Ah - well I have a powder that is out of date but functional (tested this arvo). As it's powder, to get it certified costs more than it's worth - so I'll keep it to hand for use when welding or grinding.
I'm used to coping with fires - a childhood in the Australian bush does that - it's just to make other people comfortable.
as above some extinguishers once operated discharge the full gas cylinder into the extinguisher body and although they have an operating valve for you to use they are not designed to keep stored pressure for any legnth of time so beware
If im on a job welding near dry timber work, especially old dusy stuff, i tend to wet it down afirst with a garden type pressure sprayer.
A water extinguisher will be what you require.
As Wyn said, if youve discharged a bit from it, good chance it may lose pressure now, and not work very well.
Sounds like you need someone fire spotting for you. I've seen a few near misses like this working solo. When your wrapped in a mask it's easy to miss that somethings burning nearby. Is heat transfer to wood the issue or spatter etc..? Even disc cutters in a confined space are a fire risk for the same reason.