Weightru (United Kingdom) - Weighbridges are purpose-built machines to weigh large industrial vehicles with a load.
Usually, this is for commercial purposes, like trucks or rail containers where the movement of goods is done via vehicles.
And so, reliable and accurate weighing is crucial to give the industry the exact figures to ensure compliance and maintain goods inward and goods outward.
From time to time, external factors can cause your weighbridge to produce inaccurate readings.
But before we jump right in, let’s just discuss how a weighbridge works.
Whilst a weighing system can take many forms, a weighbridge tends to have one or more load cells that support the platform, a junction box and a weight controller.
Then, when load is applied to the platform, the load is distributed over each load cell, which then sends an electrical signal to the junction box before it is converted into a weight reading.
With that in mind, here are 4 things that can cause problems with your weighbridge and affect your system’s weighing accuracy.
1. Load Cell:
As a quick explanation, a load cell is a piece of metal that bends with the load’s force and converts this force into an electrical signal.
For absolute accuracy, selecting a top-quality load cell for your weighing system is crucial.
And as long as the load is applied to the correct area of the load cells, it should give you an accurate reading every time.
For a load cell to provide accurate weight information, it needs to have the following specs:
- Nonlinearity: ±0.018% of the load cell’s rated output
- Hysteresis: ±0.025% of the load cell’s rated output
- Non-repeatability: ±0.01% of the load cell’s rated output
- Creep: ±0.01% of the load cell’s rated output in 5 minutes
- Temperature effect on output: ±0.0008% of the load per degree Fahrenheit
- Temperature effect on zero: ±0.001% of the load cell’s rated output per degree Fahrenheit
Please note, not every specification will apply to your specific weighing system. But, it’s important to understand these to determine the load cell’s accuracy.
Finally, the load cell’s response time is another factor to consider: the load cell should settle in less time than the required weighing time.
2. Loading Goods:
It’s important to make sure that each load is placed to each load cell as specified by your weighbridge manufacturer.
Consequently, an improperly applied load can cause the strain gauges in the cell to send a signal that is not the actual load’s weight.
In other words, all load cells need to support the entire weight of the load for it to be measured.
So, if you need to use bumpers to keep the weight vessel from swaying, make sure that it does not support any of the load itself.
That said, be sure to correctly align each load point and that each load cell is level and on the same plane to ensure they share the load evenly.
Moreover, you need to make sure that the structure beneath the load cells is strong enough to hold the weight of the vessel and its contents without bending.
3. Environmental Factors:
External forces like wind loading, vibration and temperature changes can produce errors in the load cell signal.
- Wind loading: let’s say your weighbridge is located outdoors. If the wind is at 30mph that day, this will exert force on the load cells which has nothing to do with the weight.
- Shock loading: this is when a heavy object is chucked onto the weighbridge, causing a force bigger than the system’s capacity and causing damage. Loads placed onto the machine need to be controlled to prevent this.
- Vibration: vibration from other pieces of equipment that is near your weighbridge can cause the load cells to measure the weight and the vibration that’s transmitted to it. To prevent this, isolate your weighbridge and use a system with algorithms that remove vibration effects.
- Temperature changes: large temperature changes can cause the weigh vessel to expand or contract, which can cause errors in the weight reading as well as cause damage to the load cells.
4. Signal Transmission:
Not only is it important to monitor the load cells, but it’s equally as important to make sure that the weight controller is only measuring the load cell electrical signal and nothing else.
In essence, RFI (Radio Frequency Interference), EMI (Electromechanical Interference), moisture and temperature can affect the electrical signal.
- RFI & EMI: sources include lightning, two-way radios, power lines, static electricity and more. To prevent these electrical noises from causing interference, it’s recommended to isolate the load cell in a shielded cable, then route this separately.
- Moisture: if moisture enters the weighbridges junction box, it can seep into the cables of each load cell and affect how it sends a signal and thus the accuracy.
- Temperature: fluctuations in temperature can cause resistance changes in the cable and in turn, cause load cell signal changes.
The Final Say:
To achieve complete accuracy, there are a lot of different factors to consider: both mechanical and operational.
That said, it’s important to select quality components, specifically tailored to your application to ensure your weighbridge performs as you need.
It’s also worth noting that weighbridges come with “worst case” specifications and their actual performance is a lot better.
Nevertheless, always pay close attention to how your weighbridge is installed then operated, to prevent any external forces or electrical noises from affecting your weighing accuracy.