A fire extinguisher is a metal cylinder that holds water or a smothering substance that, when the safety lock is released and the handle is squeezed, is forced out of the extinguisher nozzle at high pressure. Handheld extinguishers typically weigh from 0.5 to 14 kilograms (1.1 to 30.9 lb) and are designed to be easily operated by one person. Cart-mounted extinguishers are more substantial, weighing from 23 kilograms (51 lb) to more than 40 kilograms (88 lb). The latter are more common in industrial settings such as aircraft hangars and heliports.
Different kinds of fires require different extinguishers to put them out. The rating on a fire extinguisher tells you which kind of fire it is rated for. The last letter of the rating number indicates which class the fire is in, such as A, B, C, or D.
Each type of fire extinguisher is used to fight a certain group of fires, but there are also overlapping areas between classes that can be extinguished with any of them. For example, a Class A fire could be put out with either water or dry chemical, depending on the fuel being burned.
Water-based extinguishers put out Class A fires by cooling the burning material and removing the heat from it, which stops the chemical chain reaction that causes fires to continue. Foam-water extinguishers have a similar effect and are used to put out Class A, B, and Class C fires.
Carbon Dioxide fire extinguishers discharge a compressed gas, which smothers Class B and Class C fires by excluding oxygen from the fire. These are often used in laboratories or food preparation areas where expensive and delicate electronic equipment might be located, since the agent will not leave a residue on delicate electronic components. Carbon Dioxide fire extinguishers are not recommended for fighting cooking oil or grease fires, because the agent will make these more difficult to control by causing them to glug and splatter.
Dry chemicals are a second method of fire suppression that is effective against Class A, B, and C fires. Monoammonium phosphate, commonly known as ABC dry chemical or tri-class, is the most common of these. It is pale yellow in color and works by separating the three parts of the fire triangle, preventing the chemical reactions that cause flames to continue. It is less toxic than carbon tetrachloride, the first dry chemical to be widely used in fire extinguishers, which was replaced by the more environmentally friendly liquid chlorobromomethane (CBM) in 1969.
Most fire codes require a fire extinguisher on every floor of a home, and at least one in the kitchen. It is also a good idea to have an extinguisher in any room that might be susceptible to electrical fires, such as the garage or shed. Be sure to maintain these extinguishers as they are needed, by having them inspected and recharged annually by an approved servicer. This is especially important, as the large manufacturer of ABC extinguishers recalled over 70 million units dating back 20 years or more due to faulty seals and other problems.