QA advice and best practice for the correct selection of calibrated test samples for your application of inspection systems according to BRC, EFSIS and HACCP.
A test piece is just a test piece?
Not exactly – just like inspection systems one size really doesn’t fit all. After all you haven’t invested the time and resources into inspection capital equipment to detect the presence of a test piece. You need to validate the fact that the inspection system will keep your products free from foreign body contamination.
Contaminant type and shape
If you look at the test pieces you are using now, you will notice that the contaminate embedded is a sphere and the diameter is shown in mm. Most metal detectors work on the principle of a balanced coil, as contaminates and product passes through the system it unbalances this arrangement.It is this unbalance that determines whether the contaminate will be detected. As well as the material type the cross sectional area of the contaminate plays the most significant part in this. That’s why we use spheres – no matter what orientation the test sample enters the metal detector the cross sectional area will not vary.
The most common materials used to test inspection systems are Ferrous (Fe) iron, Non Ferrous (NFe) typically brass, and stainless steel (SS) typically non magnetic 316 grade.
Unfortunately in the real world the most common contaminants will not be spherical, metal is most likely to be in wire or shaving form. Remember the metal detector works on cross sectional area? You could in theory get a long thin “needle” type contaminate – if it was only 1mm in diameter and your metal detector detection limit is 2.0mm it could be 12″ long and you still may not detect it.
X-ray systems operate differently, they work on density, the amount of energy absorbed by the contaminate together with the shape determines detectability. You could have a 1mm thick blade and if your X-ray is set to detect 2.0mm you may miss the contaminate even if it was 1″ in diameter if it was to lay flat perpendicular to the X-ray fan shaped beam. When selecting glass contaminates for test samples – care should be taken to understand the type of glass sphere being used. Density varies greatly from one glass type to the next. Commonly Soda Lime Glass (SLG) is used as it provides a semi standard. If you package your product in glass jars, the most likely glass contaminate is probably going to be your jar glass, following a breakage etc. it may be worth considering having custom samples made that use your glass type.
Contaminant Size Used for Testing
Most commercially available test samples use calibrated spheres of different materials. When you select your size specification to test with it is important to choose one that is reliably and repeatably detected. Too small and the inspection system may miss it, too large is not a representative test. As a rough rule of thumb if you have a wet or salty product (reactive) and your metal detector will detect 2mm Ferrous, it will be around 1.5 times this for Non Ferrous so 3mm. Double the ferrous for stainless steel (as its non magnetic and non conductive) so 4mm approx.
If you have a dry product – such as grains, or deep frozen product the Ferrous and Non Ferrous will be the same and the stainless steel approximately 1.5 times this.
X-ray systems are not affected by the frozen/thaw state of a product, working on density (X-ray absorption) so Ferrous, Non Ferrous and Stainless Steel will be the same size. Hence most X-ray systems are only tested with Stainless Steel and Glass. As mentioned earlier different glass has different density so ensure you know the type of glass in the test sample.
Your product and application will affect the size of contaminants detected. When you write up your HACCP procedure ensure you are using the correct size contaminates. Your metal detector or X-ray system provider will be able to assist with determining the sensitivity of your system. Thereby establishing a realistic specification. Your inspection sample provider should also be able to assist you with setting sensitivity and arranging routine testing procedure and annual calibration.
Calibrated Test Sample Format
As mentioned earlier the purpose of your inspection system is to detect unwanted foreign bodies in your products – to ensure final product quality for your consumers. Laying a test piece on the belt of a metal detector may be a repeatable test, but do you know how that relates to real world metal detection? Consider your reject type and ensure a contaminated product can be rejected positively, so consider the position of the test sample.
You should consider a test piece format that allows you test your system in a repeatable, representative manner. For example if it is a free flowing product the sample needs to move with your product. If it is a conveyor based system with a pneumatic or mechanical reject then ensure that the contaminate can be located anywhere within the packaging and still be detected and rejected. On the subject of location – product itself can shield the effects of the contaminant. A 2mm ferrous test piece that is easily detectable on the outside of a block of cheese may not be detectable at all inside. Consider inserting the test piece so that the contaminate sphere is embedded in the product.
Do not use plastic test pieces on X-ray systems – the way the contaminate spheres are embedded makes it far easier for the X-ray to “see” them. The image analysis software is looking for density differences and an area of high absorption (contaminate) surrounded by low absorption (plastic test piece) makes it highly detachable, even if embedded in the product. A real world contaminate would most likely be surrounded by higher density product. Use flat X-ray test cards to test X-ray systems they are supplied in flat card form to minimize the effect of the contaminate “holder”.
The center of a metal detector aperture is the least sensitive part, closer to the lining (top, bottom and sides) is more sensitive. Consider this when you determine where to place the contaminate for your routine testing. Make sure you can recover the test piece following the test, you don’t want to cause a contaminate problem with testing. If you make up test packs make sure they are clearly marked and do not get mixed up with good product.