An extinguisher on the wall does nothing if nobody knows whether it still holds pressure. The job is to name one owner for the visual look, one owner for the service or replacement call, and one place where each look is written down. Those three jobs are different, and they fail in different ways.
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Read the Gauge on Every Visit
Look at the pressure gauge on the front of the cylinder. Most gauges have a colored band marking the operating range, and the needle should sit inside it. A needle outside the band, or a gauge that has dropped out of it since your last look, means the unit needs service or replacement under the label instructions. Do this on every visit you make, not only the visits where something seems wrong.
Check the Pin, the Seal, and the Body
The pull pin should sit in its locking position with the tamper seal intact. A broken seal or a missing pin means the unit has been used or disturbed. Look at the cylinder for dents, rust at the base or valve, and cracks in the hose or horn. Confirm the label is readable and that any service tag attached to the unit is filled in and current.
Keep It Where Someone Can Reach It
Keep the unit near an exit and within reach of the kitchen, not behind a closed door, under a sink, or in a closet that stays shut. Make sure the wall bracket is fastened to the wall, not resting on a shelf or the floor. If the unit has been moved from its bracket, put it back and note the move, because a unit nobody can grab in a hurry does not do its job.
Name the Owners
Pick one person who does the visual look and writes the result on the tag or in one shared note that everyone can find. Pick a second person who owns the service or replacement call to a fire equipment company or the fire department, since the look and the service are separate jobs. If the unit was discharged at any point, even partly, the label and the manufacturer decide whether it can be serviced or must be replaced. Do not assume a partly used unit is still good. Keep the manufacturer's contact and the purchase record with the house paperwork.
When Dad Says No
If Dad will not let you look at the unit, ask him to show you where it hangs and to lift it from the bracket for a quick look. If he refuses both, the visit still counts: write down what you could see from where you stood, and tell the person who owns the service call that the unit has not been checked up close. Being honest about what you could not see is part of the record.
Next move: walk to the extinguisher you have, read the gauge and the seal, write down what you saw on the tag, and name the person who will do the look on your next visit.