How to Cheaply and Easily Measure Dust Collector Performance

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Blog entry by Redoak49 posted 08-04-2016 03:43 PM 16548 reads 5 times favorited 17 comments Add to Favorites Watch

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.

17 comments so far

View pintodeluxe's profile


6488 posts in 4057 days

#1 posted 08-04-2016 04:53 PM

Interesting discussion. How do you actually take the measurements?

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View Redoak49's profile (online now)


5392 posts in 3233 days

#2 posted 08-04-2016 06:05 PM

As mentioned above, you just need to measure the difference in the height of the two sides. It is very simple and you can use a ruler or anything that you want. One side of the manometer is attached to your dust collector and the other is just open.

View pintodeluxe's profile


6488 posts in 4057 days

#3 posted 08-04-2016 06:46 PM

What I meant was how did you attach the hose to the dust collector? Just inside a DC hose, or in the body of the collector itself?

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View Redoak49's profile (online now)


5392 posts in 3233 days

#4 posted 08-04-2016 07:12 PM

I understand…where did I connect the tube? I did it near the inlet just before a hose is connected. The best way is with a barbed connector but drilling a hole just smaller than the tube and shoving it in works as long as there is no leak. Or make a short section of pipe or metal duct to put on the inlet of your dust collector. Attach your dust collector hose on the other end. You can attach the manometer tube to the short section of pipe.

You can also look at the blog I recently post about measurements. The schematics show where I connected on my system.

View htl's profile


5577 posts in 2403 days

#5 posted 08-04-2016 08:03 PM

Funny you put this up just as I was posting my cyclone build that I made lost year.
I sure could have used it when I had 3 completely different type systems and trying to figure out which one and what was working best.
May have to get the old safety cone cyclone off the shelf and test it out with my wood one. lol
Thanks for the How2 redoak49!

-- An Index Of My Model making Blogs

View Mark Shymanski's profile

Mark Shymanski

5623 posts in 4957 days

#6 posted 08-04-2016 09:57 PM

Very interesting, thanks for posting this.

-- "Checking for square? What madness is this! The cabinet is square because I will it to be so!" Jeremy Greiner LJ Topic#20953 2011 Feb 2

View AandCstyle's profile


3306 posts in 3501 days

#7 posted 08-04-2016 10:00 PM

Red, thank you for this. I will find some tubing tomorrow and begin to better understand my DC’s performance. LJs gives a “Daily Top 3” for projects, but this should get a “Best Blog” award because it is one of the most helpful I have read.

-- Art

View htl's profile


5577 posts in 2403 days

#8 posted 08-05-2016 12:34 AM

LJs gives a “Daily Top 3” for projects, but this should get a “Best Blog” award because it is one of the most helpful I have read.

- AandCstyle

I want to second that vote, a very useful post for any vac ahalic.

-- An Index Of My Model making Blogs

View EarlS's profile


4754 posts in 3592 days

#9 posted 08-05-2016 01:56 AM

I got a chuckle out of this thread. I’ve been heavily involved in designing several light phase pneumatic conveying systems at work. Basically, the important factors are the cubic feet per minute of air flow and the pressure (inches of water column) that the blower can generate. The blower generates a pressure differential across the fan so a higher outlet pressure say from a dirty filter bag will result in less suction on the inlet side of the fan.

From there, the pressure drop through the system, tee, elbows, slide gates, length of runs, etc. drive the performance of the various end users such as a table saw or a router table, planer, or whatever. Your fan will perform along the system curve with higher pressure and lower flow or higher flow and lower pressure. If you leave a lot of open or slightly open slide gates (think of buildup in the slide) then your pressure drops because the flow through the system has increased. As you get closer to the inlet of the blower the vacuum increases. That pressure differential is what moves the air and subsequently the saw dust. Too much saw dust all at once (like a planer) can result in sawdust building up in the low spots, elbows, or other places where the velocity is lower.

A secondary cyclone slows the velocity enough that the saw dust settles out because of the density (weigh per volume) of the larger dust particles. The really fine stuff continues through the fan into the bag where it accumulates. This issue is also why a lot of small shop vacuums can’t keep up with larger hoses, the velocity is not high enough to convey the saw dust. That is also why there is a pile of sawdust in the bottom of your table saw, the velocity in the corners isn’t high enough to move them out. Additionally, the dust blown off the saw blade happens because the rotation of the blade generates a slight positive pressure along the blade carrying the dust back over the top of the blade unless you have a really high negative pressure inside the box of the saw.

Does all of this make sense?? I’m kind of rambling.

-- Earl "I'm a pessamist - generally that increases the chance that things will turn out better than expected"

View Redoak49's profile (online now)


5392 posts in 3233 days

#10 posted 08-06-2016 02:38 PM

Wow…getting a chuckle from the thread. This was NOT a thread about theory. There are a number of threads on that subject. Or maybe, you could post a blog about the theory.

