Industrial metal detectors are used in the pharmaceutical, food, beverage, textile, plastics, chemicals, and packaging industries. Contamination of food by metal shards from broken processing machinery during manufacture is a major safety issue in the food industry. Metal detectors are not new to food processors. Metal contamination has always been a major problem. Sources include mechanic's tools and pieces broken off equipment, tags and staples carried into the plant with ingredients, and personal effects like jewelry and pens. Companies have installed systems in their plants for years to protect expensive production equipment from damage and to produce metal-free finished products .
Why should a food processor consider a metal detector?
Factors that affect sensitivity :
How a metal detector works?
Rejection mechanisms
• Product safety
• Equipment protection
• Contract (customer) requirement
• Regulatory compliance
Metal detectors are installed in three basic configurations: pipeline, conveyor and free-fall. Pipeline detectors are used for any product conveyed in tubes or pipe.
SENSITIVITY:
Sensitivity usually refers to the smallest sphere of a particular metal that the unit can detect as the sphere passes through the center of the metal detector aperture. The sensitivity of a metal detector is adjustable, and the user generally sets it to the largest setting that will detect the expected contaminants.
Many factors affect sensitivity:
TYPE OF PRODUCT. :
Dry products, such as sugar, salt and cereals, are relatively easy to inspect and generally do not pose any unusual problems. Wet products, on the other hand, create an interference signal in the metal detector that will show up even when no metal is present. The signal is caused by salt or acid that makes the product electrically conductive, and the detector's electronics must be able to eliminate or at least reduce it for the detector to perform satisfactorily.
TYPE OF METAL CONTAMINANT. :
Ferrous metal, which is both magnetic and a good electrical conductor, is easy to detect. Stainless steel, particularly the grades used in the food industry, is non-magnetic and a poor electrical conductor; thus, it is much more difficult to detect. Non-ferrous metals, such as aluminum and copper, are non-magnetic but are good conductors; they are generally easy to detect.
SHAPE OF METAL CONTAMINANT.:
As long as the smallest dimension of the metal contaminant is larger than the spherical sensitivity of the detector, shape is not a concern. However, for thin pieces of metal, such as a piece of wire where the diameter is smaller than the spherical sensitivity of the detector, the orientation of the piece in the product will affect its likelihood of detection. For example, ferrous wire is easiest to detect when it is in line with the direction of movement and most difficult to detect when it is across the direction of movement. Stainless steel and non-ferrous wire are just the opposite.
HOW TO DECIDE THE APERTURE DIMENSIONS.
A small aperture is more sensitive than a large aperture, and the sensitivity increases from the center to the corners. Therefore, for best performance, the aperture should be sized as close as possible to the dimensions of the largest product that will pass through it.
What can be detected
All metals are either ferrous, nonferrous or stainless steel. The ease of detection will depend on their magnetic permeability and electrical conductivity. The size, shape and orientation (with respect to the detector coils) of the metal particle also is important. Since size, shape and orientation of metal contaminants is not possible to control, it is best to operate a metal detector at the highest possible sensitivity setting.
How a metal detector works?
Most metal detectors use a balanced, three-coil, system to detect small particles of non-ferrous and stainless steel. The coils are wound on a non-metallic frame, each parallel with the other. The center coil is connected to a high frequency radio transmit¬ter. Coils on either side of the transmitter coil are receivers. As these two coils are identical and placed the same distance from the transmitter, they receive the same signal and produce an identical output voltage. When the coils are connected in opposition, the output is cancelled, resulting in a zero value. A schematic of the coil configuration is shown in figure 1.
When a particle of metal passes through the coils of a metal detector, the high frequency field is disturbed under one coil, chang¬ing the voltage by a few microvolts. The state of balance is lost and the output from the coils is no longer zero. It is this phenomenon that is used to detect metal.
Rejection mechanisms
The purpose of rejection mechanisms is to remove the contami¬nants from the product/process stream. The mechanism must be designed to remove 100 percent of the detected contaminants along with a minimum amount of salable product. The contaminants are removed and stored in a manner that eliminates any possibility of their being reintroduced into the product or process stream.
Metal detectors should be operated at the maximum sensitivity setting for a given product. The maximum acceptable sensitivity setting will allow the detector to perform reliably for extended pe¬riods of time without excessive false rejects. Scheduled testing of the detector and reject device (with ferrous and nonferrous metal samples) will confirm proper operation.
Metal detectors are an important part of a comprehensive contamination control program. If they are specified, installed, operated and maintained correctly, they will contribute to improv¬ing product quality and reducing losses. A properly used metal detector should not be considered as insurance against metallic contamination. Rather, it is a diagnostic device that can help to discover accidental metallic contamination. Plans and procedures should be in place to prevent contamination. If contamination does occur, contaminants must be identified immediately, traced back to their source and eliminated
