Frequently Asked Questions

Want to know more about installing a metal detector system? Below is a list of common questions/concerns relating to metal detectors, their usage, applications and basic trouble shooting.

Q. Why do we need a metal detector, I always thought they were more trouble than they are worth?

A. Well designed and supported metal detector systems are a necessary part of the production process. They are there to protect expensive equipment in the event of the presence of damaging metal, and should be viewed as an insurance policy that is sure to quickly pay for itself by preventing costly downtime and repairs.


Q. We only get clean sources of material in our mill, so why would we need a metal detector?

A. Clean sources of material do not guarantee the absence of tramp metal in the manufacturing facility. Metal can be introduced in the conveyor through the material or through equipment breakage, clean-up of floor scraps, misplaced tools and employee carelessness, to name a few. A Metal Detector can be a necessary part of the production process and at the very least an insurance policy that is sure to pay for itself.


Q. Do we need a technician from MDI to install this equipment?

A. MDI Metal Detector systems do not require a factory technician present at start-up. Manuals are provided and free technical support is available by phone or email. Electrical or maintenance personnel will have all the information required in the manual. However, a technician is available upon request at the prevailing service rates plus direct travel expenses if requested.


Q. Does Installation require any special tools?

A. No special tools are required. Standard tools readily available at most manufacturing or mill sites are all that is needed.


Q. How do you determine what search coil is used?

A. The coils are manufactured in many sizes and configurations. What is being conveyed, type of conveying system, sensitivity needed, and what equipment it will protect, all play a part in determining which system is best for the application. The experts at MDI will help you determine the right system for your application.


Q. What type of metal detection technology do you use?

A. The technology we use is the Phase Shift or Balanced Coil method. This method reduces the susceptibility of interference, and has the advantage of a certain “filtering capability”. In combination with digital signal processing, the transmitter-receiver principle is the more intelligent, flexible, and accurate method.


Q. Our metal detector trips when there is no metal present, what causes this?

A. This is known as False or Nuisance Tripping and many things can cause this. The most common is intermittent shorted turns. A close second is Radio Frequency Interference (RFI). Lightening and Arc Welding interference fall under RFI also. Variable Frequency Drives (VFD’s) are also a source of false tripping. Sometimes when the metal detector is overly sensitive it will pick up smaller metal than the customer wants to be bothered with. MDI has many years of experience in helping reduce the False or Nuisance Trips caused by RFI and VFD’s through filtering in the Digital systems.


Q. We are considering a metal detector system for our whole log line but there is no room after the debarker, can it be installed before?

A. Yes, although installing a metal detector after the debarker is more ideal. When installed after the debarker, the process of debarking the log would have already removed much of the exterior metal in the bark and will reduce the number of nuisance trips.


Q. What are Intermittent Shorted Turns, or Intermittent Loops?

A. An Intermittent Shorted Turn or Loop is a conductive metal path near the metal detector search coil with an intermittent break in it. This conductive path acts as a secondary energy loop reflecting into the search coil’s electromagnetic field as a load-unload false trip-out signal whenever this secondary loop is connected or broken.

Any structurally connected metal parts or touching metal to parts, such as beams, cross pieces, conduit, pipes, idler rolls etc. may form this Intermittent Loop. To eliminate the effect of this loop, make a permanent solid CONNECTION or a definite BREAK in the loop. Make a weld at the break, or weld a shorting bar across the loop.  If appropriate, place a permanent insulator between the metal (such as touching conduit or pipes).

A visual examination often identifies possible Intermittent Loops.  More than one Intermittent Loop may be present.


Q. What needs to be considered before purchasing a metal detector?

A. There are 4 main areas that should be addressed when considering a metal detector.

1) Rejection of Contaminate:  Plans should be made for how the contaminate will be removed from the conveying line once it is detected.  The two most common methods are using a marking system to reject material downstream, or stopping the conveyor upon detection to remove metal or contaminated material before it moves on.

