Cumbersome elevated ramps at the base are sometimes seen in high security applications because of this problem. Transit speed Some walk through detectors will alarm only in a narrow range of the speed with which a metal object is carried through. A very slow passage, such as an elderly frail person going through, may result in no weapons detection. Conversely a very fast speed, a weapon tossed through the archway for example, may not be detected either. Sensitivity drift Most walk through detectors suffer from a constantly shifting sensitivity level, which often results from changes in ambient humidity and temperature.
Some manufacturers recommend checking and resetting sensitivity up to several times per day.
The detection of a given metal object in most walk through detectors is quite variable depending on the orientation of the object within the archway opening. For example, a small weapon carried through in a particular position may be detected, then not detected after rotating the weapon 90 degrees. Re-bar effect Most detectors' sensitivity to metal at floor level is seriously impaired when the unit is placed on concrete slab constructions containing steel reinforcement bars. Likewise, walls, ceilings, and other environmental surroundings can also have an adverse effect. Discrimination Ferrous metals such as steel and iron generate a slightly different signal pattern than do non ferrous metals such as copper and aluminum when carried through a detector.