Coffee Break Training: Portable Extinguishers: Minimum Hydrostatic Test Pressures for Extinguishers [open pdf - 233KB]
"Other than rudimentary hand-pump devices, most portable fire extinguishers employ some sort of internal or cartridge-applied pressure to discharge the fire extinguishing agent. To minimize the risks associated with catastrophic container failures, high-and low-pressure portable extinguishers must be periodically pressure tested."
Coffee Break Training - Fire Protection Series No. FP-2010-3
United States Fire Administration: http://www.usfa.fema.gov/