Thread by @azonenberg

New year, new... fume hood filter?

Not how the saying usually goes, but that's how things go in my lab. This is a ductless hood with a stack of two filters, a doped carbon for organic vapor/acid gas followed by a HEPA to catch carbon particles, sanding debris, etc.

Here's the service plenum with the access cover removed. I change the carbon filter annually and the HEPA only when I notice it's clogged enough to impair airflow.

This is pretty rare since I don't generate a lot of particles in the lab and the overall air quality is excellent.

After loosening the compression bar on the filter mount, I can remove the HEPA and save it for re-insertion over the new carbon filter.

If I used the hood for anything really nasty like asbestos I'd wear a respirator and bag the filter during this stage to prevent contaminating the lab.

But this filter is mostly for carbon particles and occasional paint spray or sanding debris, so no need for extreme care.

I did bag the used carbon filter, since it tends to shed carbon particles when handling it. The acid gases are converted to non-volatile salts by the alkaline doping on the filter, so no worry about those offgassing.

And the only other stuff used in the hood is things like acetone and IPA. Trace quantities of those outgassing isn't a significant air quality concern. Nothing super toxic or carcinogenic, the acid gases are the main reason I use it.

Empty filter plenum awaiting insertion of the new OV/AG filter.

Note my hand for scale in the last photo. While it's chemically the same as an OV/AG respirator cartridge like a 3M 60923, the quantity of media is massively greater!

All done, new filter inserted and existing HEPA put back on top of it. Just need to fill out the service sticker and do a flow velocity check.

Airflow velocity looks perfect. The moving-vane indicator in the hood reads slightly over 100 ft/min and double checking with an anemometer at the face reads slightly under. Right where we want it.