Coffee Break Training: Portable Extinguishers: Portable Fire Extinguisher Hydrostatic Testing [open pdf - 200KB]
"Periodic hydrostatic pressure testing is performed on portable fire extinguishers to evaluate the containers' strength against unwanted failure. The tests are performed by qualified service technicians in a controlled environment to minimize the risk of injury or damage. Hydrostatic testing is performed by filling the container with water or another non-compressible fluid that is compatible with the fire extinguisher container. The pressure is then increased on the container to a specific point as required by the manufacturer or Federal safety standards, and the container is inspected for potential failure points. The pressure must be maintained for at least 30 seconds. National Fire Protection Association (NFPA) 10, 'Standard for Portable Fire Extinguishers', provides the following hydrostatic test frequency based on the different types of portable fire extinguishers."
Coffee Break Training - Fire Protection Series No. FP-2010-9
United States Fire Administration: http://www.usfa.fema.gov/