What is a weighbridge?
Truck weighbridge also known as a truck scale, is system of several components that work together to provide weigh readings when a truck drives on to the scale.
Components of Truck Weighbridge or Truck Scale
Five main components for a truck weighbridge or truck scale:
1. Foundation – Permanent installations use a concrete foundation.
2. Steel Platform – This is the structure that creates the driving surface for the trucks
3. Load cells – The key components that uses a weighbridge in order to make the weigh measurement is load cell. A load cell is a transducer which converts force into a measurable electrical output.
4. Terminal – sometimes referred to as an indicator, the terminal is the control panel for the scale. It displays the weight value to the operator, and often serves as the connection point for other scale peripherals.
5. Cables – the signal from the load cells must be transmitted to the terminal. In most cases this is done with cables.
How do truck scales work?
The entire steel platform is mounted on load cells. Each load cell is comprised of a durable material such as steel or concrete with one or more strain gauges attached to or embedded in it. A strain gauge consists of a wire (or wires) that transmits a mild electric current.
As the cell is subjected to weight, the wire in the load cell strain gauge is altered or compressed slightly. The change in the wire results in a difference in the resistance to the current passing through it.
The analog signal from each load cell is sent to the junction box. Junction box combine the signals of multiple load cells and then transmit the summed signal to weighbridge controller. The controller digitizes the signal from the junction box using analog to digital converter (ADC) and weigh is registered on LED display.
The weight of the payload, including the weight of the truck, is measured by first driving the truck onto the weighbridge; this weight is referred to as the gross weight. To obtain the weight of the payload (the net weight) the tare weight (the weight of the empty truck) must then be subtracted from the gross weight.
Truck Weighbridge or Truck Scale On-Site Calibration Testing Methods
1. Zero Calibration Test
Empty weighbridge platform is tared and calibrated to Zero.
2. Range Test
It is very important to calibrate the weighbridge with maximum known weight.
General thumb rule to calibrate any weighbridge system is with 10% of its total capacity. For example, a 100 tonnes weighbridge may be calibrated with 10 tonnes reference standard test weights.
3. Repeatability Test
To ensure the repeatability of the weighbridge system by ensuring that the same weight is displayed at all times when applying a given calibrated load in any direction.
4. Load cell Eccentricity Test
All load cells in a weighbridge may produce unequal output which gives corner error and result weight error on every weighment.
General thumb rule for eccentricity test is to test each load cell with the load of total weighbridge capacity / number of Load Cells. For example, a weighbridge with a capacity of 60 tonnes with 6 load cells may be tested each load cell with 10 tonnes weights.
5. Linearity Test
To test the entire weighbridge capacity with reference standard test weights along with dummy loads.