Sources of interference
Even the best portal metal detector will fail to operate properly if it is not located in an area that minimizes outside interference. There are many different shapes and forms of interference to a metal detector. School administration and security staff should be aware of potential problems. Below is a partial list of possible interference sources (see also exhibit 3.8):
- A metal stool or metal trash can placed close to the portal.
- Fluorescent lights located directly above the operating area of the portal and within 1-2 feet of the top of the portal.
- Motors or anything that causes a spike of electromagnetic energy nearby (within a few feet).
- An elevator motor. If it is a large motor, the elevator can cause interference even up to 10-15 feet away.
- Nearby air ducts in the wall with metal components that expand/contract slightly when the cooling/ heating system is in operation.
- Plumbing within a nearby wall such that the pipes vibrate when water is running through them.
- Chain link fencing.
Most nearby metal structures will not prohibit use of a portal metal detector. However, the instrument sensitivities of the detector should be set to allow for the presence of these structures. Any change in position of the portal in relation to nearby metal structures can affect the equipment's sensitivity.
Research Report: The Appropriate and Effective Use of Security Technologies in U.S. Schools