Thanks, I'll watch the video when I get a chance. My idea was to basically reproduce the setup in Joel Amilcar's PhD thesis (
https://ir.uiowa.edu/etd/2002/) using an off the shelf air quality sensor (PM2.5 with the hope of being able to extrapolate below that size), using a salt spray nebulized with vape gear. I didn't have any good ideas about how to control the particle size (this isn't my area at all) so had hoped to get some advice at some point (let me know if you have any thoughts). I had remembered reading about a guy on Youtube who did some tests like that but never managed to find the channel. Maybe it was yours! Someone else here mentioned Aaron Collins so I will have to check his videos too.
I think it would be great to have easily reproduceable, low cost test equipment available at shops and clinics because the quality of masks (even of a specific make and model) is likely to vary between units. You might find a good mask with your tests, and then find that your next order of the exact same mask is crap. So you have to keep testing. Maybe it is less bad now. I was interested in this more when masks were in very short supply, but then I managed to get a few boxes of them, and set the testing idea aside.