John has summarised it well.
BS 5306-3 details the standard required for competent maintenance on extinguishers and states 3 types of service:
In the UK, three types of maintenance are required:
* Basic Service: All types of extinguisher require a basic inspection annually to check weight, correct pressure (using a special tool, not just looking at the gauge) and for signs of damage or corrosion, cartridge extinguishers are opened up for internal inspection & check weighing of the cartridge, labels are checked for legibility, where possible dip tubes, hoses and mechanisms checked for clear free operation ;
* Extended Service: Water, Wet Chemical, Foam & Powder extinguishers require every five years a more detailed examination including a test discharge of the extinguisher and recharging- on stored pressure extinguishers this is the only opportunity to internally inspect for damage/corrosion. by recharging fresh agent is used as they all have a shelf life, even water goes foul inside an extinguisher;
* Overhaul: CO2 extinguishers, due to their high operating pressure, are subject to pressure vessel safety legislation and must be hydraulic pressure tested, inspected internally & externally and date stamped every 10 years. As it cannot be pressure tested a new valve is also fitted.
Three types of extinguisher companies exist:
1 - Those whom are not competent and do not fully service an extinguisher (wipe & swipe) thus appearing good value to the user through cheap prices, but meaning an extinguisher may not operate when required or even be dangerous.
2 - The very competent firm who carry out all required checks, but are either overpriced on new equipment or over zealous in condemning extinguishers or in numbers required
3 - The fair priced, yet competent company, that does not over sell or do unnecessary work
Third party accreditation & trade body membership will ensure that you do not get company type 1, but not stop you getting type 2 - if you Private Message me with who you use I can advise further