TEMPERATURE VS. PRESSURE
The pressure gauge on a pressurized fire extinguisher also acts much like a thermometer! As the temperature of the atmosphere surrounding a pressurized extinguisher increases, the pressure inside the cylinder also increases. Conversely, as the temperature of the extinguisher decreases, the pressure within the extinguisher also decreases.
Each year during extreme seasons where temperatures are unusually high or low, we get a lot of telephone calls about extinguishers that are either "overcharged" or "low in pressure." If there is any doubt about the pressure reading on the gauge of the extinguisher, the easiest way to check is to simply place the extinguisher in a controlled environment (an air conditioned room) for 24 hours. If the pressure gauge returns to its normal position, then the pressure fluctuation was due to temperature. If the pressure reading continues to be either too high or too low, then there may be a need for maintenance on the extinguisher in question.
Another method for checking the pressure within an extinguisher is to turn the extinguisher gauge into a "differential pressure gauge." Remove the adhesive dot on the lens of the extinguisher gauge and using an Amerex Gauge Tester pressurize the gauge case to the extinguisher operating pressure. The pointer on the extinguisher gauge should move from the pressurized area to zero. When the pressure in the extinguisher gauge is released and returned to atmospheric pressure, the pointer should return to its proper position in the green zone.
Amerex dry chemical extinguishers are shipped from the factory with the yellow pointer of the gauge slightly above or exactly on the white mark within the green zone. This corresponds to 195 psi ±4%, the allowed UL tolerance. The yellow pointer should not be below the indicated pressure (the white mark within the green zone) at 70°F.
The green zone on the face of an extinguisher gauge corresponds to the pressure in an extinguisher between -40°F and approximately 135°F if the extinguisher was pressurized to its exact operating pressure at 70°F. Any variation in the starting pressure or in the temperature of the extinguisher outside the ranges given above will result in the yellow pointer also moving outside the green zone.