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Metal Detection 101

Jeff Herne, Product Manager,

Advanced Detection Systems Mining

and Aggregate Division

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Metal Detection 101

Understanding that mysterious machine you call a (!@#%$%!) metal detector.

Gustave Trouvé invented the first metal detector in 1874, and Alexander GrahamBell developed a portable metal detector in 1881 to search for the bullet inPresident James Garfield’s body (he was unsuccessful). The first standardized useof metal detection was in 1919 after World War I, searching for unexplodedordnance.

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Metal Detection today has developed into both a hobby and a science,

and although the applications are widely varied, the principles behind the

science are the same. In our world, industrial metal detection first took

hold in 1947 in the food industry, and in the aggregate world in the 1970s.

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Things to consider and remember: KNOWLEDGE

• Metal detection will never be 100% perfect.

• Metal detectors must be maintained like any other piece of

equipment.

• Metal detectors should be used in conjunction with other

equipment, specifically magnets.

• Metal detectors, no matter how complex, will have

performance limitations.

• A bargain-priced metal detector will inevitably create not-so-

bargain-priced repairs.

Understanding the Machine

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Balanced field – tuned like a radio to a specific frequency.

Introduce metal and the result is chaos.

Product Effect

Product Effect is the magnetic and conductive properties of the

rock on the conveyor. Product effect must be tuned out in order

for the metal detector to achieve optimum performance.

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Understanding Product Effect

• The magnetic and conductive properties of the

material upset the magnetic field.

• A properly calibrated detector will factor product

effect into the balancing equation, effectively ‘tuning

out’ the signature of the material on the belt.

• It is physically impossible to detect a contaminant

whose value is less than the product effect of the

material on the conveyor.

• The more constant the product effect, the more

sensitive the metal detector can be.

No one said we’d be doing math!

Sensitivity value + Product Effect + Contaminant = Detection

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Contaminant orientation

How a contaminant is oriented when it enters the magnetic field

is demonstrated perfectly by these two “Olympic” swimmers. In

this instance however, a smaller splash means no detection,

whereas a larger splash makes contaminant detection much

easier.

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Metal Detector Installation

Things to remember - installation

• Location, location, location. Magnet, metal detector, waste gate/reject.

• 24 inches of metal-free zone in each direction from centerline.

• Non-metallic rollers, new bearings.

• Vibration mountings.

• RF interference, ambient noise.

• Proximity of high-voltage lines and cables.

• Install aperture low enough to prevent a loaded belt from scrubbing the

bottom of the plate, and overburden from hitting the upper plate.

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OUCH

• No protection

• Downtime

• Repair costs

• Man-hours lost

• Revenue lost

• Potential Damage?

• Were we

chasing

sensitivity?

As the aperture height increases –

sensitivity decreases!

Bigger is not always better – plan accordingly when sizing your detector!

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Conveyor shroud – a big electrically charged blanket

that will suffocate your metal detector…think about it!!

Can you spot the problems?

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“My metal detector goes off constantly, even with no material on

the belt, I think its defective. It also doesn’t detect small stuff.”

Don’t be overwhelmed by your metal detector.

A former US Air Force pilot told me his F-15 was easier to understand than his metal detector…

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• Monitor your variables and adjust your detector accordingly.

• Metal detectors are not “fire and forget”. If your product changes, chances are

your metal detector settings will require changing, too.

• The smaller the aperture (opening) of the metal detector, the more sensitive it will

be. Think of the effects of dropping a golf ball into a bowl of water, versus

dropping a golf ball in a pond…

• Your metal detector can’t distinguish between a ½” mill ball and a crushed soda

can – they both project similar magnetic and conductive profiles.

• The smaller your sensitivity requirement, the more susceptible to false trips, and

nuisance trips your detector will be.

• Your detector should always be your last line of defense against damaging metal.

• If something DOES get through, it might not be the detector’s fault (or my fault).

Questions?

Thank you for

attending!