There are many threads about dust collectors and modifications made to them. Unfortunately, there is relatively little information on the actual dust collector performance and the effect of the modifications. I did some measurements using some expensive instruments that most people will not have access to. There has to be an easier way of checking the performance.
The easiest way is to use a manometer to measure the static pressure of the system. You can make your own manometer cheaply, quickly and it will be accurate. I made one using a 6” by 24” piece or ½” plywood, a length of ¼” clear tubing, zip ties and colored water.
I marked off the board every two inches and drilled holes large enough for my zip ties to go through and then attached the plastic tubing to the board.
I then filled it partially with water colored with blue food dye.
On the right, the dust collector is off and the difference in heights is zero inches.
In the middle, I opened two gates and measured a difference in height of 8.5”.
On the left, I had only one gate open and measured a difference in height of 10.3”.
(Note – I checked my measurements with this water manometer against a digital manometer and found very good agreement.)
So using this information, I looked at the performance curve for my dust collector. I found the 8.5” and went across to the curve and then down to the axis and it says that my flow was about 950 CFM. Doing the same thing with the 10.3”, the graph shows 625 CFM.
One of the popular dust collectors is the Harbor Freight 2 HP unit. I found a performance curve for it in a Wood Magazine Article and copied it. To measure static pressure, one could drill a hole the size of the plastic tube near the dust collector and insert the end of the tube.
As an example, if you measured a difference in height of 6.0” you could use the curve to find that it corresponds to about 400 CFM. Or if the difference in height was 3.0”, the flow would be about 600 CFM. Doing this would allow one to determine the effects of various hose lengths or the flow that you would get when connected to various machines.