How do I select the right capacity load cell?

Before choosing load cells for a specific application, the user should verify it is possible to achieve sensible results from the planned weighing system. It is not possible for a typical weighing system with a total capacity of 80,000 pounds to weigh in increments of 1 pound (1/80,000). Strain gauge load cells should not be expected to work beyond a minimum stable/repeatable weight increment of 1/10,000th of the load point capacity.  With newer technology found in the HI4050, HI6300 sereis and HI6500 series this minimum has been expanded to 1:40,000.

The final choice of load cells for your system will be somewhat of a compromise based on the standard capacities of Hardy’s load sensors and your application requirements. Hardy load sensors have outputs from 0.9 to 3 Milli Volts per volt (mV/V). With 5 volts excitation, the full scale output for your Hardy load cell will be anywhere from 0 Millivolts to a maximum 15 Millivolts. This output is the full scale output from no load to full capacity load. Full scale capacity takes into account the weight of any vessel or platform on the load cells, usually referred to as the “Dead Load”. This dead load will use up a portion of the load cell output, and as a result should be as low as possible compared to the “Live Load” (active weighing range). In practice this is not always easy to do, and it will depend on the overall resolution or accuracy required of your weighing system. If multiple load cells are connected in parallel, then the total output will be the summed average of the outputs from the individual load cells.

When the range of your system has been calculated, this information can be related to electrical output in Millivolts (mV). This should then be related to the minimum weight increment or scale division which is required by the user.

EXAMPLE:
4 – number of load cells (at 2 mV/V)
75 kg – capacity of each load cell
300 kg – total capacity of system (4 X 75 kg = 300 kg)
5 VOLTS EXCITATION * 2 mV/V per load cell = 10 mV full scale output
1 mV = 30 kg (300 kg/10mV) 
100 kg – dead load 
3.33 mV – Electrical output for dead load weight
200 kg – available live weight capacity applied equals 
6.66 mV – total electrical range for live weight, starting at a base of 3.33 mV

Using your specifications rather than the example above, you can determine the electrical output per division on your system and compare that to the specification of your Hardy controller to ensure you are not expecting more than your system can deliver. If the signal is too low, then your system may need to be redesigned with respect to the live load weight and dead load, or load cell capacity.

The calculation of load cell capacities for vessels will usually be different from that of floor scales. It is unlikely that an individual load cell on a weigh vessel will be overloaded under normal working conditions (if calculations are correct). But on a floor scale it is possible for one corner to carry a high percentage of any load cell placed on the weigh deck, so the calculation to determine correct capacity is doubled to prevent load cell damage. One method for determining the required load cell capacity for a particular application is outlined below:

VESSEL/SCALE USING LOAD CELLS
Load cell capacity = (live weight + dead weight) / # of load cells

PLATFORM FLOOR SCALE
Scale capacity = (live weight + dead weight) * 2

Hardy standard load cell capacities will then dictate which load cells should be used. When calculating capacities for platforms floor scales remember to double the normal capacity for safety. When calculating the capacity consideration must also be given to:
a) Any adve