The tamper seal is a small plastic or wire tie through the safety pin. It has one job: to show at a glance that the pin has not been pulled since the unit was last serviced. It is the cheapest and most informative part of an extinguisher, and it is the single item the monthly visual check exists to look at.
Why a broken seal is not a cosmetic problem
The pin can only be pulled by breaking the seal, and pulling the pin is the first step of a discharge. A broken seal therefore means one of three things: the unit has been partially or fully discharged, somebody has handled it in a way that could have discharged it, or it has been interfered with. None of those can be ruled out by looking at it, which is why the unit comes out of service until an engineer has checked it.
A pressure gauge is not a substitute
A stored pressure unit can lose a fraction of its charge without the needle leaving the green, and a carbon dioxide unit has no gauge at all because it is weighed instead. The seal answers a question the gauge cannot: whether the operating mechanism has been touched.
What the monthly check covers alongside it
That the unit is present and on its bracket or stand, that nothing is stacked in front of it, that the body is free of dents and corrosion, that the hose and horn are attached and undamaged, that the label is legible and facing outward, and that the service label is in date. The seal is checked first because it is the one that most often tells you something.
Recording it
The check is recorded in the fire log book with the date and the person who did it. A record of checks with no findings over several years is less credible than one that shows an occasional missing unit or blocked point, because those things genuinely happen in occupied buildings.
Questions people ask about fire extinguisher seals
Can a broken seal simply be replaced?
Not by the person who finds it. Refitting a seal without knowing whether the unit was discharged hides the very thing the seal exists to reveal. Replacement is part of a service.
What if the pin is missing entirely?
The unit is out of service immediately. It cannot be relied on and it is also liable to accidental discharge while being moved.
Do all extinguishers have seals?
Every portable with a pin operated mechanism carries a tamper indicator of some kind. The material varies between manufacturers and the function does not.