How do i verify that my scale system is Balanced?

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Courtesy of Hardy Process Solutions

Have you ever wondered how to check the mechanical or load cell signal balance on your weighing system? Here's how. Verifying that your scale system is balanced helps to provide you with accurate readings.

Load sharing is important to ensure stability of the vessel and scale. Any imbalance between load cells would have to be dramatic to make the scale unstable.

For example, standing on a floor scale and shifting your weight to cause a rocking motion in the scale base would cause an instability that would be solved by balancing the corners. Normally, an imbalance is caused by motors and other attachments to the scale. Sometimes a counterbalance is required to keep a scale vessel upright.

How to Verify Scale System Balance

Using a multimeter, check the connector J1 on the weight controller load cell connector. If your instrument has INTEGRATED TECHNICIAN from Hardy Instruments, a participating Encompass Product Partner in the Rockwell Automation PartnerNetwork, the measured values can be read from the front panel display, the embedded Web server or via a programmable logic controller (PLC) human-machine interface (HMI). Follow these steps:

Confirm that the excitation voltage is correct.

  • 10V DC (Hardy Instruments models HI 2151/20WC, HI 1771WS and HI 2160RC).
  • 5 V DC (Hardy Instruments models HI 2151/30WC, HI 1746WS, HI 1756WS, HI 1769WS, HI 200DNWM, HI 4050 and HI 3000 series).

Confirm the millivolt (mV) signal is a valid positive MV reading.

  • 0-30 mV on the HI 2151/20WC, HI 1771WS and HI 2160RC
  • 0-15 mV on the HI 2151/30WC, HI 1746WS, HI 1756WS, HI 1769WS, HI 200DNWM, HI 4050 and the HI 3000 series (which includes the HI 3010, HI 3030, HI 3300 and HI 3600).

Load sharing includes load cell operation and verification.

  • Assuming the load cell wiring is complete and correct, apply weight. Does the mV signal increase in a positive direction?
  • A negative signal reading on the connector indicates the load cell is upside down or defective. Check to see if the load cell is correctly mounted.
    The arrow goes with the direction of force. Use the load cell installation manual to verify the correct orientation. If there isn't an arrow or instructions, you must manually verify the correct direction.
  • Load cells in tension won't reflect a negative reading if installed upside down. If upside down, the force applied by the cable would be included in the weight readings.
  • If you have received positive results, continue.

If your vessel has multiple load cells, verify all the mV readings individually. At the summing junction box with the controller powered up and the excitation wires connected, do the following:

  • Disconnect the signal side of the load cell cables.
  • Verify a positive reading from each load cell.
  • Record the mV readings and compare each corner for proper load sharing.

If you have an HI 215IT INTEGRATED TECHNICIAN junction box, the cables don't have to be disconnected, so skip step 4a. The values can be read on the front panel display, the embedded Web server or via a PLC HMI.

Proper load sharing should see only a difference of approximately +/- 1 mV.

  • Larger differences, caused by motors and piping, should not be more than +/- 4 mV.
  • If there isn't any piping to explain the mV difference, adjust the corners and balance the mV readings using shims, or if equipped, by adjusting bolts on the load cell-mounting hardware.

Drawing a load cell map will help to determine the correct leg to adjust and the direction in which it should be adjusted.

Three load cells should self-level like a three-legged chair.

  • Using a spirit level, verify that the vessel is vertically and horizontally level.
  • Verify whether any height change will affect the attitude of adjacent vessels or piping.
  • Adjust each leg to dynamically match mV outputs.
  • Verify the mV readings and physical level when complete.

Four load cells or more present a challenge.

  • Using a spirit level, verify that the vessel is vertically and horizontally level.
  • Verify whether any height change will affect the attitude of adjacent vessels or piping.
  • With all signal wires connected, measure the mV output. This is the sum of the load cell signals and your target mV setting for each load cell.
  • Disconnect the individual signal wires so you only read the output of individual load cells.
    Using a map, chart the individual load cell readings. Record the mV reading of each leg (load cell).
  • Adjust the load cell to the lowest reading to dynamically match the target mV readings previously obtained.
  • Read the mV readings from each load cell to verify a proper correction.
  • Repeat step d and e as necessary to achieve the correct load sharing of your vessel.
  • Verify the mV readings and vessel level when complete.

Proper load sharing produces mechanical soundness and a solid weigh vessel. Weight drifting and fluctuation usually are signs of a mechanical or a damaged load cell.

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