Applied Science: Sniffing Out the Enemy

They all laughed when the U.S. Army introduced the "E63 Manpack Personnel Detector" for experimental use in Viet Nam. The device, a 24-lb. chemical and electronic version of a nose, was designed literally to smell the body odor of concealed enemy troops. Now the G.I.s—and presumably the Viet Cong, too—are more impressed than amused. The "People Sniffer," as it is known in the field, has demonstrated that it can locate hidden enemy forces and has been ordered for dozens of U.S. infantry and artillery units.

Developed by General Electric and the Army's Limited War Laboratory, the E63 is based on a phenomenon often demonstrated in high school chemistry classes: if open bottles of ammonia and hydrochloric acid are placed close together, a white cloud of ammonium chloride particles forms above them. These particles serve as nuclei for the condensation of the air's water vapor into tiny, foglike droplets.

B.O. Beeps. In Viet Nam, the ammonia is produced by groups of perspiring men. Urea, a component of perspiration, is attacked by bacteria on the skin and decomposed into odorless carbon dioxide and ammonia gas. Thus the air in the vicinity of a large group of men—especially in hot and humid climates—contains high concentrations of ammonia. To detect the ammonia, the E63 scoops up air, passes it over a wick saturated with hydrochloric acid and into a humidifying chamber. If the air contains any ammonia, a fog forms, changing the amount of light shining on a photoelectric cell and varying the amount of electric current that it produces. The current variation in turn increases the frequency of a beeping sound in the operator's headphones and produces a higher reading on a meter, warning him that there are men near by. Higher concentrations of ammonia—presumably produced by larger numbers of men—produce a denser fog and higher meter readings.

Although it was originally designed to be carried on the back of a foot soldier, the E63 has been used most effectively aboard a helicopter. Flying above the treetops at speeds as high as 100 m.p.h., helicopters equipped with People Sniffers have detected groups of men in jungle-covered "free bombing zones"—areas cleared of civilians where anything that moves is regarded as fair game. On the basis of E63 readings, artillery fire has been called in and flights of B-52s have been dispatched to saturate the suspicious area with bombs.

The device is effective only when operated into the wind. Carried by the leading man in a patrol, for example, the E63 will pick up the odor of patrol members themselves if the wind is at their backs. But it is sensitive enough to pick out an upwind enemy sniper lying in ambush at distances greater than the range of most rifles. "There's no question about it now," says Lieut. Colonel Alvin Hylton, chemical officer of the 1st Infantry Division. "It works."