Unless it's a consumer disposale model, they are much cheaper to have them serviced.
Look in yellow pages for fire extinguishers, several places that will do a test & recharge. they will even come to your place & take it with them, I think they also will leave you a loaner in most cases.
Here is from Industrial Accident Prevention Association
Maintenance
Extinguishers must be properly maintained to
ensure that they will work when needed, and that
they are safe to use. A carbon dioxide extinguisher,
for example, can build up a high static charge if it
is used when there is a breakdown of the insulation
around the discharge horn. This can cause electric
shock
Adequate maintenance of extinguishers consists of
regular inspections, recharging as needed, and a
complete annual checkup and servicing. Records
must be kept of all maintenance work carried out,
including inspections.
Testing and servicing is usually carried out by a
service agency. If employees in your company look
after testing and servicing, they must be trained and
fully qualified to do so.
Inspections
Fire extinguishers must be inspected at least once a
month, and more often where needed. Inspections
are visual checks to determine that:
The extinguisher is well supported:
− hangers are fastened solidly.
It is accessible:
− can be easily reached;
− location signs are clear;
− class markings are clear;
− operating instructions are clear.
It is in working condition:
− discharge opening is clear;
− is fully charged;
− has not been tampered with;
− is not damaged;
− hydrostatic testing has been done.
The ring pin is in place.
The seal is intact.
Recharging
Recharge spent extinguishers immediately and
return them to their locations. Follow
manufacturer’s instructions for recharging.
Servicing
Completely examine each extinguisher at least
once a year, and whenever your monthly
inspections indicate that this may be needed.
Replace defective parts and extinguishers,
recharge extinguisher as needed, and ensure that
hydrostatic tests are carried out according to the
manufacturer’s instructions.
Extinguishers that contain Halon 1211 or 1301
must be serviced according to the requirements
of O. Reg. 413/94 under Halon Fire
Extinguishing Equipment, of the Environmental
Protection Act.
Set up a maintenance schedule for extinguishers
so that they are not all out of service at the same
time.
Record Keeping
Attach a durable tag to each extinguisher that
shows:
dates of monthly inspections, recharging,
and servicing;
name of servicing agency;
signature of person who performed
the service.
Maintain a permanent record for each fire
extinguisher that shows:
serial number and type of extinguisher;
location of extinguisher;
inspection date;
description of maintenance work or
hydrostatic tests carried out;
date of next inspection;
date of scheduled annual servicing;
inspector’s comments;
inspector’s signature.
