Technologies that screen liquids have been available in the commercial market for years. About the size of tabletop coffee makers or smaller, the machines deploy energy through a bottle of liquid and use a variety of scanning technologies, including microwave, acoustic, ultrasonic and infrared spectroscopy, to measure the density and detect a threat. But most airports have been reluctant to install them for several reasons, including questionable reliability of the equipment, high costs and logistical burdens. Critics argue that most liquid-explosive detectors are complicated to operate, have high rates of false alarms and still require security agents to individually screen items.
