If I may, I do have an opinion here. I owned a small (one-man operation) fire extinguisher service for 14 years. By the time I sold the business, I was inspecting or servicing about 6000-7000 fire extinguisher per year.
There is nothing really wrong with the advice given so far. Just some things some things I'd like to add or clarify.
DO:
Check the gauge on a regular basis. If it is low on pressure, take it to a licensed fire extinguisher company for repair, or just replace it. (if it doesn't have a gauge, get rid of it and buy one that does - see below)
If you use a dry chemical extinguisher, even a little bit, take it in for service or replace it.
The hydrostatic test interval for dry chemical fire extinguishers is 12 years. If you've had your extinguisher that long, and never had it recharged, then take it in for service or replace it. (the rubber o-ring and valve stem seat will eventually dry rot and need to be replaced)
Dirt daubers and spiders love the nozzles and hoses. And yes, a nest can block the hose. Remove the hose regularly and blow out with compressed air. NOTE: you can prevent this with a piece of thin tape. (freezer tape or scotch tape) Unless you wrap it up tight with gorilla tape, it will just blow off when you discharge the extinguisher. Alternatively, you can use a thin balloon. (you have 195 psi on a charged extinguisher - plenty enough pressure to pop that tape or balloon)
DON'T:
Please do not keep your home fire extinguisher above the stove, above the clothes dryer, on top of your water heater, or anywhere else that you might have to use it!!!
Same with the one on your tractor or other equipment. If you have a cab, mount it inside. If not, I like to mount them on the fender.
Don't worry about it:
As Ray said, a properly filled extinguisher won't have moisture in it and the powder will not settle
IF it's just hanging on the wall or sitting on the floor. However, the powder will pack from vibration. (equipment mounted) NOTE: the easy way to fix this is to mount the extinguisher horizontally. Then, when you pick it up, the powder will naturally fall to the bottom and be ready to go. That's it. That's all you have to do. Period. However, if you can't mount the extinguisher where it is laying on its side, then, YES, by all means, you need to take it out of the bracket, turn it upside down and whack it with a rubber mallet until the powder breaks loose.
And, if it makes you feel better to pound on that extinguisher that's been just sitting under your sink, then go ahead. It won't hurt anything.
I would like to add that most of the extinguishers that you can buy at retail box stores are JUNK. STAY AWAY from them. In most cases, you can purchase a commercial grade extinguisher from a reputable fire extinguisher company for just a few dollars more. They will also be able to sell you a heavy duty vehicle bracket that will actually hold the extinguisher.
And, no reputable fire extinguisher service company, that I know of, will work on those plastic valve pieces of junk either. (I use the word "reputable" here for a reason. There are crooks in any line of business, fire extinguisher companies included. Ask the local fire department who they use.)
BTW, if you are thinking that I'm exagerating, just take a good look at the extinguishers on the wall in the big box stores. They will NOT be the same ones that they have on their shelves. That's right. They won't use that kind of junk in their store, but they will sell it to you.
Did I mention that they were JUNK???
In case you're wondering which brand to buy, I recommend Amerex or Badger. Not doing a commercial here, just telling you which ones are the best quality. (and I've seen them from the inside-out) Those were the only ones that I would sell and use myself.
JMHO.