What it is about is helping and encouraging people to measure their systems in a cheap way. Many are making modifications with no idea as to the effects. This was a way that they could check their results.

View EarlS's profile


4754 posts in 3592 days

#11 posted 08-07-2016 01:28 PM

Easy RedOak – the chuckle is from how many times things I do at work pop up here at the same time. Just goes to show how much engineering there is in woodworking. As far as theory, most of the comments I provided were relating to practical explanations of why dust collection systems don’t work as expected. A manometer is nice to make, but the real question is how to properly use it to get the best performance out of your dust collector. Maybe your next post on the topic could include a discussion of how you used it to better your dust collection system.

-- Earl "I'm a pessamist - generally that increases the chance that things will turn out better than expected"

View Redoak49's profile (online now)


5392 posts in 3233 days

#12 posted 08-07-2016 06:17 PM

I am sorry if there is confusion. I have written several blog entries on my system and measurements showing the effects of various duct and hose configurations.

EarlS …. I would appreciate it if you would post about your dust collector at home and share your results. My experiences are that what I did in an industrial setting was not what I could afford to do at home.

View robscastle's profile


8272 posts in 3448 days

#13 posted 09-07-2016 11:18 AM

Very interesting material Redoak49

I agree the industrial workplace and home work place differ greatly

For example I guess if your dust collector is remotely located from the collection point I can see the requirement for measurement and performance indicators.

Otherwise as is with mine its right next to my work area so I can tell by ear or by a presents of chips that the collector is full, blocked or oherwise not performing correctly.

By clearing the bags lines crusifex etc a good indication of suction can be benchmarked by the level of suction on your hand and the velocity noise of air flow.
Once this is known I can by feel with my hand and ear tell if the DC is working OK or needing attention before even starting work.

Then from experience how soon I need to service it comes into play.

Sanding work produces little particle bulk but is the most frequent reducer of performance as it clogs the filter bags and restricts air flow rather than bulk content.
The next would have to be thicknessing and jointing as both produce a high volume of particle and quickly fill collection bags,

Followed by Table sawing, and dado blades.

This works for a small co located work area like mine but once you get into a large multi outlet system and posibly remote system the situation changes.

This one is at a school we service.

A two man job just to get the filters out.

Consideration to clear ducting may also be an advantage as the passage of material removed being able to be visually seen passing becomes sight sound and feel adding to the MMI man machine interface or human factors

-- Regards Rob

View Pitzerwm's profile


5 posts in 2821 days

#14 posted 06-16-2017 01:16 AM

I wanted to thank everyone on this tread for the info. I thought that I would also throw in what I have learned by trial & error.

I started out tapping into my central vac system, for my house which used a carwash vac. It had 2” flex hoses and I didn’t bother with sweeps and Y’s. It worked mainly for clean up. Starting to do more lathe work and wood working in general and starting to worry about my lungs, I started thinking about doing a better job.

I found a 1.5hp Cincinnati Fan unit with a cyclone and 55 gal drum. no bag. I picked up 4” piping and fittings and started putting things together. At some points I had to step down to 2”, but made these 2” runs as short as I could.

After reading here, I picked up a CFM/FPM meter : to see what I had. It uses a phone app and it is a direct read, no extra math needed. The Cincinnati Fan site had some good info too.

This unit, had a max CFM of 875, so in my mind that gave me a starting point. The CF unit had a 6” in with a 4” wye and the outlet 6” ducted down to 4”. My reading at the unit was 706CFM, 8086FPM, at one of the 4” opening. Since I had to install it outside, I added a 4” 90, and 6’ of pipe for the outlet which got it over the roofline. The new measurements were 597CFM, 6843FPM, in and 517CFM, 5923FPM out.

My piping is pretty simple the 4” coming into the back wall into a wye, and alone the back wall, 20’. There is a drop that goes to the center of the garage, for the table saw, which I will also use for the lathe, router, planner and jointer when using them. At short end of the backwall a drop for the bandsaw/drill. At the far end of the main run is a 1.5 run to the front of the garage to vac out the cars.

Here are the readings, at the 4” drop for the BS/drill 539CFM, 6174FPM, dropping down to 2” 203, 9315FPM. At the table saw 4” 370, 4235FPM.

The 2” drop at the far end of the main run, 212, 939FPM, and the 1.5 car vac, outlet 94, 7653FPM.

Since I didn’t need a bag, I took a small computer muffin fan and mounted it in the exhaust flow, hooking it to a free HF digital volt meter, I show 18.5 ACV , with almost nothing in the barrel.

I hope this info is helpful.

View ericbroder's profile


4 posts in 2702 days

#15 posted 08-10-2017 05:13 PM

Dumb question…

Can anyone post a picture of how the manometer is attached to a point in your dust collection system. For example, in a 6” duct, do you cut a 6” circle with a tube-sized hole to block the duct and direct all suction to the tubing?

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