Fake weapons do not bring real results in testing metal detectors. Here are a variety of other materials used in the development of improved testing standards. Source: N. Hanacek/NISTThe use of metal detectors has expanded well beyond airports to a variety of facilities, including courthouses, sports venues, and even schools and houses of worship. The National Institute of Standards and Technology (NIST) has researched and developed four metal detection standards to keep up with the increased usage of these machines and ensure that they work correctly and can be trusted to detect weapons and other threats. This effort represents the first product conformity standards created for metal detectors. Three have already been published by ASTM International.

In addition to increasing confidence, NIST expects the acceptance and implementation of these standards to shorten the time needed for testing new products, resulting in lower costs for users.

"We were able to reduce the time required for exhaustive walk-through metal detector testing from almost 9,000 hours to just 66 hours by eliminating redundant and often unnecessary procedures," said Nick Paulter, whose research group at NIST conducted the work.

Walk-through metal detectors (WTMDs), commonly used at checkpoint areas, work by generating an alternating magnetic field that is altered by metal passing through it. Testing involved noting when metal triggered an alarm. Humans with no metal on their clothing and no medical implants have been used as "clean testers," but because of the difficulty they have consistently recreating their exact movements each time they pass through the WTMD, numerous test runs had to be done to compensate for the variation, which made testing expensive.

Robots began to replace humans because they could reliably follow the same path, but the gain in consistency meant a loss in real-life representation of human movement. Using robots that moved more like humans proved to be as time-intensive and costly as using humans themselves.

Numerous variables, such as height, weight, speed and the path a person takes through the WTMD, impact the alarm signal. The NIST research ultimately found that to successfully determine if the WTMDs met baseline standards of performance, robot testers needed to travel only one path, moving at one consistent speed, but should include six different objects with one orientation for each.

The NIST team also helped to create documentary performance specification and test method standards for hand-held metal detectors (HHMDs), which are waved around the person being searched, and for hand-worn metal detectors (HWMDs), which make contact with the person being searched. Both very small objects, such as razor blades and keys, as well as larger objects, such as fake knives and guns, were found to be problematic for testing due to variation in size and orientation.

The solution was to use an object of consistent shape and size. The researchers discovered that steel or aluminum spheres of different sizes made the perfect test stand-ins for a variety of threats, from small contraband to knives and guns. An added bonus is that the spheres left at the test center do not pose a risk if stolen, as might be the case with even a fake knife or gun.