Warning: Compressed air is dangerous. You are responsible for your own safety. Fire extinguishers can cause massive damage if activated indoors.
Canned air is nice for blowing clean PCs and other things for dust, but the cost of those disposable cans quickly add up.
A fire extinguisher should be replaced or have an expensive service after 10 years. Service and refilling costs the same as a new one, so what to do with the old one? The short boring answer is to deliver it as special waste at the nearest depot, but are there better alternatives?
The tank of a standard fire extinguisher is usually rated at 15 bar. How about a handy compressed air tank for the small jobs where the compressor is cumbersome to carry around? Converting a fire extinguisher to a portable compressed air tank is easy and straightforward.
You will need:
- Discontinued old fire extinguisher
- Quick connect coupling with 1/4 ” thread
- Coupling 1/4″ with internal threading (female)
- Coupling 1/4 with external threading (male)
- Thread tape (or locktite or simular)
The first step is to empty and wash the appliance. This MUST be done outdoors. Extinguishing powder is something really messy and it should be delivered as special waste. You may want to ask the nearest fire station or company that services fire extinguishers for help. If you choose to do it yourself, be warned. It will be a lot of powder. The powder does not dissolve in water, and dusts violently. Do it far away from houses, cars, etc, etc. Turn the appliance upside down. Release all pressure as carefully as possible so as not to blow out too much of the powder. When the pressure is completely equalized, then the entire top can be unscrewed. Also unscrew the hose, you do not need it anymore. Pour the powder into a tight bucket with a lid and depose of propperly. Then wash all the parts well while you think about what I said that it would have been a good idea to get help from some professionals for this.
Cut off most of the internal riser so that it does not absorb debris and condensation from the inside of the container. When everything is dry, just screw on the top again. Screw in the quick coupling where the hose was. Use thread tape to make it airtight.
Screw two couplings together (use thread tape again). This is needed to fill the 
container with a standard compressor. Then just connect the compressor and push down the handle to fill the tank with compressed air. I also drilled an extra hole in the handle so that the safety pin can be used to keep the handle in the open position.
I also use a pressure regulator with water / oil separator to get a little cleaner air. This is just a standard regulator with connected quick connectors. When I refill air, I reverse it get clean air both ways.
