All depends on various factors. For example, at one terminal, the WTMDs are bolted to the floor and not bolted down at another. The ones that are not bolted down are more susceptible to vibrations that result from bumping into the machine.

Another factor is the width of the body as you walk through. A thinner person wearing a metallic wristwatch, for instance, may not necessarily trigger the alarm on the WTMD. However, a person with a wider build may trigger the alarm because the watch is now passing much closer to the sensors located along the walls of the WTMD. This isn't really a big thing. In this instance, there may be other metallic items that accumulate to the point where the watch is now a factor between alarming and not alarming the WTMD.

It is very difficult to strip yourself down to the point where you have absolutely nothing metallic on your body.