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Gravity compensated scales (onboard ship scales)

Load cell scales measure weight, i.e. the force exerted on the object being weighed. Therefore if the value of g varies then so does the weight of our object.

Normally scales do not move around, so once they are calibrated at a particular point on the earth’s surface they will continue to provide accurate weighing.

Now consider a scale onboard a ship. Today’s large fishing vessels have complete processing and packing lines onboard and the weighing scales incorporated in these lines have to work while the ship is in motion. Not only does g vary as the ship changes position, but more significantly, the rise and fall of the ship means that the scales can see changes in g from +0.3 g to -0.3 g in rough conditions. It is clear that normal load cell scales would be unsuitable under these conditions.

Specially designed gravity compensated scales incorporate two load cells. One load cell has the platter connected to it in the normal way, either acting as a single point or as part of a flexure system.

The second identical load cell has a fixed, known mass attached to it. Whatever happens to the value of g , the system knows that this second load cell should always give a constant output.

Therefore by monitoring the change in output from this reference load cell as g changes, the output from the first load cell can be ratiometrically adjusted through the electronics to provide a true weight reading which is independent of the change in g caused by the ship moving up and down.

On an historic note, the first patent on this type of weighing equipment was granted in 1967 to Eric Allison carrying out work for the White Fish Authority in Hull in the UK. Patent No 35457/67 describes an electromechanical weighing device subject to movement. Between 1969 and 1971 the first prototype of a gravity compensated scale, developed by Hunting Engineering, was tested onboard ‘The Corialanus’. Unfortunately, through lack of funding at the time, the project did not progress. It was not until the 1980,s that significant advances were made and products became commercially available.

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