Posted by Brian G on 20/09/2019 07:39:54:
The fire station at the refinery I worked at used to take all the extinguishers and BA sets that needed servicing and use them for staff training. Quite interesting to be faced with a large tray of burning oil and a dry powder extinguisher that has been outdoors for 3 years or so. It took me three tries to find one that wasn't clogged up.
I guess the way to look at the mess is that, like getting old, it beats the alternative.
Brian
As one who used to be involved in the safety side of car rallies, one of the first things we did on arriving at a stage start, was to upend the dry powder extinguisher and listen with your ear pressed against it.
After a long drive, the powder clumps together, and may not come out when you trigger it in an emergency.
I'd urge anyone with a dry powder extinguisher in the workshop to do the same occasionally. Normally just upend it on your shoulder with your ear pressed against the cylinder and you will hear and feel the powder running to the other end. If you're not physically capable just invert it on a bench, but make sure you have your ear against it, and have a rubber mallet handy in case the powder needs freeing off.
N.B. make sure the pin's in first. 
Also, for those of us with very cold workshops in the winter, remember your foam (AFFF) or water extinguishers need a suitable anti-freeze in them.
Bill