What Blocks A Metal Detector?
Electromagnetic Field Interference
At the heart of every metal detector is the principle of electromagnetism. Essentially, these devices generate an electromagnetic field, typically through a circular accessory known as a search coil.
When this electromagnetic field comes into contact with a metal object, it induces electric currents within that object, known as eddy currents.
These eddy currents generate their own electromagnetic fields. The detector senses these secondary fields and reacts by producing a signal—often a beep or a buzz—which alerts the user to the presence of a metal object.
Metal detectors detect the current induced in a metal object by their changing magnetic field.
Any shield, would be in itself inherently magnetic when placed in the field of a detector.
Anything that conducts electricity or reflects or conducts or absorbs magnetic fields will be detected. Anything that interacts with the detector's magnetic field in any way will trip a metal detector.
Foil Packaging
- Foil acts as a conductive layer that interferes with the electromagnetic field of a detector.
- Traditional metal detectors rely on detecting disturbances in this field — so the foil packaging essentially “blinds” them.
- Metal detectors, the detector identifies the foil as a metal contaminant.
- That’s why many manufacturers struggle to detect real foreign objects inside such packaging.
- Some materials can interfere with the readings the machine detects.
- This is usually enough to change the reflections enough to distort the size/shape/density of the object, but not enough to completely block the return signal or make the object appear transparent.
The machine can see that something is there, but may not be able to show a clear picture of the shape or probable material it’s made from.
For multi-layer packaging or aluminum foil products, specialized detector of metal systems are essential.
Mineralized Ground
- Electrically conductive minerals
- These materials can come from burning mineral fuel leftovers, such as slag, fuel coke, and clinkers. These minerals typically have a wide signal.
- For this reason, they can mask the signs that gold nuggets release, making it difficult for gold detectors to find their targets.
- Soil minerals
- Oxidation can alter the ground balance in areas with an intense fire. As a result, it can be more difficult for gold detectors to detect their target.
- A common term in metal detecting is hot rocks, it designates soil that has high iron and mineral content.
- This plays havoc on a metal detecting requiring ground balancing and sensitivity tuning.
- Mineralized soils
- The type of soil or medium also plays a role; mineralized soils can interfere with detection.
Metal Properties
- Firstly, ferrous metals contain iron and are strongly magnetic, making them easier to detect.
- Common examples of ferrous metals include iron, steel, and nickel.
- Non-ferrous metals, on the other hand, lack significant magnetic properties and are more challenging to detect.
- Some examples of non-ferrous metals are copper, aluminum, brass, and lead.
- Stainless steel has low magnetic permeability, meaning it doesn’t allow magnetic fields to pass through it as easily as other metals.
- While many non-ferrous metals, such as gold, copper, and aluminum, can be detected, they might not be as easily discernible as ferrous metals, which contain iron.
The detection capability largely depends on the metal detector’s frequency, calibration, and the metal’s size and shape.
Aluminum conducts electricity so it creates a magnetic field.
Metal detectors do not find metals using magnetism. Instead, it utilizes electrical conductivity.
Depth, Size, and Conditions
Deep soil and distance can prevent detection.
It’s also worth noting that a metal object’s depth, size, and orientation can affect its detectability.
Suppose the gold is buried at least 8 to 10 ft. In that case, it will be more difficult for the metal detector’s receiver coil to receive its electromagnetic field.
Gold that is buried relatively deep is difficult to find.
Detection capability largely depends on the metal detector’s frequency, calibration, and the metal’s size and shape.
- Product temperature or moisture
- Metalized or foil packaging
- Vibration or electrical noise in automated production systems
Several factors can impact how well a metal detector performs.
