Fire extinguisher

A fire extinguisher is, in the most common usage, a portable device, usually but not always handheld, for extinguishing undesired fires. Some extinguishers may be on wheeled carts or even truck-mounted, for use on large-scale fires.

Basic principles
While there may be exceptions, the general rules for using an extinguisher are:
 * 1) Call for professional help
 * 2) Select a position for attacking the fire, from which you can escape if the extinguisher does not contain the fire
 * 3) If the extinguisher has a safety pin or ring, remove it.
 * 4) If the extinguisher will displace breathable air, be sure you have a source of oxygen. People have become unconscious very quickly when using a carbon dioxide extinguisher in a poorly ventilated room.
 * 5) Actuate the extinguisher, aiming its output at the base of the fire, using a sweeping motion. While the operator can advance if the fire dies down and the stream will not reach the still-burning area, never advance so far that a rekindling of the fire could trap you.

Types
While some extinguishers can cope with a wide range of fires, no single type of extinguisher can put out all causes of fire. The basic classification of the National Fire Protection Association defines extinguisher types as:


 * A: Flammable wood, paper, etc.
 * B: Burning oils, gasoline, etc., that do not mix with water
 * C: Approved for safe use on electrical fires; dry powder type A are safe, for example, but water-based type A are not
 * D: Flammable reactive metals

For many years, the principle of putting out fires was to break one or more sides of the "fire triangle" consisting of:
 * 1) Fuel
 * 2) Oxidizer, usually oxygen
 * 3) Heat

More recent insights show there is a fourth component, the propagating combustion wave front, and interfering with the wave front also can stop the fire. Extinguishers may interfere with multiple sides of the fire polygon. Type A water extinguishers both cool the heat level and cut off oxygen. Type ABC extinguishers, using dry powder, primarily separate the fuel and oxidizer. Type D actually start a controlled fire that creates a hard oxide coating over the metal, keeping air from it.

Type A
While the original type A extinguishers were water-based, do not assume that merely because water extinguishers will not work on certain fires, large-scale firefighting equipment, used by trained personnel, cannot be used on other types. Fog nozzles on large hoses, powered by fire engine pumps, can be very effective on oil fires.

Soda acid
One of the first types of water-based extinguishers is called "soda-acid", or, especially in a truck-mounted version, "chemical". In a handheld soda acid extinguisher, most of the extinguisher is filled with a water solution of a carbonate or other compound that generates carbon dioxide when mixed with acid. At the top of the extinguisher is a bottle of acid, with a stopper that will fall free when the extinguisher is inverted.

There is a handle on the original cylinder bottom, to be used to hold the extinguisher; the hose and nozzle, once the extinguisher is inverted, will be at the base of the cylinder. Guide the nozzle with one hand while holding the cylinder handle.

Soda-acid extinguishers need a good deal of maintenance and have other undesirable characteristics; they are becoming rare and deservedly so. As a historical note, "chemical engines" were used in the 19th century, before there were reasonable portable steam or gasoline pumps; an operator manipulated acid and water valves to create a stream from a fire engine. These are totally obsolete.