Why are we seeing weighing errors using hard calibration?

We are trying to perform a hard calibration using 3000 lbs test weights. After Hard Cal our Hardy controller reads 3002 lbs with 3000 lbs applied, 1999.3lbs with 2000 lbs, and 995 lbs with 1000 lbs. At 0.0 the unit will not display a tenth of a pound without first applying about 8 lbs. When we suspend a test weight of 30 lbs on the cell the controller reads only 22 lbs. Why is there an error?

When I used to calibrate a 4000 lb cell on our older HI 2151/20WC, we never saw more than a few tenths of a pound error throughout the test range 0-3000 lbs. Why the difference between the HI 2151/20WC and newer controllers?

With the Hardy instruments, if the displayed weight does not show the correct span weight when you exit calibration you should repeat the calibration to ensure the scale had settled prior to accepting the calibration. It is important to wait 20-30 seconds at the zero and span menu selections so that the readings are stable. During calibration the instrument automatically reverts to a "AVERAGE" setting of 200 requiring a 10-12 second minimum wait period. After calibration is complete and exited properly the "AVERAGE" setting changes back to the previous settings. Exiting calibration using exit in the HI 2110WI, HI 2151/20WC, and HI 2151/30WC will leave some parameters incorrectly updated, you need to return to End Cal and press enter.

This may be an indication that the load cells is stressed or damaged.  Refer to the associated  WebTech answer for troubleshooting load cells.  One load cell may have a large negative offset that is summed. 

When you do not read any weight change with a small weight, there may be a slight negative signal offset in the load cell. Some controllers do not read negative signal offsets like the older HI 2151/20WC controllers did. This could be problematic when calculating the correct counts per engineering units for calibration. To overcome a negative offset install additional dead weight on the scale to overcome the negative millivolt signal level, then go into hard calibration and zero the scale. Then add the 3000 lb test weight, wait the 20-30 seconds and span the scale at 3000 lbs. When you exit the calibration menu the weight reading displayed should read 3000 lbs. This additional dead weight should not be removed as the added weight applied will overcome the negative offset. Lifting the 3000 lb test weight with the extra weight should now give a correct weight reading.