Then a piece of rubber hose is pushed over the bushing.  The hose holds the bushing in place and serves as a flexible connector for the vacuum cleaner hose.

Modify the 3/16” vacuum tees by plugging two of the three holes in each tee with small screws.  Then drill four .040” holes (#60 drill bit) in each Tee as shown.  Epoxy the vacuum tees into the holes.  Be careful not to plug the small .040” holes with epoxy.

The 4” test plugs will be used to make orifice plates of various sizes to suit the flow requirements of whatever you wish to test.  You will select an orifice that can provide a flow that will result in a reading on the inclined manometer that falls somewhere within about 55 – 85 % at the desired test pressure (ideally 15” H2O) read on the u-tube manometer.

Here's a picture of a blank test plug.

Orifice sizes and their associated flow are as follows:

51.85 mm = 209.5 CFM
42.38 mm = 139.5 CFM
29.98 mm = 70.4 CFM
21.16 mm = 35.5 CFM
9.92 mm = 7.8 CFM

You can make orifice plates with several sizes of holes and combine the flows to get a desired total flow.  For instance, if you use a plate with one 21.16 mm hole and one 9.92 mm hole, the orifice plate flow rating will be 43.3 CFM (21.16 + 7.8 = 43.3).  When you do a test, if you want to get a feel for how much air you are moving simply read the percentage indicated on your inclined manometer and multiply the orifice flow rating by the percentage.  Keep in mind that the contraption isn’t calibrated.  I also did not follow all the specifics outlined in the DTec article.  It’s intended primarily to show whether mods result in improved flow, and to be able to do meaningful comparisons.  If you are looking to capture precise flow numbers, this ain’t the tool for you.

The orifice plate gets installed next.  The size will depend on what you are testing.  More on that later.

Next install the upper section of 4” ABS pipe.

or adapters to suit whatever you are testing.

Don’t worry, the holes are easy to plug.  If you don’t want to spring the big bucks for the valves, you can make inexpensive bleed valves from PVC pipe and fittings.