Position: In addition to ensuring that the weighing load cells measure only the desired weight, it's equally important to ensure that the weighing controller measures only the weighing load cell electrical signal. Radio frequency interference (RFI), electromechanical interference (EMI), moisture, and temperature can all interfere with this electrical weighing signal.
Moisture interference. Moisture that enters the weighing system's junction box can wick itself into the cables to each weighing load cell and reduce the capacitance between signal lines. This causes the weighing load cell excitation lines (the lines carrying electrical energy to the cells) to couple with the signal lines (the lines carrying the weighing cells' signals back to the junction box), creating electrical noise that can affect the weighing accuracy. To avoid this, use a waterproof NEMA 4-rated junction box and plug any unused junction box holes. If moisture is present in your environment, also use weighing load cells that are hermetically sealed at both the strain gauge area and the cable entry. The strain gauge area should be welded shut. The cable entry, which is the most vulnerable to moisture because moisture can wick up through the cable, should have a welded fitting that includes a glass-to-metal hermetic header.
RFI and EMI. Just as vibration is mechanical noise (that is, interference) to a weighing cell, RFI and EMI are electrical noise to the weighing load cell signal sent from the cells to the weight controller. RFI and EMI sources include lightning, portable two-way radios, large power lines, static electricity, solenoids, and electromechanical relays. One major step toward preventing these electric noise sources from affecting your weighing accuracy is to isolate the weighing load cell low-voltage signal (typically equal to 1 millionth of a penlight battery's output) in a shielded cable and then route the cable in a conduit separate from other cables. But be aware that the weighing load cell cable shield can also be an open door for electrical noise. To prevent the noise from affecting your weighing load cell function, properly ground the shield by tying it at only one end to a true ground, which will prevent the shield from forming a ground loop.
Temperature interference. A weigit batching control system's weighing load cell cable conduit that's subject to large temperature changes or that runs more than 50 feet from the junction box to the weight controller can be affected by temperature fluctuations, which cause resistance changes in the cable. This can cause excitation changes, in turn causing weighing load cell signal changes. To prevent these temperature problems, use six-wire weighing load cell cable, which allows the weight controller to make ratiometric readings of the weighing load cell signal that ignore excitation-change-induced changes.
