Why isn't my weight reading stable after hard calibration?

After trying to do a new hard calibration on a previously working system, my weight reading is not stable and my Hardy Instrument will not calibrate properly. What could be the problem?

Instability can be from a poor calibration, unstable mechanical or structural components, or the load cell capacity dictates a larger weight increment than you intended.

On the Hardy units when the weight is unstable and your Hardy controller does not accept the calibration it normally indicates that a calibration was attempted and the load cells were not reacting as they should, or the span weight was not placed on the scale before the cal high command was given.  Check the troubleshooting sections of the manual for error codes and troubleshooting hints to follow. You can download a manual for the Hardy Controllers at  http://www.hardysolutions.com

Select your controller type and the Docs and Program tab.

Try repeating the calibration.  You may have pressed the Zero or Span command to quickly and did not let the scale settle for 15-20 seconds.  There could have been motion in the scale.  Check for vacuum, motors, and vibrators that were left on by mistake. Check the wiring to verify the load cells are connected properly.

If the weight is unstable you might need to increase the motion tolerance to a higher level, so that the unit no longer gives an indication of motion. The unit will not calibrate if it is indicating it is in-motion. Next, try doing the calibration in reverse, that is, do the span with weight on the scale first, then remove the weight and do the low cal. Once the calibration is done, decrease the motion tolerance back to a normal setting and do a save command.

Next verify the scale motion or instability is not more than one part in 10,000. One part in 10,000 is a point when a scale is considered stable and should be repeatable to that level. If your scale was 1000 lb total the weight should be stable to 0.1 lb. If you have more decimal points on your scale this will not increase accuracy or stability. Increasing the graduation size to match the 1 part in 10,000 will help make the scale stable as it was intended.

Other problem possibilities is the summing junction box was wet, the grounding and shields are not correctly connected,  EMI/RFI, Static discharges, collapsing fields from relay coil activation, and high voltage cables to close to the low voltage cabling. (See shielding, grounding, and static WebTech answers)