Every extinguisher cylinder is a pressure vessel, and pressure vessels fatigue. The hydrostatic test is NFPA 10's answer: on a fixed schedule, the cylinder is emptied, the valve comes out, and the shell is pressurized with water well past its service pressure to prove it can still hold. Pass and it goes back into service with a fresh clock. Fail and the cylinder is condemned — destroyed, never refilled.
The part that catches people is the schedule. There isn't one hydro interval; there are two, and the agent inside the cylinder decides which one applies.
The two schedules
From NFPA 10's hydrostatic test interval table (§8.3):
Every 5 years:
- Stored-pressure water
- Water mist
- Wetting agent
- AFFF and FFFP foam
- Loaded stream
- Carbon dioxide
- Wet chemical (Class K)
Every 12 years:
- Dry chemical — stored-pressure and cartridge-operated
- Dry powder (Class D)
- Halon 1211
- Halotron and the other clean agents
The pattern isn't arbitrary. The 5-year list is dominated by water-based agents — everything on that side either is water or behaves like it, and water sitting against steel for years is a corrosion problem. CO2 is on the short cycle for a different reason: those cylinders run at far higher pressure than a stored-pressure dry chem, so they get proven more often. The 12-year list is the dry, non-corrosive agents in mild-steel or aluminum shells at moderate pressure.
If you remember one heuristic: wet or high-pressure = 5 years, dry = 12 years.
When the clock starts
The first hydro is due at the interval measured from the date of manufacture stamped on the cylinder — not from when it was installed, and not from when your company picked up the account. Every subsequent test runs from the date of the last test.
This matters when you take over a serviced building. A 2015-manufactured CO2 unit you inherited in 2026 isn't due for hydro in 2031 — it was due in 2020 and again in 2025, and if there's no test record, it's overdue the day you tag your first unit in that building. Onboarding an account means reading manufacture dates, not just last-service tags.
How hydro interacts with the internal exam
The hydro schedule and the internal-exam schedule are designed as one system, and this is where hand-tracked schedules usually go wrong:
- Agents on the 12-year hydro cycle (stored-pressure dry chem, halon, clean agent) carry a 6-year internal exam — deliberately the midpoint. Opened at 6, pressure-tested at 12, repeat.
- Agents on the 5-year hydro cycle carry no separate internal-exam clock at all. The hydro empties the cylinder and examines it internally by definition, so the standard doesn't schedule the same teardown twice.
- Cartridge-operated dry chem is its own animal: internally examined every year (that's what its annual maintenance is), but still hydro'd at 12.
Practical consequence: if your tracking system schedules a "6-year internal" on a CO2 or a Class K unit, it's inventing work the standard never required. If it doesn't schedule the 6-year on a stored-pressure ABC, it's missing a required teardown. Both errors come from treating the clocks as independent when they're actually one interlocked table keyed by agent.
What a passing test has to leave behind
A hydro isn't done until it's recorded. For low-pressure cylinders, that's a durable label on the shell carrying the month and year of the test, the test pressure used, and the agency that performed it. High-pressure cylinders (CO2) are stamped per DOT rules instead. The unit also gets its verification-of-service collar, since the valve came out. I covered the full paperwork stack — tags, collars, labels — in the service tag article.
Condemnation
A cylinder fails hydro by leaking, distorting, or letting go entirely (tests are conducted with the cylinder in a protective cage or filled with water for exactly this reason). A failed shell is condemned on the spot — marked and rendered unusable so it can never wander back into service. There's no repair path for a failed pressure vessel.
Also worth knowing: certain conditions condemn a cylinder without a test — significant corrosion or pitting, fire damage, repairs by welding or soldering. If the shell wouldn't be safe to pressurize, the hydro is failed before the water goes in.
Tracking it across a real account base
One building with thirty units might have ABC dry chems (12-year hydro, 6-year internal), a couple of Class K wet chems in the kitchen (5-year hydro, no internal clock), CO2s in the server room (5-year), and a Class D in the shop (12-year). Four different service profiles in one hallway walk, every clock anchored to a different manufacture date.
That's the tracking problem the Inspections & Compliance module exists to solve: register the unit once with its agent type and manufacture date, and the 5-vs-12 split, the internal-exam interlock, and the non-rechargeable 12-year replacement rule all fall out of the standard automatically — and roll forward correctly every time a tech records a service. However you track it, the schedule above is the one the fire marshal will check your tags against.