Double weighing
Page 65
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My attention has been drawn to the letter from Mike Pearce (CM, February 19) and I should be most grateful if you would allow me an opportunity to offer some observations on his comments.
While I have every admiration for the technical consideration given to the subject of "double weighing" by Mr Pearce I would respectfully suggest that he is hardly qualified to decide whether "double weighing" is a valid method of weighing or not.
Now, having said that, I do not wish to discount much of what Mr Pearce has said or to regard it as tommyrot—far from it. I do feel, however, that it is important that the system of weighing motor vehicles, under circumstances commonly known as "double weighing", should be properly understood and correctly related to the use of road weighbridges.
Over many years, the changing lengths and carrying capacities of road transport vehicles have meant that weighbridge platforms were frequently unable to accommodate the new and longer vehicles and, since the installation of new weighbridges failed to keep pace with this changing pattern, the practice of "double weighing" vehicles developed.
Where a vehicle is too long for the weigh platform of a weighbridge, or too heavy for the machine to accommodate, the practice of "double weighing" involves weighing the vehicle in two operations—first the front half and then the rear half. The sum of the results of these two weighings provides the weight of the vehicle.
It is very important to recognize that the weighing objective in this dual weighing operation is to obtain the total weight of the vehicle and, admittedly under these conditions, the road surface approach levels to each end of the weighbridge platform can, and does, affect the accuracy of the summated weight of the two weighings.
In recent months, however, the description "double weighing" has been applied by many people to the routine method of using special "axleweighbridges for the purposes of enforcing the overloading provisions of road traffic law. The weigh platforms of these special "axle" weighbridges are of a limited length and are designed purely for the purpose of enabling enforcement authorities to check the "weight transmitted to the road surface" by the individual axles of goods vehicles. This operation has a visual similarity to "double weighing" in that the whole vehicle cannot be accommodated on the weigh platform and, recently, considerable prominence has been given to the incidence of error which this pattern of weight is reputed to introduce.
In these circumstances the weighing objective is not to weigh the whole vehicle but merely to weigh one axle at a time, each weighing being a distinct and separate weighing. Ideally, for these weighings to be free from criticism, the approach surfaces to each side of these weigh platforms should be as level as possible.
Now, if one accepts the premise of Mr Pearce's academic technical exercise, one would reject any suggestion that "double weighing" procedures could be acceptable even if the approach surfaces were free from any errors in level, and the same reaction would apply to the use of "axle" weighbridges with level approach surfaces. It is on this particular point that I feel Mr Pearce's comments are open to criticism for the following reasons.
Weighing equipment can only be as accurate as its design and construction will permit and, even for the purposes of Weights and Measures law, weighing machines are regarded as legally correct if the weighing error, under test, falls within specified tolerance limits.
It is possible for a 30-ton road weighbridge to be regarded as correct for legal purposes even though, when tested by an inspector with standard test weights, it reveals an error of more than 50Ib. The real point behind my reference to these tolerance limits is to emphasize that accuracy in weighbridge weighing is not the precise accuracy which forms the basis of Mr Pearce's theorizing.
The normal use of weighbridge equipment involves an acceptance of weighing error which is due to, and governed by, mechanical construction factors. I want to suggest that the incidence of errors introduced by doubleweighing procedures, or the operation of "double weighing", is affected by the fact that the road approach surfaces to the weigh platform are not truly level then, surely, the extent that the -out of level". condition does affect the true weight should be established in practical terms to see whether it can be regarded as an acceptable error or not.
No detailed research into the incidence of error due to "double-weighing procedures" has, to my knowledge, been carried out since the last war but this pattern of weighing has been with us for a very long time and a very thorough and expert inquiry was carried out sometime before the war to see what errors could arise from a variation in road surface approach levels to the weigh plate of a road weighbridge.
These tests were carried out by a Chief Inspector of Weights and Measures employing very precise methods on a weighing machine which had been specially prepared and tested for accuracy.
The inquiry took account of two main variables: a) The effect, in +in. incremental stages, of variations in level from lin. to 5in. (10 separate level conditions).
b) The effect of changes in the height of the load over the axles of the vehicle—the range taking into account of 10 height variations from 15in. to a maximum of 42in. above the axle.
Reference to the results of this specialized inquiry reveals that the greatest error occurred when the approach level was 5in. in error and when the height of the load above the axle was at the maximum height of 3ft 6in. Under these extreme circumstances the error amounted to 57Ib per ton of load on the weighbridge platform—an error of approximately 21 per cent. Lesser proportions of "out of level" condition produced significantly lower errors.
Few modern weighbridges, and certainly none of the axle weighbridges installed by the Ministry or by local authorities, have approach surface errors anywhere approaching this magnitude. Indeed, the specification for most axle weighbridges is very precise indeed and calls for correctly level approach surfaces extending to a distance of 40ft on either side of the weighplate and finished to good surface standards.
If one accepts and relates the results of this inquiry to double-weighing operations, carried out under conditions where the approach road levels are not significantly in error, then it seems reasonable to suggest that any weighing errors disclosed would be minimal and could be virtually discounted.
Criticizing these weighing procedures can be a very stimulating academic exercise so long as it does not get so far out of perspective as to suggest that errors exist which may not be present. Much of the enforcement of the overloading provisions of road traffic law is the responsibility of Weights and Measures officers who, I would suggest, have a special expertise and wide experience of all matters associated with weighing equipment. It goes without saying that every attention is given to the accuracy and reliability of all enforcement weighing procedures and to the approach levels of road weighbridges. Inspectors are purists in the mechanics of weighing goods and vehicles and, Mr Pearce can take heart in the fact that, in general, most of them would be opposed to "double weighing" any vehicle where the necessity for this procedure could be avoided. Nevertheless, the above comments would govern any use of "double-weighing" methods and would influence the acceptance of the results obtained thereby.
R. E. PARKINSON, Chief Inspector, Weights and Measures Department, Devon County Council