The extinguishers are activated by a signal from the control unit. The extinguisher valve can be completely opened in 5 milliseconds to provide rapid flooding of the confinement with discharge velocities that can exceed 100 feet per second. The extinguishers are filled with a suppression agent, generally dry chemical powders or halon gases. Dry chemical powders commonly used are mono-ammonium phosphate or sodium bicarbonate. Sodium bicarbonate is generally preferred over mono-ammonium phosphate for grain handling systems because it is compatible with food and is water soluble.
Sodium bicarbonate is stable under long-term storage conditions and is nontoxic and noncorrosive.
When a dry powder is used, the extinguisher is pressurized with carbon dioxide or nitrogen. Halon gases have an advantage over dry chemical powders in that they completely dissipate after each discharge. This eliminates the messy cleanup after an accidental discharge of dry chemicals. However, high concentrations of halon gas are toxic to humans and should never be used where workers are present. This problem is minimized, however, when halon gas is discharged inside a bucket elevator leg and dissipated below potentially harmful concentrations.