Thursday, 17 April 2014

So how accurate is your system?

There are some very confusing phrases used to describe accuracy of weighing systems. 

Sometimes deliberately to hide the fact that a machine isn't actually that accurate. What exactly does OIML R60 Class IIII Approved mean for instance? It sounds impressive but it's not a very clear way of describing the accuracy of the system. In fact R60 is a standard that refers only to the load cells used not the whole machine

Our dynamic axle weighbridges for instance are certified to OIML standard R134, Class I and Class 0.5 depending upon what they are being used for. The R134 standard is a much better indication of how good the weighing machine is as it refers to the system's overall accuracy. But even then just simply quoting what appear an impressive set of numbers doesn't tell the whole story.

You could buy a system with the most accurate load cells on the planet but if it's been installed incorrectly, the system as a whole simply won't weigh accurately. But if the whole system has to achieve a required standard after a test on site then you can be sure the accuracy is right.
The most accurate axle weighbridge in the world.

Something else some companies do that sounds impressive is to promise a factory deadweight test to capacity. Given that the approach concrete for a dynamic axle weighbridge is a vital part of the system which has a direct bearing on the accuracy, testing a system at the factory is only a small part of ensuring accuracy.

We visited a customer a while back who had bought an axle weighing system from a competitor. Despite only weighing 2-axle rigid vehicles, the easiest to weigh accurately, and the original manufacturers changing the load cells and weight display completely, the system was always inaccurate.

However, with our specialist knowledge it took us about 30 seconds to establish what was causing the problem and suggest the solution. And it was nothing to do with the accuracy being claimed for the system which we are sure was correct when it left the factory.

Most companies quote accuracy as a percentage. But a percentage of what? The capacity of the machine is the answer. So a system with a 40,000kg capacity and a 1% accuracy could be as much as 400kg in error and still be within specification. Or potentially 800kg when weighing a 2-axle vehicle.

Usually, VOSA will take action on overloads of 10% or more. But they use the gross weight of the vehicle to calculate their percentage. So a 3,500kg van should not be more than 350kg over that limit. 

So an axle weighbridge with a 40,000kg capacity and 1% accuracy could advise that that van is legal when in fact it is on the brink of a prosecution.

And what of our R134 Class 1 and Class 0.5 accuracy? They mean that our system can weigh to within 0.5% or 0.25% respectively depending upon the application.

And with a capacity of 15,000kg, more than adequate for the largest road going vehicles, that means we would weigh your 2-axle van to within 75kg or 37.5kg. Making our system the most accurate of its type in the world and ensuring your vehicles do not leave your site overloaded.


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