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Dousing Worries of Extinguisher Blowing Up

Question: Automotive experts always seem to be saying that drivers should keep a fire extinguisher in their car as a safety precaution. This makes sense, but the warning label on small extinguishers says not to expose them to temperatures greater than 120 degrees. I drive a Datsun 280Z with no trunk and a black interior that heats up to well over 120 degrees on hot days. Am I risking an explosion in my car by keeping a fire extinguisher in there?--P.S.

Answer: The possibility of an explosion is almost non-existent, according to experts at several manufacturers of fire extinguishers and at Underwriters Laboratory, the organization that created the 120-degree standard.

Underwriters Laboratory, a product testing organization, requires that fire extinguishers be safely stored at between minus-40 and plus-120 degrees, a temperature range that is posted on all extinguisher labels. But the laboratory also has a requirement that extinguishers survive an exposure of 175 degrees for seven days, which is not stated on the label.

Fire extinguishers operate under high pressure and the metal cylinders are supposed to be made to exacting standards. Carbon dioxide extinguishers operate up to 1,800 pounds per square inch. More common consumer extinguishers operate at 100 pounds per square inch, still a pressure higher than in virtually any other consumer product.

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For this reason, each individual cylinder is tested with water pressure at a level three times higher than the pressure with which they are later filled. In addition, they are designed to withstand six times their normal pressure. It is impossible for the heat inside a car to drive up the pressure higher than these limits, according to engineers and Underwriters Laboratory.

Having said all this, the Underwriters Laboratory test does not guarantee that the fire extinguishers will be effective after exposure to high temperature. It is possible that the valve could leak, slowly releasing pressure or that the heat could alter the chemical composition of the fire retardant.

To minimize the wear and tear inflicted by heat on a fire extinguisher, you should store it low in the passenger compartment and out of the sun. Even if your car has a trunk, it is better to store it in the passenger area where it is readily accessible.

Fire extinguishers sold to consumers have two basic types of retardant--halon gas or dry chemicals. Halon is a gas related to Freon, the inert gas used in refrigerators that is suspected of damaging the Earth’s ozone layer. The dry chemicals used are sodium bicarbonate or monoammonium phosphate.

Engineers say that either type of extinguisher will work on car fires, but you’ll need a larger capacity extinguisher with halon. If you opt for dry chemicals, a 5-pound cylinder should be the minimum acceptable size. For halon, you would want a 5-to-9-pound size. The tiny hand-size cylinders are nearly worthless. In addition, the dry chemical extinguishers create a mess of their own that will have to be cleaned up.

Fire extinguishers are given ratings of A, B or C, depending on the type of fire they will put out. An A rating is for paper or wood, B is for flammable liquids and C is for any class of fire that involves a live electrical circuit. Many extinguishers carry ABC ratings, which is the most desirable for a car.

You should be cautioned that it is dangerous business to attempt to put out a car fire. Most engine fires involve the combustion of gasoline, which can detonate explosively. Most experts warn against attempting to raise the hood of a burning engine.

If you are intent on using an extinguisher, you should attempt to spray the retardant through the car’s grill or up into the engine compartment from the ground. And keep your distance. When the car’s gasoline blows, you don’t want to go with it.


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