Filtration tools

[LKJoel]

Tying to expand my knowledge on IAQ, so please bear with me while I ask a few questions and try to gain a little more knowledge

My Question is, what IAQ tools do most of you carry and what are the bare essentials to begin stocking. Specifically I am interested in tools that can
measure the effectiveness of installing HEPA filters and UV lamps. Something that I can take before and after readings with and show the home owner how much cleaner the indoor air is. I have been looking at some particulate counters, but wonder what you guys recommend using.

Thanks in advance

[homebrewski]

I know this isn't what you mean, but I suggest a decent digital camera, to show customers the results of bad iaq, and lack of maintenance; that you come across at job sites.