This type of extinguisher contains a charge of sodium bicarbonate powder. The container of this type extinguisher holds a steel capsule containing liquid carbon dioxide. On breaking the capsule, by depressing the knob, the gas forces all the powder out of the discharge nozzle. The powder blankets the fire in a cloud of dust, cutting off the supply of oxygen. If the fire is hot enough the sodium bicarbonate will decompose to form sodium carbonate, carbon dioxide and water all of which will aid the smothering operation and give some cooling effect. The extinguisher discharges in only 15 sec and it is thus important to aim the appliance accurately. The operating pressure of this container is higher than that of the soda-acid type. It is consequently hydraulically tested at a pressure of 35 bar.