How can I tell if my load cell is good without using Integrated Technician?

I am using older load cells from Hardy Process Solutions (or from another company) and I want to make sure they are functioning properly before I use them in another application. I don't have the Hardy feature "Integrated Technician" in my junction box or controller, so I checked the installation/troubleshooting guide which says to check the resistance between the signal wires and the excitation wires and the resistance of each should be around 350 ohms. Well, the resistance between the signal wires on all 8 load cells was right at 350 ohms, but the resistance between the excitation wires on all 8 load cells was around 387 ohms. So, are all of these load cells bad, or is it ok for the excitation resistance to be 387 ohms?

The true test of a load cell is the load cell's signal millivolt output at no load, mid range and full load. Attached is a tech note on load cell troubleshooting that should answer your question.

There are troubleshooting guides in chapter eight of the individual weight controller manuals. There are also other articles available in Hardy WebTech Knowledge base that addresses other methods to determine the health of your scale system.

Using information provided or derived from different sources can provide readings and assumptions that can determine the health of your weighing system.

For this example we will use:
3 Load cells with capacities of 1000 lb each. Full scale output (FSO) readings for each load cell (acquired from load cell certificates) for our example are: 
#1= 3.0003 Mv/V
#2= 3.0034 Mv/V
#3= 2.9994 mV/V
Millie volt reading at empty = 5 mV
Excitation voltage = 5 VDC

1. Adding the individual load cell weight capacities (lb or kg) will determine the scale system's rated capacity (3000 lb in this example)

2. Average the full scale output of each load cell to determine the scale's mV/V output at rated capacity. (3.0010 in this example)

3. Multiple the Excitation voltage by the average mV/V to determine the milli-volt output at rated system capacity. (5.00 V x 3.0010 mV/V = 15.0050 mV @ 3000 lb in this example)

4. Measure the empty vessels load cell output at the signal + and signal - terminals. (2.25 mV in this example)

5. To determine the amount of load cell capacity used by the empty vessel, dead load, divide the scale system's rated capacity (lb/kg) by the milli-volt output at rated system capacity (found in step 3) Multiple this finding by the milli-volt reading of the empty vessel (found in step 4) (3000/15.0050 x 2.25 = 449.85 lb dead load in this example)

6. A secondary calculation gives the live load of the scale system. (Dead load - scale capacity 449.85 - 3000 = 2500.15 system live load)

7. A general test could be performed by adding a known weight to the scale and measuring the mV signal output increase. This would give a general indication if the scale is linear. Mv readings to the second decimal place would be sufficient for this test. Adding 450 lbs to our test scale should result in a signal reading of 4.50 mV.

8. Divide the mV@FSO by the rated scale capacity lb = mV/(lb) times the test weight, plus the original mV reading at empty should equal 4.50 mV. (15.0050 mV/3000 lb x 450 lb + 2.25 = 4.50 mV in this example)

9. Knowing the capacity of the load cells, excitation voltage, and a known weight the average mV/V rating of the load cells can be determined.

10. A 450 lb test weight increased the Mv reading by 2.25 mV in this example. (A 2.25 Mv increase / 450 lb x 3000 lb scale capacity / 5 v EXC = 3 Mv/V)

For this example it was determined the load cells were 3 Mv/V with an accuracy of two decimal points.   Most meters are OK to two decimal points on the Milli-volt scale.  More decimal points than two require a high quality meter with a recent calibration sticker.