I have posted several threads that have detailed dust collector testing and along with effects of ducts and hoses. I borrowed the test equipment to do this as it is not cheap especially the digital thermal anemometer.
Now that I have the performance curve for my system, I can use a simple homemade manometer to determine the static pressure and read from the graph for my system the approximate flow.
As I read the comments and questions, it would be really nice if there were an easier way for others to evaluate their system. Many people are not going to have access to the instruments that I used or want to go through all of the work I did.
Several people have posted about using a handheld fan type anemometer. I have done that using a “HoldPeak HP-866B” anemometer but have not been happy with the variability in the results. It seems that holding the fan anemometer in the right location at the end of a hose or duct is difficult as the dust collector is trying to suck it in and any movement of the anemometer results in larges changes in air velocity.
I thought that there had to be a better way than hand holding the fan anemometer. I used a piece of ¾” plywood and made a cut out for the anemometer and secured it in place using zip ties.
The 4 inch dust collector hose was secured to a piece of plywood to keep it from moving and the anemometer holder was held in place with a clamp. The distance was varied by using blocks of wood as spacers.
This system made of scraps of wood held things in place securely and it was possible to center the anemometer on the hose.
I measured the flow with the anemometer at several different distances from the end of the hose and then plotted the data with a curve fitted to the data in Excel.
The results show that the air speed measurement is very sensitive to the distance between the end of the hose and the fan anemometer. The speed goes up rapidly as the anemometer gets closer to the hose end. So what is the right distance to measure the speed? My tests with the same hose and conditions using the digital thermal anemometer gave about 490 cfm and 5621 fpm. Based upon the graph, it appears that holding the fan anemometer about a half an inch from the end of the 4” hose would be about right.
However, when working with your own system, it probably does not make a difference if you are 3/8” to 1/2” as long as you do it the same way every time. It would work well in trying to evaluate any modifications that are made.
Once you have the flow, you can also measure the static pressure. I used a homemade water u-tube for some of my previous work. It was simple to just measure the difference in height on the two sides to determine the static pressure and gave the same results as the digital manometer.
Now, with a little work and not too much expense (the anemometer I used is $23 on Amazon), you can evaluate your dust collector and any changes or modifications.
There are quite a few people on the forum with a wide range of dust collectors that are making modifications to them to enhance the performance. It would be extremely helpful if a few would take the time to evaluate the baseline for their dust collectors and then report on the results of modifications. I am certain that it would be a huge help to others seeking to do similar improvements.