (during some maintenance operations for example).
Table 2 : Load Cells Capacity definitions
|Load Cell Measuring range||Range for which the reading should be within the maximum permissible error
This is the expected weight to be measured by the process (example the max weight to be dosed for a recipe)
|Safe Load Limit||Max load that can be applied to the load cells without causing damage.
Rule of thumb : 150% of Measuring range
|Ultimate load||Max load that can be applied without leading to physical damage. Accuracy may however be impacted if such loads are reached.
Rule of thumb : 300% of Measuring range
|Safe Side load||Max load that can be at 90 degree (on the side) without causing damage.
Rule of thumb : 100% of Measuring range
Some manufacturers are also proposing tension modules. The load is then pulling on the load cell instead of pushing on it, like in most of the applications.
Events that can lead to overload include : shock loading, agitators in movement (dynamic load), off center load, overloading a scale.
Calculation formula should be applied to take into consideration the situation in which the load cell will be installed and then select the right load cells capacities.
Figure 1 : Capacities and loads - graphical representation
For an industrial process, in an industrial environment, an accuracy of 0.1% of the applied weight can be reached. Load Cells of high precision could reach around 0.03% of the rated capacity (max measuring range).
The error of the scale is the combinations of calibration, linearity, hysteresis and repeatability errors. It is possible to perform a calibration sequence so that the calibration sequence is reduced to around 0, as well repeatability errors should be very miniminal (less than 0.03% of Rated Capacity). In practice, the accuracy of the scale system will be equal to the combined error.
Combined error = Non linearity + Hysteresis = 0.02 to 0.03% of Rated Capacity. This is for 1 load cell.
All reading, from 0 kg to Rated Capacity, should be within the accuracy (Combined error * Rated Capacity * Number load cells). This means that at low load, the error (as % of the weight applied) should be higher than at high load.
Figure 2 : Load cells performance Graph
On top of this, the repeatability error, which should be very minimal, should be controlled after installation to make sure there is no issue.
Another important characteristic is the minimum verification interval (vmin). It is the minimum amount of mass that must be applied to the load cells without exceeding the max permissible error. It is to be related to the combined error which is expressed as % of the Rated Capacity. During design it must be checked that (vmin=Emax/gamma <h; minimum weight expected on the scale).
The resolution of the scale must also be defined. It is related to the number of digits the scale will display, although it is not directly related to the load cells accuracy (it is an illusion to ask more digits than the scales accuracy).
Summary : important load cells characteristics to define when designing an installation
Table 3 : Definition of design values for load cells
|Combined error||The Combined error is giving the best precision one can expect from load cells properly calibrated. For individual load cells, it should be in the range of 0.03% of the Rated Capacity (Emax). As a consequence, accuracy should be expected better at high load than at low load|
|Minimum verification interval (vmin)||This is the minimum weight that must be applied to the load cells to stay in the design accuracy|
|Resolution||This is the number of digits that can be displayed by the load cells. It is not related to the accuracy.|
To perform load cells calibration, it is necessary to have a reference. This reference is usually provided by test weights that have been calibrated by recognized institutes.
It is then necessary to follow a procedure defined by the supplier in order to load the scale with defined mass, check and record reading, then correct potential errors. In certain situation, for very large scale, it may be necessary to fill the scale with water to perform the calibration.
The load cells need then to be checked with the following tests, to be done in the order below :
Table 4 : Definition of calibration tests for load cells
|Friction test||The test is performed by adding and removing several time the same weight to the scale.
The display shown must be the same and should not change by more than 0.02% of the scale Rated Capacity
The friction test will make sure that there is no deviation of the repeatability (explained above) due to some interactions of the environment with the load cells (the scale is in friction with another part for example)
|Creep test||The test is performed by adding a weight on the scale and waiting for a defined time.
During the waiting time the reading should not change by more than 0.01% of the scale Rated Capacity
The creep test will detect any tension that will relax due to the application of the weight (a tension to a flexible for example)
|Linearity and Hysteresis (=Combined error)||Weight are added step by step on the scale, then removed. Value read are compared to calibrated value of the weight.
It will detect any deviation of linearity of histeresis that could be the sign of an incorrect calibration of the load cells or interaction of the environement with the scale
It may not be possible to test the whole scale, in this case, a minimum of 10% of the Rated Capacity should at least be targetted
The tests and reference values given above are general and recommendations of suppliers could vary. They are given as 1st indications and to perform sense check. The verification of suppliers specifications and manual should always be done. The 3 tests given above are the minimum to be performed, other tests may be requested by manufacturers, especially for precision feeders.
Load cells are very sensitive to current. A particularly important point is to ensure that load cells are electrically isolated if any welding work is on going close to the load cells. A good practice would be to install load cells once all welding works have been completed. If the load cells is not properly isolated or if the earthing of the welding station is not properly placed, leak currents can damage it.
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