I wanted to keep track of modifications to my new HF DC, so I built a very simple manometer this morning. My only mod so far is the Wynn filter. Pictures below. Initial measurements
As close to the HF inlet as I can measure (see picture staticp.jpg), I get 6" H20, which if my calculations are correct, translates to 975 CFM. Note that I live at 7,200' altitude, so any DC will suffer a lot in performance. I'm also using a 4" pipe, a big no-no according to DC experts, but it makes hooking up to my tools a lot easier.
I then measured at the end of a Rockler 4' long expandable hose. See pic hose.jpg (it's unhooked in the picture) and Dust Right® 4'' Dia. Expandable Hoses | Rockler Woodworking and Hardware
This drops the performance to 2.5" H20, or about 630 CFM. Ouch. This was with the hose compressed and no bends. So, I guess I gotta change how I bring the DC to my tools.
Anyway, as I modify my DC, I want to keep track of performance changes. Some details
The manometer itself 1/2" tubing with filled with water, with green food coloring. I step that down to a long section of 3/8" tubing (to reach anywhere in my shop), then to 1/4" tubing. I drilled a 1/4" hole in a 4" hose splice, and that's where I measure static pressure. Again, see staticp.jpg. The markings on the manometer are 1/2" apart. Pretty simple.
My measurements may be a bit off, because I need to seal the attachments. But I doubt that they're off that much.
Converting inches of H20 to CFM requires some assumptions, the biggest being current air density. But that doesn't really matter. The bottom line is you want to maximize inches of H20 (the difference in the water levels), and the manometer lets you measure any changes to the DC system.