Load cells are constructed using electric resistance metal foil strain gages bonded to an elastic flexure element. The load cell is a passive analog device with continuous resolution limited ultimately by noise, due to electron motion on the order of 10-9 volts (1 nanovolt). Therefore, practically speaking, resolution is limited by the type and quality of the electronic instrumentation used, rather than by the load cell itself. Many reasonably priced instruments can resolve 0.8 to 1.0 microvolt/count as a minimum signal level.
For example, consider a load cell with Rated Output of 3mV/V. Assume that 10VDC excitation is used. At Rated Output, the signal level produced would be:
3mV/V x 10V = 30 mV
If the indicating instrument can resolve 1 microvolt in the rightmost digit of the display, then:
Resolution = 1 µvolt/30 mV
= 1 µvolt/30,000 µvolt
= 0.000033, fraction of Rated Output
= 0.0033 % of Rated Output
If, for example, an MB-5 (5 lbf Rated Capacity) load cell were being used, the resolution in pounds could be calculated as:
Resolution = 5 lbf x 0.000033
= 0.00017 lbf
If an instrument capable of 0.5 microvolt resolution were used, the resolution would be approximately 1 part in 60,000 or 0.000083 pounds for the 5 pound capacity cell. Maximum resolution may be limited by the instrument to the total number of counts that can be displayed.
Another typical example would be the case where only a portion of the range of the load cell is to be used. If the maximum load on the MB-5 were to be 3 pounds, then the output would be:
3 mV/V x 3 lbf / 5 lbf = 1.8 mV/V
Using 10V excitation provides a signal of 18 mV output for 3 pounds input. If the instrument displays is to display 30,000 counts a signal strength of
18 mV / 30,000 counts = 0.6 microvolt/count
results in a display of 0.00015 pound/count resolution. Of course, the instrument must have a sensitivity of at least 0.6 µvolt/count for this example.
It can be seen from the above examples that the sensitivity and stability of the electronic instrumentation is critical, when high resolution is required. High electronic gain alone will not achieve good results if the zero stability or gain stability is poor because the readings will drift with time or temperature changes.
Also, keep in mind that excessive resolution can be detrimental in situations where the stability of the applied force is low, as in some hydraulic systems.
Generally, it is desired to read physical units instead of counts. Most instruments provide a count-by feature of 1, 2, 5 or 10 to facilitate this. For the above example, an instrument could be set up to read 30,000 counts by 2 for the 3 pound load, providing resolution of 0.0002 lbf Premium instruments are available that offer as good as 0.001µvolt/count.