2) Possible Product Effect:  If the material we are scanning produces a signal that could be detected by the metal detector, then a sample of the material would need to be sent to MDI for testing and evaluation, to determine if a metal detector is feasible.

3) Room to Install Equipment:  You will need room for the fiberglass/non-metallic conveyor section and adequate room to install the metal detector, so that the contaminant, once it has been detected, can be removed before reaching the equipment to be protected.

4) What Equipment is around the Conveyor:  It’s good to take note of anything that may interfere with installation, such as another conveyor above or below, cables, or other equipment that needs to be considered during installation. Examples would be: Cutoff Saws, Catwalks, Walkovers, other Metal Detectors in close proximity, Log Kickers, Grinders, Crushers, etc.


Q. What is included in your metal detector system?

A. There are three main components:

1)  Metal Detector Search Coil (Head)

The coils are manufactured in many sizes and shapes, such as flat for under conveyor applications, four-sided and multiple-sided. What is being conveyed, sensitivity needs and the conveying system being used help determine the size and shape.

2) Electronic Control Enclosure (Panel)

The Electronic Controls are remotely mounted and connected to the search coil with a provided cable. The Standard cabinet to house the electronics is a NEMA 4/12 enclosure. A Stainless Steel NEMA 4X enclosure is available upon request.

3) Metal Free Area (Non-Metallic Section)

The transport system (conveyor) used to pass the product over or through the metal detector search coil must include a non-metallic section. A metal free area must be provided so that the metal signal detected is the metal contaminate, not the conveyor. On formed pan belt conveyors and vibrating conveyors a portion of the steel pan is replaced with a matching fiberglass section in the metal detector area. On Idler conveyors usually the Idler rolls are set farther apart or the rolls are removed to create a metal free area. For a roll case conveyor, one or more rollers will need to be removed to create a metal free area. In addition, you may need to create a break in metal framework as well.


Q. What type of metal can be detected?

A. MDI Metal Detector Systems will detect all types of metal. There are three basic categories: Ferrous (magnetic), Non-Ferrous (non-magnetic), and Stainless Steel.

1) Ferrous Metal is any metal that can be attracted to a magnet. This is iron and steel. In time it will rust when exposed to air and water. Ferrous metal is typically the easiest metal to detect and usually the most common contaminant in industrial environments. Examples include paperclips, thumbtacks, pins, staples, most screws, nails, washers, welding slag, rust, abrasions from metal to metal contact, and tools dropped into the conveyor.

2) Non-Ferrous metal is non-magnetic metals (copper, aluminum, brass, lead, etc.) It will take approximately 50% more of a non-ferrous metal to be as detectable as a ferrous metal. Manganese is also a non-ferrous metal and difficult for most metal detectors to detect. MDI’s digital flat and surround systems are very effective in detecting manganese.

3) Stainless Steel is always the most difficult metal to detect due to its poor electrical conductive qualities. By definition stainless steel has low magnetic permeability. A stainless steel sphere would have to be 50% larger than a ferrous sphere to produce the same signal strength on the metal detector.


Q. Why do I need a Metal Free Area?

A. The metal free area is the portion of the conveyor that is made of a non-metallic material, usually fiberglass. This is used to pass the product over or through the search coil without other metal interfering with the metal detector's performance. In other words, it creates the working environment so the metal detector can detect metal in the product, not the conveyor itself.


Q. Can just the bottom of the conveyor be fiberglass or wood and leave the sides of the conveyor metal?

A. No, The vibration of the conveyor would cause the sides of the conveyor to be seen by the metal detector and continual false tripping will occur.


Q. Will a spray marking system pin-point the exact location of the metal?

A. Spray marking systems can only indicate metal is in the log, not exactly where it is located. This is because a small piece of metal near a search face may have the same signature as a larger piece of metal deeper in the log. Type of metal such as ferrous, non-ferrous or stainless steel also affect the signal. It’s best to use a hand held metal detector to help pinpoint the metal once the log has been removed to a metal free area for scanning.