Harbor Freight Rikon Impeller Upgrade – Is It Worth It? #2: Airflow Measurement Setup

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Blog entry by clagwell posted 12-03-2019 01:25 PM 1623 reads 0 times favorited 2 comments Add to Favorites Watch
« Part 1: Intro and Bottom Line Part 2 of Harbor Freight Rikon Impeller Upgrade – Is It Worth It? series Part 3: Help! My motor's smoking! »

Test Duct
My test duct is a 10’ length of 6” PVC drain pipe (ASTM 2729). This is the type of duct often used for dust collection in home workshops and will most likely be repurposed for that in the future (after I plug all of the holes!).

At the midpoint of the pipe is a measurement port for a hot wire anemometer or pitot tube. There are three pressure ports located at 17” from the duct inlet, 57” from the duct inlet, and 22” from the fan. Performance curves used pressure data from the port closest to the fan.

The pressure ports are implemented as four tap piezometer rings.

Another forum had discussed the Testo 405i hot wire anemometer and it’s ability to record data on a phone via Bluetooth. That led me to their 510i manometer. It looked like the pair would save me a lot of time since I could record both flow rate and pressure and then dump the data to a spreadsheet. I took a chance and bought one of each.

Unfortunately the 405i turned out to be very wrong for use in a 6” duct and was demoted to thermometer.

I ended up using the 510i with a pitot tube for the flow measurements and had to use my old generic manometer for pressure. In retrospect it would have been better to buy a second 510i instead of the 405i.

Airflow velocity is not uniform in a duct so to get accurate numbers it’s necessary to sample the velocity at various points within the duct and then use the average. In a round duct this is done by testing at several specific points along a diameter and then repeating at different angle(s). There are industry standards that specify the location of the points. A single pass across a diameter is called a traverse.

I’m a bit of a stickler about repeatability so I 3D printed an indexing jig for doing the traverse.

There are various techniques for restricting the duct inlet. One that you see often is a cone that is an adjustable distance from the entrance. It can be fine tuned if you are trying to set a particular pressure or flow. Another is to use various sizes of orifice. You’re limited to a fixed number of restriction values but the repeatability of the restriction is excellent. I chose repeatability over fine tuning and 3D printed a number of orifice plates.

-- Dave, Tippecanoe County, IN --- Is there a corollary to Beranek.s Law that applies to dust collection?

2 comments so far

View Redoak49's profile


5393 posts in 3234 days

#1 posted 12-03-2019 04:11 PM

Very Good setup and will be interested in the results.

View a1Jim's profile


118258 posts in 4822 days

#2 posted 12-03-2019 04:19 PM

Wow very scientific my test is as basic as it gets “does it work for what I need it to do” It’s great that you have the equipment and know-how to test this.


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