My guess(tm) is because the device is looking only for large particles (>1 micron) such as cigarette smog and not small particles such as mold and bacteria. The device has been around for a long time, possibly before cheap UV emitters and before GaN, InGaN and AlGaN detectors were commonly available. However, it might also be something as simple as the age of the Nikken device, which probably preceded cheap UV components.
No need for a class and there are only a few tricks:
- Use Google images. If you're looking for a schematic of something, search for schematic images, not for text.
- Know thy buzzwords. The proper selection of search terms is paramount. I usually dive into the Wikipedia article first to extract the necessary buzzwords, and then search for them either individually or as a group.
- Don't rely completely on Google search. I like to use Blekko when I'm stumped, but which does have a learning curve:
- If you want research papers and/or patents, use Google Scholar search:
- When posting a link, try to reduce it to only the necessary parts, removing all the tracking info, and irrelevant detail. That's not easy on some searches and does require learning how the URL's work. I guess that might be a worthy topic for a web page (but not a class).
The rest is just reading. I must admit that I'm more into speed than accuracy, which results in my missing some obvious points, but overall seems to work well.
I also like to read pages backwards, starting at the bottom. Many web authors have the common affliction of describing problems and solutions in reverse order. They write like a mystery story, where the details are supplied first, and the important conclusions at the end.
I hope this helps.
I would have posted a link if I had found something relevant. I'll try again, but not for a few days. Busy with other projects.
I can't do anything to compensate for filth in the ducting, but measuring air speed is fairly trivial with a hot wire anemometer. I've built several for weather stations that are quite useful even at low wind speeds. If fan bearing (more likely bushing) wear is deemed a problem, then a feedback loop with the hot wire anemometer controlling the fan speed should compensate for any variations in speed. Of course, altitude and temperature enter into the equations, so this isn't really trivial, but methinks can be accomplished.
The problem is that you seem to be designing an "instrument" rather than something for home use. The requirements for each are quite different in areas of accuracy, calibration, repeatability, etc. It's much easier to take a precision instrument, and downgrade it for home use, than to convert something crude into a precision instrument. For example, your question about identifying particle composition might be better addressed with gas chromatography or possibly a mass spectrometer.
Marginally related anecdote: You don't really need all the technology to count dust particles. I once had to deal with a serious airborne pollution problem, twice. I'll leave out the details (unless someone wants them). The problem was to measure how much dust was being transported by the HVAC ducting. I didn't have time to rent air quality instrumentation and just needed something fast. I purchased some sticky back window shelf paper[1]. I placed some 100 cm squares in the air stream, and gave it about 30 minutes to collect some dust. With a microscope, reticule, and my miserable math, I estimated the particle density from several small sample area. The microscope also gave me a clue as to what I was counting. I made an order of magnitude math error, but the dust problem was sufficiently bad that it didn't matter. In one case, the dust was coming from a nearby industrial plant with a malfunctional electrostatic precipitator. In the other, the 1989 earthquake had broken open the HVAC ceiling ducting joints which allowed the introduction of nearby blown insulation into the air ducts.
[1] Use paper, not vinyl as the vinyl will build up a static charge from the air flow.