I don't want to hijack this thread, but how many of you service them annually?
That is great info. So should we shake them up from time to time to prevent settling or is that powder too sticky and basically ruined once compacted?
Worked my way thru this whole thread and these are the only two mentions of this. The dry powder type need regular maint. or they're useless. The powder is so fine that just the vibration from sitting on the floor of your garage for a year will compact it to the point that when you need it all you'll get is the propellant and none of the powder.
I talked to my past employer's fire extinguisher service guy about this. That is the real reason for the service, to break up the clump of powder in the bottom of the tank.
Now think about the vibration of a vehicle mounted extinguisher. I'll bet that powder is compacted at the bottom of the bottle in just a few months. I lay my vehicle mounted units on their side and periodically rotate them 180° (about time to do that again). Should probably do it every time I even think about them, just as a habit.
Figuring that the Coast Guard has a pretty good idea of what a boat needs and their extinguishers are a requirement, and that boats are pretty much the same in their fire fighting needs as a 4x4, I buy my extinguishers from a marine supply. I could likely pay less elsewhere, but this is easy and I know that I'm getting something specific to the need. Time has value too, I'd rather be doing something fun or earning me income than sorting out exactly what I need in a fire extinguisher and then trying to find a local supplier. I'd probably end up where I am now anyway.