Ahhhh Metal detection the black art of food production.
Look at a metal detector as a difference detector, it is not capable of finding metal it finds a difference between the settings and your product.
Metal detectors have a few basic settings depending upon the manufacturer but all are essentially the same.
Sensitivity, threshold: These are the trigger points for the contaminant or test sample.
Phase angle, compensation: These are the settings for the product.
How to set up:
The detector will probably have an automatic calibration feature which you should follow. This is normally a good start point. When calibrated you should then look at the signal produced by the product each time it is "scanned" by means of the display. This is either represented by a number or scale of led's. When the product produces little or no effect then the product is tuned out.
Next you can look at the sensitivity or threshold. Pass the product through the detector and introduce your contaminants in 0.5 mm increments to find which sample size works for the product. Do this for each type of metal, Fe, NFe and Stainless 316 or 304. You can then determine the repeatability of the system by watching in production and checking whether your product is consistent. Inconsistency in the product will produce false triggering which will require further tuning. A differential between product and product plus test sample must be achievable or else you will get a system that rejects good product.
Why do you get differing results for different products?
A metal detector is looking for either a conductive or magnetic effect from the product. A good conductor is something like gold silver or copper but another good conductor is salt and water. Since your product is made up of both of these then you have a product effect which requires attention. Product effect and the non ferrous and stainless samples have similar signals as each other so this is where your woes begin.
In the UK the supermarkets produce a requirement for performance based upon the aperture size of the system but sometimes this is not achievable. I will dig out the table for reference for you later.
Kind regards
Brian