After a discharge or at an extended service a unit is either recharged and returned to service or taken out of service permanently. The decision is made on the condition of the pressure vessel and its components, not on how much agent is left in it, and it is made by the engineer rather than by the person who used it.
What condemns a unit
Corrosion on the body, particularly around the base where moisture sits. Dents or gouges that could have weakened the shell. Damaged threads at the neck. A failed internal examination. A body beyond the age at which the manufacturer supports it, or one for which spares are no longer available. Any of these ends the unit regardless of what it contains.
When a refill is the right answer
Where the body passes examination and the unit is within its supported life, recharging is normally cheaper than replacement and is the environmentally better outcome, since the steel shell is the bulk of the embodied cost. This is the usual result for a unit discharged in training or in a genuine incident in its early years.
Why buying new can still be right
A building replacing an ageing provision often finds that the combined cost of extended services and pressure tests across a set of old units, spread over the next few years, exceeds the cost of new equipment that resets the whole schedule. That is a portfolio decision rather than a per unit one, and it is the conversation worth having when a certificate lists several units approaching an overhaul.
The part that is not optional
A discharged unit is not a unit. From the moment it is used, the provision at that point is gone and the risk assessment's assumptions no longer hold. Replacement or recharge is urgent rather than routine, and an interim unit should stand in the gap if the turnaround is more than a day or two.
Questions people ask about refill fire extinguisher
Can a unit be topped up on site?
Water and foam units are normally recharged on site by an engineer with the right equipment. Carbon dioxide units are exchanged, because recharging a high pressure vessel is a workshop operation.
Is buying new fire extinguishers cheaper than servicing old ones?
Not year on year, but sometimes across a replacement cycle. The comparison to make is the total of the forthcoming extended services and pressure tests against new units with a fresh schedule.
What happens to a condemned unit?
It becomes waste and is dealt with under the waste duty of care, which usually means the servicing contractor removes it and documents the transfer.