# Accuracy of weighbridges. Not as good as you might think



## Chudders (Jul 5, 2008)

Have recently weighed the motorhome a couple of times at different weighbridges. ( I am paranoid about being overweight) I noticed that the weights always came out at relatively round figures. Last time 3700 Kgs. (Not for example 3712 etc) So I enquired of the operator why this might be. I was informed that the weighbridge rounded the figure automatically to the nearest 20Kg,s. I also enquired of the previous weighbridge and was told the same.
Now I was confused by this so I have spoken to Trading Standards, weights and measures and directly to the man who does the certifying and checking the accuracy of weigh bridges and also carries out roadside checks with VOSA and the police.
I am informed that digital weighbridges fall essentially into two categories. Those that work to 10 divisions of accuracy and those that work to 20 divisions.Most digital weighbridges in this area work to 20 Divisions of accuracy (Cornwall) What is a division, it is 20Kg,s (Hence the rounding of the figure) They are legally allowed up to 3 divisions of inaccuracy, so that is 60Kg,s on a 20 division weighbridge (10 on a 10 Division weighbridge. 30 Kg,s) The old analogue weighbridges used to give the reading shown on the dial so would not round the figure. Digital ones, which are now most of them are allowed to work in this way.
He did say that most modern weighbridges are pretty accurate and he rarely finds them with anything like the maximum permitted discrepancy but they do automatically round to the nearest 20Kg,s. He advised to use weighbridges that look clean and well maintained and which are regularly used. Heavy rain can affect the weighing mechanism not only from standing water on the platform but also what is known as 'floating' of the mechanism underneath. Operators should check this. Some have automatic pumps to prevent this.
So that was an interesting discussion from the horses mouth so to speak. When you get weighed the figure you are given may not be as accurate as you thought it might be. I recommend you ask if their weighbridge rounds the figure to the neares 20Kg,s I think you will find that they all do from what I understand. Have a look at you weighbridge certificates and see if they appear to be nice rounded figures, if so that,s the reason why. The accuracy or inaccuracy does of course have to apply over the entire range of the bridge. For example many weigh up to 50,000Kg,s, some more, so the inaccuracy is likely to be proportionately a lot less at a lower weight such as a motorhome but it does still round the figure.
Regards, David.


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## H1-GBV (Feb 28, 2006)

ALL digital devices are unreliable with their final figure, including, for example, my voltmeter. I always assume it could be one digit either way.

With a 2 000kg motorhome, 20kg would mean an imprecision of 1%, with a 50 000kg vehicle, it would 0.04%. Hence "better" for the heavier vehicle, not for the light motorhome.

Either way, an analogue weighbridge would probably not be read to that degree of precision. Not being aware of how strictly the regulations are enforced, I would be surprised if VOSA worried too much about you being 60kg over on one of their bridges. 160kg might be a different story! I MUST get mine weighed soon.


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## Chudders (Jul 5, 2008)

Just noticed, how did I get this topic under 'France Touring' must learn how to use the website . what have I done wrong. Can it be moved. Sorry to everyone for not getting it in right place
David.


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## Pudsey_Bear (Sep 25, 2008)

It'd be interesting to know how much leeway Vosa actually allows you, I know they allow 5% for water in the load of a truck, and some MH's have a small lake on top when it's raining.

Kev.


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## Chudders (Jul 5, 2008)

Kev_n_Liz said:


> It'd be interesting to know how much leeway Vosa actually allows you, I know they allow 5% for water in the load of a truck, and some MH's have a small lake on top when it's raining.
> 
> Kev.


I believe I was told that they must allow at least the tolerance of a weighbridge where you may have got your vehicle weighed. Also as H1-GBV has pointed out the percentage of inaccuracy is perhaps not too great. On my weight of 3700Kg,s 20Kg,s is only about 0.54% ( I think ! !)
David


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## Pudsey_Bear (Sep 25, 2008)

I borrowed a weighbridge on Sky over the bank hols (Mol road, Sligachan end) I was 30kg over, so I've taken more than that off since getting back, it was mostly in the garage, I'm also going to move the Spare under the fixed bed, to take a bit more off the back axle, should give me a bit more payload.

Kev.


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## Chudders (Jul 5, 2008)

Chudders said:


> Just noticed, how did I get this topic under 'France Touring' must learn how to use the website . what have I done wrong. Can it be moved. Sorry to everyone for not getting it in right place
> David.


Thanks for moving this post of mine, I promise to buck up my ideas and post correctly in future
Sorryyyyyyyyyy
David


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