It has taken the x-ray inspection industry several years to repair the damage wreaked by a minority of suppliers who oversold the technology, claiming it could detect all imaginable contaminants and replace the checkweigher on production lines.
Even today, although most of the disillusionment around x-ray systems has dissipated, Loma Systems UK sales manager Tony Bryant says he is still occasionally asked by prospective customers whether x-rays can give the same level of weighing accuracy as checkweighers.
“The simple answer is that no, x-ray systems cannot deliver the same level of accuracy,” says Bryant. “Take a 100g pack; a checkweigher will typically deliver accuracy of between 0.2 and 0.5g, whereas with an x-ray, accuracy will be in the region of 5-10%.”
Some of the confusion around x-ray’s suitability as a weighing device probably stems from its ability to carry out product integrity checks in addition to identifying contaminants.
“As well as precision detection of foreign bodies, such as metal fragments, glass, mineral stones, calcified bone, high density plastics and rubber, x-ray systems are capable of performing a wide range of product integrity checks such as identifying broken or missing components in food and pharmaceutical products. All of these quality checks are carried out simultaneously at high speeds,” says Neil Giles, marketing communications manager at Mettler-Toledo Product Inspection.
As an example, an x-ray system can be used to ensure consistent fill level across all of the pots in a yogurt multipack, identifying packs that contain either under- or over-filled pots and removing these pots from the line. To understand why an x-ray will not provide precise weight data, it is necessary to look at the principles on which it operates.
The fundamental difference between an x-ray and a checkweigher, as Colin Maher, product manager inline inspection with Sartorius Industrial Weighing, points out, is that x-rays don’t actually weigh products. “Weight is a determinant of mass. X-rays determine volume by density,” he says.
X-ray weighing is achieved by inspecting the density of the product via a grayscale image. The equation of density equals mass over volume becomes the starting point for evaluating the mass (weight) of the object. Sophisticated software algorithms are then applied to the product being inspected to produce a weight reading and production statistics.
Checkweighers, on the other hand, rely on the force of gravity exerted by an object on a weighing platform. Checkweighers weigh dynamically and the scale is in the form of a conveyor that transports the product from one end of the machine to the other. This conveyor is typically mounted on a loadcell which deflects or moves in proportion to the ‘weight’ of the object. Heavier weights cause more of a deflection than lighter weights and this deflection is converted into an electrical signal, which is processed by an on-board computer. The weight is then displayed.
It follows, therefore, that x-ray will only really be accurate if the product is uniform and homogenous throughout because the mass measurement is taken from the greyscale image. “You need a homogenous product like butter to get an accurate measurement; you wouldn’t be able to determine the density of a box of cornflakes with any accuracy using an x-ray,” says Maher.
But when the product is uniform and homogenous, Chris Keenan, sales manager at Selo UK, claims x-rays can sometimes provide more accurate results than a checkweigher. “Product of a consistent shape and density, such as blocks of cheese, are ideal for producing accurate and reliable results using x-ray technology,” he says.
Regulatory issue Even if a product does lend itself to ‘x-weighing’, there are other barriers which prevent producers from doing away with an end-of-line checkweigher, not least the regulatory situation. “In most countries, you need a checkweigher to satisfy weights and measures regulations. All products sold by weight and carrying the ‘e’ mark have to be weighed by a checkweigher or a static scale,” explains Torsten Giese, marketing manager with Ishida Europe.
The reason for this regulatory position goes back to the distinction between the way x-rays and checkweighers work.
“Because x-rays determine weight by measuring the mass, rather than using a weighcell, they do not comply with e weight legislation, whereas checkweighers use a weighcell that’s regularly calibrated and complies with the necessary directives,” says Chris Keenan. “To summarise, x-rays cannot be used for weighing retail products that choose the e weight logo, but in some cases, can be reliably used for weighing products for internal use.”
And Loma’s Tony Bryant doesn’t think the legal position is ever likely to change, saying: “Trading Standards will never be converted to thinking that a piece of software could take the place of a conventional weighing device.”
Regulatory restrictions aside, another barrier to relying on x-ray for weight readings is speed. Giese estimates that an x-ray would be 30% slower than a checkweigher at calculating a reading. And with some checkweigher suppliers investing R&D resources in maintaining weighing precision at higher throughput speeds, this speed gap isn’t likely to close any time soon.
“Advancing checkweighing technology to maintain weighing precision at higher speeds has been achieved via the use of real time control systems able to process information from the weigh cell more rapidly than ever before. This increases weighing frequency, or the number of measurements taken by the system as the product passes over the weigh cell, which enhances the precision of the final weight,” says Mettler-Toledo’s Giles.
The increasing realisation among producers that checkweighers are an unexploited source of production data will also help assure their position on the production lines of the future.
“There is a lot of data that can be extracted from a checkweigher that can provide valuable information from a production point of view. This information is being used more than ever to determine line efficiencies and data capture,” says Selo UK’s Keenan.
His observation is shared by Colin Maher, who estimates that 80% of checkweighers sold by Sartorius are linked to data capture systems, and says Sartorious ‘hasn’t sold a tallyroll printer for five years’. Ishida’s Giese is also excited about the possibilities that exist for his company’s Data Capture System (IDCS). “It is possible to link checkweighers into a factory’s efficiency measurement software and measure line speed, downtime and rejects, and from that you can determine how efficient that line is,” he says.
He does, however, admit that at present, these capabilities are being under-utilised by UK companies, saying: “this is definitely an untapped source of efficiency gains.” For the moment it seems data capture isn’t driving checkweigher sales any more than x-weighing is driving purchases of x-ray systems.