One thing about the dry powder extinguishers -- the powder settles and compacts -- won't work even if the gauge is in the green. Add to your semiannual preventive maintenance list: dismount the unit, turn it upside down, and smack the heck out of it on the bottom a few times with a rubber mallet to loosen the powder.
If you need to have one recharged because of age (hasn't been discharged), don't dump the powder. It's not cheap, but is reusable.
Dan & Liz H
"I'd rather be happy for just one day than miserable for the rest of my life."