How can I determine the capacity of a load cell?

I am replacing a load cell on an existing scale and unable to read the label. Is there another way to figure out what the capacity of the load cell is?

Normally you can look at the label to find the capacity of your load cell. Many times the label has been removed or is unreadable. In these cases you will need to determine the milli-volt per volt rating in order to calculate the capacity. Since the mill-volt per volt rating on load cells can differ between manufacturers (1, 2 ,3 or 4 mV/V) you may need to run the following calculations four ways and then determine which is correct.

Being able to predict the millivolt change is the reverse of a scale calibration.

Place a known weight on your scale and record the millivolt reading with a minimum of two decimal point resolution. You will need to know the SENSE voltage level from your controller. Knowing the sense voltage and the four possible mV/V ratings you can then perform the following:

1. Add test weights until you see a mV change of 1 mV (50 lbs for example).

2. Guessing you are using a 2 mV/V rated load cell with 5 volts excitation/sense voltage. The full scale would be 500 lbs. (5 v excitation X 2 Mv/v X 50 lbs, or 5 X 2 X 50 = 500. So assuming you have a 2 Mv/v and have verified you have 5 v excitation, the capacity of your load cell is 500 lbs.

In other words:

2 mV/V times 5 dc excitation/sense = 10 Mv full scale. 50 lb = 1 Mv (or 10% of full scale) 10 times the 50 lb = 500 lb.

Normally these don't come out to a full millivolt reading. If the capacity is some odd value try converting to KG or use another mV/V setting.

Using the trial and error method you should be able to determine the load cell capacity and sensitivity when the calculations represent a standard capacity.  Measure the gauge resistance.  When trying to purchase a replacement load cell use this method.  Match the millivolt/Volt and the resistance readings for the input and output circuits.  Review the data sheet of the prospective replacement load cell along with physical dimensions as a hint if this is a good replacement load cell for your existing system.

Please see the attached technical note for additional information.