Weigh Indicator Issues.
Understanding the environmental factors that can contribute to indicator issues.
Weigh Indicator Issues.
Mechanical and operational issues can and will occur during the life of scale ownership. Scale manufacturers have certain standards when it comes to scale care and scale calibration. When operating your legal for trade scale and indicator, there are factors that will add to potential issues but can be prevented with an understanding of which issues can arise. This will help you prolong the lifespan of your scale system and also help you avoid other common problems. You can ensure that your weighing system performs accurately by choosing components suited to your application and taking steps to control environmental and other forces acting on the system. This article discusses the factors that can affect your weighing indicator’s accuracy and provides advice and DIY fixes for potential issues.
UNDERSTANDING THE FLOW OF COMMUNICATION
A weighing system and weighing application can take any of several forms but typically includes one or more load cells that support (or suspend) a weigh vessel or platform, a junction box, and a weight controller. Sometimes you will see certain setups with a remote display or scoreboard. When a load is applied to the weigh vessel or platform, a portion of the load is transmitted to each load cell. Each cell sends an electrical signal proportional to the load it senses via a cable to the junction box. The load cell signals are summed in the junction box and sent via one larger cable to a weight controller, which converts the summed signal to a weight reading. This weight reading's accuracy can be affected by the system components' quality and the system's installation and operation in your environment.
INTERFERENCE WITH INDICATOR
Weigh indicators communicate with the Load cells by receiving an electrical signal, which is processed by some type of weight controller that then outputs the weight onto the scale’s digital indicator. If it is an analog or electronic load cell there are factors that can add to inaccurate weight measurements. Interference occurs during any part of the process can lead to inaccurate weight readings between the load cell and scale indicators.
Below are just a few of the more common types of interference that we see when operating your scale and indicator.
Temperature – As temperatures increase wire resistance increases as well, causing voltage to drop, which can lead to the scale controller displaying results that are actually different from the load cell’s output.
Moisture – From time to time, condensation or moisture can build up on the load sensor, which can affect electrical outputs and cause interference in the signals being sent from the load cell to the display indicator.
RFI and EMI – These can create additional input signals, or noise that can also cause the scale controller to display an incorrect weight.
Temperature: A factor that some can overlook is the impact temperature has on your indicator. A 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 indicator can be affected by temperature fluctuations, which cause resistance changes in the cable. This can cause excitation changes, in turn causing load cell signal changes. To prevent these temperature problems, use six-wire load cell cable, which allows the weight controller to make ratiometric readings of the load cell signal that ignore excitation-change-induced changes. Essentially the extreme temperature changes can cause a chain reaction from the cable to junction box, thus sending an inaccurate signal to the indicator.
INTERFERENCE WITH SIGNAL TRANSMISSION
In addition to ensuring that the load cells measure only the desired weight, it's equally important to ensure that the weight controller measures only the load cell electrical signal. Radio frequency interference (RFI), electromechanical interference (EMI), moisture, and temperature can all interfere with this electrical signal.
RFI and EMI. Just as vibration is mechanical noise (that is, interference) to a load cell, RFI and EMI are electrical noise to the 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 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 load cell cable shield can also be an open door for electrical noise. To prevent the noise from affecting your 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.
Moisture. Moisture that enters the weighing system's junction box can wick itself into the cables to each load cell and reduce the capacitance between signal lines. This causes the load cell excitation lines (the lines carrying electrical energy to the cells) to couple with the signal lines (the lines carrying the 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 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.
At American Scale we strive to bring you informed and useful content on all things scale systems related, including products from the top scale manufacturers. Be sure to check out our truck scale articles about truck scale foundation or how much do truck scales cost. We even have a guide to warranties. To learn more about bench scales, it would be good to check out our other articles such as “Bench Scale Basics'' and “How Much Does A Bench Scale Cost?”. These articles will help with the basics of bench scale ownership. Also, take a look at our“Common Problems With Bench Scales'', to help prolong the lifespan on your scale.