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Are My Load Cells Reacting Correctly to Weight?

Learn how to calculate and verify your weigh vessel’s performance.

By the Hardy Instruments Technical Support Team

To figure out if your load cells are reacting correctly to weight, apply a test weight to each corner. Make sure each load cell shows a positive increase in mVs proportional to the weight applied.

How to Record Millivolt Signals

Calculating and verifying performance of your weigh vessel includes determining system capacities, dead load (amount of load cell capacity used by the empty vessel) and live load. Using information provided or derived from different sources can provide readings and assumptions that can determine the health of your weighing system. We’ll use the system in Figure 1 to illustrate the process:


Figure 1. This sample system provides a basis for our example in this article to determine if your load cells are reacting correctly to weight.

Load cells 1,000-lb. capacity; Full Scale Output (FSO) for each load cell:

  1. Adding individual load cell weight capacities (lb. or kg) will determine the scale system’s rated capacity (3,000 lbs. in this example).
  2. Average the Full Scale Output (FSO) of each load cell to determine the scale’s mV/V output at rated capacity (3.0010 mV/V in this example).
  3. Multiply the excitation voltage by the average mV/V to determine the mV output at rated system capacity (5.00 V x 3.0010 mV/V = 15.0050 mV @ 3,000 lbs. in this example).
  4. Measure the empty vessel’s load cell output at the signal + and signal – terminals (2.25 mV in this example).
  5. To determine the dead load, divide the scale system’s rated capacity (lb. or kg) by the mV output at rated system capacity, found in step 3. Multiple this finding by the mV reading of the empty vessel, found in step 4 (3,000/15.0050 x 2.25 = 449.85 lbs. dead load in this example).
  6. A secondary calculation of dead load - scale capacity gives the scale system’s live load (449.85 – 3,000 = 2,500.15 system live load in this example).
  7. As a general testing method, you can add a known weight and measure the mV signal output increase. This would give an indication if the scale were linear. Readings to the second decimal point are sufficient for this example. Adding 450 lbs. to our example scale should result in a 4.50 mV signal reading.
  8. Divide the mV@FSO by the rated scale capacity lb. = mV/(lb.) times the test weight, plus the original mV reading at empty. (15.0050 mv/3,000 lb. x 450 lbs. + 2.25 = 4.50 mV for this example).
  9. Knowing the capacity of the load cells, excitation voltage and a known weight, you can determine the average mV/V rating of the load cells.
  10. In this example, a 450-lbs. test weight increased the mV reading by 2.25 mV. It can be determined that the load cells are 3 mV/V with an accuracy of two decimal points (2.25 mV increase/450 lb. x 3,000 lb. scale capacity/5 V EXC = 3 mV/V in this example).

Rockwell Automation Encompass Product Partner Hardy Instruments, San Diego, designs and manufactures process weighing instrumentation. For more information, visit www.rockwellautomation.com/go/p-hardy.