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There are many reasons why your weight reading could suddenly start drifting. One of the most common reasons for drifting appears at certain times of the year when we are swamped with phone calls from customers experiencing this problem. Almost every one of these customers is experiencing drifting due to static buildup on their load cells and/or weight controller. As the humidity changes, so does the static charge in the air. Your weight controller is looking for very small changes in input voltage, and static discharges can cause the weight reading to drift.
The most effective way to determine what is causing your weighing system to drift is to isolate the problem to the load cell(s), the controller, junction box wiring or the cables. The problem could also be caused by external mechanical binding. Please refer to the attached tech note to help you isolate the cause of drifting on your weigh system. The easiest thing you can do is swap controllers with a known good one. If the weight still drifts check all the load cell connections to make sure all connections are tight. Check in the junction box and on the back of the controller. You should also make sure there is no moisture in the summing junction box. If water or moisture gets into the junction box the load cell cable and/or load cell may have to be replaced due to the fact that cables can "wick up" the moisture and damage the load cell (and cause drifting symptoms).
Weight drifting over time can happen to any instrument, and it is usually not the instrument that is the cause, it is more often the scale input to your controller. You will first need to perform a calibration with the scale empty and then record the Mv/V levels from the load cell(s). You can find the Mv/V levels by going into the Integrated Technician section of your Hardy controller. The following Hardy weight controllers have the Integrated Technician (IT) feature: HI 1746WS, HI 1756WS, HI 1756-2WS, HI 1769WS, 1756-FC, HI 1769FC, HI 1769-2WS (that use commands in the ladder logic). The HI 2151/30WC, HI 3030, HI4060, HI6300, HI6310 and the HI 4050 use the front keypad and display to run the IT programs. To utilize the IT feature you will need an IT capable controller (listed above) equipped with an IT capable junction box (HI 215 IT).
If your Hardy Instrument does not have Integrated Technician then you will need to get the mV readings using a volt meter at the back of the controller. Use the instructions below to measure the signal voltages and record the Mv/v readings for reference.
The next test attempts to establish if a load cell's millivolt output has changed when you repeat the same weight levels, if the weight drifts at a high level. Record that weight and wait for the scale to drift. This duration can change with the cause. Heating and cooling is normally a 12 hour cycle. Shorter cycle times may help determine the source of your drift. Typically a load cell is stable to +/- 0.01mv across the signal output of a static scale. Loose gauges are detected with a millivolt meter across the individual load cells signal outputs. Monitor the output and that should be stable to +/- 0.01mv. Apply a load to the load cell you are monitoring. The Millivolt signal will rise proportionate to the force applied and when the force is removed, return to the original Mv level as fast as the meter can respond. Any slow response can indicate the load cell or a mechanical bind.
Now operate your Hardy weight controller as you normally would, and when the we
