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Weighbridges play a vital role in today’s agricultural environment. They provide valuable weight data for a wide range of activities including the movements of crops and feed, checking crop yield, validating delivery quantities of product (such as animal feed and fertilisers) and helping manage crop storage facilities.

Weighbridge design

Essentially, weighbridges are very large platform scales consisting of a platform, or ‘deck’, supported on a number of load cells (typically six or eight), usually fitted in self-aligning mounting units.
There are two basic designs of weighbridge – pit-mounted and surface-mounted.
Pit-mounted weighbridge decks sit flush with the ground within a concrete foundation pit. For surface weighbridges the deck structure and load cells sit above the ground, utilising steel or concrete ramps for vehicles to access the raised deck. The size of the deck can vary considerably, ranging from smaller 7 m long up to 18 m or larger. Deck widths typically vary from 3.0 m up to 3.5 m. The most common sizes used in agriculture are typically 15 m x 3 m and 18 x 3 m.

There are three essential construction technologies used for the main deck structure:

1. All steel welded
2. Steel – concrete composite (pour on site)
3. Pre-stressed concrete

Steel construction tends to be the most common, but the choice will depend on the particular application. Steel composite weighbridges are convenient in some applications, especially if there is limited access to be able to lift in a finished steel weighbridge. Their design consists of a steel frame with a light base and reinforcing mesh which is placed in situ on dummy load cells and then filled with ready-mix concrete. Once the concrete has set, the dummies are then replaced with the live load cells.

Load cells

Load cells are electro-mechanical devices (essentially very low deflection metallic springs fitted with strain gauges), which convert applied load (e.g. the weight of the vehicle) into an accurate electrical output. For simple applications, this output, which is directly proportional to the load on the weighbridge, is then amplified, conditioned and displayed as a weight reading via a weight indicator. There are three main types of weighbridge load cell:

1. Canister load cells
2. Single beam load cells
3. Double ended beam load cells

Field results show that stainless steel canister load cells provide the best performance and reliability.
Today, both analogue and digital load cells are available, with the latter providing a number of operational and maintenance advantages. Improvements in sealing levels for load cells mean that modern devices meet the stringent IP69K pressure wash-down requirements.

Calibration

Once installed, weighbridges need to be commissioned and calibrated to ensure the displayed weight reading on the weight indicator directly corresponds with any weight placed on the weighbridge – in other words quantifying that the output is linear and accurate over the weighing range of the weighbridge (e.g. from 0-50000 kg). A typical 50,000 kg weighbridge will display in 20 kg intervals. In order to achieve this accurate calibration, certified calibration weights are placed on the weighbridge in a pre-described method and the output checked. This procedure checks the linearity, hysteresis and corner loading performance.
(Hysteresis is a measure of the correlation between increasing and decreasing loads.)

Software

In a growing number of agricultural applications, weighbridges are becoming more than just large scales for weighing vehicles. User-friendly software and driver operated terminals are bringing important time-saving automation to busy farms, crop storage facilities and waste from energy installations.
Additional peripheral equipment including traffic lights, barriers and vehicle recognition systems can be controlled via the software. This not only speeds up weighing procedures, but provides the invaluable integration of weight and vehicle data with management software systems.

Conclusion

With the wide range of available weighbridge designs, instrumentation and software, it is important that potential buyers seek expert advice and support. Parkerfarm offer a free site visit survey to ensure you receive the most cost effective and reliable weighbridge system for your application. For more details click here.