What is hydrostatic testing on a fire extinguisher?

The dry chemical contents inside your fire extinguisher are kept under pressure over long periods of time, which can eventually cause damage to the extinguisher shell. To ensure the shell, hose, and nozzle have no defects, whether due to metal fatigue or other causes, the extinguisher shells are emptied, filled with colored water, and brought up to a specified pressure over a short period of time (typically 30 seconds). The shell must be able to hold that pressure for one minute (or the time specified by manufacturer’s maintenance procedures) while the cylinder is inspected for signs of stress, leaks, bulging, and other indications of metal fatigue. Hoses on some extinguishers must also be hydrostatically tested.

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