Why is the analog output from my HI 2151/30WC or HI 2110 WI unstable?

We have installed a 4-20ma loop to a SLC500 from an analog option card. After completing the calibration the loop gives a reading that is oscillating by +/-30kg when reading a weight of 90kg, but the HI 2151/30WC display is stable. We added some filtering on the PLC input but are wondering why the 4-20ma analog output signal is so unstable?

If the readings on the HI 2151/30WC are stable and the readings on the SLC are fluctuating, that would indicate that you are picking up noise on the analog loop cable. If you put a filter on the line and it solved the problem, then the noise is coming in on the other side of the filter.

The HI 2151/30WC is sourcing the current. If your analog module is also trying to source the current it can cause fluctuation.

The ranging of the analog output can also play a part in stability. If the range set up in the analog output option is larger than the actual range of the scale, then any small fluctuation can then translate to a large fluctuation in the SLC.

Things to look at are:
1. Check to ensure the analog cable is shielded. Make sure the shield is properly grounded at only one end.
2. Ensure the analog cable is routed away from any high voltage cables.
3. Ensure your module is just receiving instead of trying to source.
4. Ensure the range of the analog output is set up based on the range of the scale.

You can also perform a test by disconnecting the analog output cable from the Hardy unit. With a multimeter (no simulator attached) measure between pins 1 & 2 for a stable voltage. Measure on the HI 2151/30WC between pins 5 & 6 for a stable current. Measure both the voltage and the current outputs.

NOTE: Be cautious of meters that average the inputs, as they may smooth out any fluctuation.

If the HI 2151/30WC is ranged from 0 to 6000 (no decimal places), you would see a change of .1 v for 60 kg and .1 ma for 37 kg change of the front displayed weight. If the front display has a stable weight, then you should have a stable reading on the meter. If it is fluctuating, then the unit would need to be returned for evaluation/repair. If it is stable, then the problem would be external noise being picked up on the cabling or the PLC. Normally this is due to a shielding problem or grounding difference between the unit and the PLC.