i've spent a few hours researching respirators lately. yeah, i'm a dork.
at first glance at 3M's respirator and accessory options, i had no idea what i needed, but after looking over their literature i was able to figure it out and get (i think) exactly what i need.
most helpful were the following .pdfs from 3M:
2006 Respirator Product Selection Guide
Half and Full Facepiece Respirators Brochure
Respirator Cartridge and Filter Selection

i ended up buying everything through Cooper Safety, simply because they had the best prices and had everything in stock.
i wanted a system that would filter out all the contaminants i plan to encounter: hydrochloric acid (HCl, in Sno-Bol, for brazing cean-up); acetone (for removing oils prior to HCl), brazing fumes, and particulates generated by grinding. i found out an Acid Gas cartridge is needed for HCl gases, an Organic Vapor for acetone, and a particulate filter is for the other two. there's about six different kinds of particulate filters; the choice is made based on whether or not the particulates are attached to oil molecules (N if not, R if sometimes, and P if yes) and what degree of "filter efficiency" is needed (95=95%, up to 100=99.97% or so). i don't really know for sure what i need in these departments. my metal teacher said N95 is fine for grinding, so i got one of those and a P100 for anything really serious.
to protect against all of these you need the following:
a mask, which straps to your face, covering nose and mouth. i got a 6000, seems adequate.
a pair of cartridges, one of which couples to each side of the mask. there's an array of them available to protect against almost anything. i got an OV/AG: Organic Vapors and Acid Gas.
to add particulate protection, there's a couple options: you can add a pre-filter and retaining ring to the outside of the cartridge, or add a filter adaptor to allow fitting circular filters to the rectangular cartridge (which i opted for, mostly because that's what they had).
i'm still not sure if the filter can be used by itself, which i'd think would be good if the cartridge causes some breathing resistance.

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