This discussion on the validity of the fan type anemometer was buried at the end of another current thread. I am starting this thread to have a topic just about it.
In 2016, I posted a thread about the results I got doing some testing with a Fan Type Anemometer called "Measuring Dust Collector Performance - Fan Type Anemometer" . What I found is using one can get wildly ranging results depending on how close you held it to the opening of your duct or hose. This does not even take into account the angle that you hold it. Lastly, the worst place to measure air flow is at the end of a pipe as you have air flowing in all directions. The presence of a large device like the fan anemometer also changes the flows.
After I posted the above mentioned thread, I spent more time reading on the Australian Forum - woodworkforums dot com. I would provide a link but not allowed here. They have a Dust Extraction Forum that is very good and an expert BobL who is a frequent contributor. I am going to put a couple of quotes from here for people to read. If you are interested, I would encourage you to read this forum to gain a better understanding.
This has to do with the capability of the 2 hp units
Yep I have measured the standard 2HP DCs from carbatech, timbecon, H&F and an older Ron Mac.
They are all the same.
Using either one of the two 4" inlet ports the pull about 425 CFM.
Removing the plastic 2 x 4" adapter reveals a 4.5" port that draws about 570 CFM
Pleated filters add a few %, less to the straight 4" port and a bit more to the 4.5" port
Modified to full 6" flow according to the generic 2HP DC thread produces about 870CFM and about 10% more with pleated filters.
These have to do with the fan type anemometers
Propellor type anemometers are not appropriate for measurement of air speeds in or around wood dust extraction ducts.
In general they significantly over estimate the air speeds and as their measurements are highly position sensitive cannot be used reliably even as a rough indicator..
Most changes that are made to DC systems produce small (a few % changes) so basing changes on propeller type measurements can end up making things worse rather than better.
I would advise against the use of any anemometers at the entrance of any DC duct to measure flow. It's dead easy to measure an air speed at a specific point but the air flow patterns at entrances are so complex and turbulent that it is near impossible to convert that into a flow. This is why a test duct is used.
you cannot place an anemometer at the front of an inlet where the air flow is highly sensitive to orientation and position and what is in line further down the air path. You can have the exact same position and orientation but something you did further along the air path alters the preferred air flow path back at the propellor and this will give you you a wrong reading.