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How to measure CFM, static pressure on dust collector, Part Two...progress and confusion

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Forum topic by Sark posted 08-08-2020 12:48 AM 1291 views 2 times favorited 20 replies Add to Favorites Watch
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Sark

316 posts in 1208 days


08-08-2020 12:48 AM

Topic tags/keywords: dust collector cfm static pressure sp velocity pitot tube anemometer manometer duct ducting

I’m starting a new thread of dust collector performance measurement. Part One, was getting a bit long, but contains a lot of information, thanks to lots of well informed posters. But now I’ve reach a new level of understanding/confusion and want to ask a few more questions.

First, Redoak inspired me to build a liquid manometer, which is very easy to build (less than an hour plus trip to HD to buy plastic tubing). From the picture you can see that the input part of the tube is glued onto a small square of plywood. This I placed over the end of the ducting, motors running, and (hooray!) measured 10” of pressure. Which I take to be static pressure, since no air was flowing. Is this a true assumption?

Next I moved this beautiful instrument to a hole on the side of the duct and measured 6”. Is this the static pressure? It’s 4” less than what was measured with the tube at the end of the duct with no air flowing.

10” of static pressure is what I would expect from a 2 HP dust collector, so I’m thinking that’s the proper value. Plus its unambiguous. The plywood is sucked against the duct with great force.


20 replies so far

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Sark

316 posts in 1208 days


#1 posted 08-08-2020 01:07 AM

Next I bought an anemometer to measure air velocity. Unfortunately, this model only goes to 67 mph or 5,900 ft/minute and I’m getting a much much higher air flow than that. (Not surprising, since I have two 2-HP blowers feeding into a single 6” duct.)

With only 1 blower operating, the reading was 77 mph, over the spec of the instrument. With both blowers operating, forget-about-it. Wildly inaccurate. I think the wind velocity must be about 120 mph. Any ideas on how to measure this hurricane’s velocity?

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Redoak49

4869 posts in 2836 days


#2 posted 08-08-2020 01:21 AM

The best discussion of measuring performance are on the Aussie site woodworkforums.com and sub forum Dust extraction. BobL has the most technical expertise of anyone I have seen on the forums.

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clagwell

293 posts in 640 days


#3 posted 08-08-2020 01:18 PM

I’m a bit confused. Don’t you already have a pitot tube and digital manometer? That’s the most accurate way to measure velocity in a duct. Much better than a cheap vane type anemometer.

The anemometer is useful for things like HVAC flow measurements. The velocities are much lower and the area much larger. At low velocities the pitot tube has low resolution. In a dust collection size duct the anemometer is big enough to reduce the effective area of the duct and so changes the flow resulting in sometimes large errors.

Your water gauge can be used to do a rough check of the calibration of your digital manometer.


First, Redoak inspired me to build a liquid manometer, which is very easy to build (less than an hour plus trip to HD to buy plastic tubing). From the picture you can see that the input part of the tube is glued onto a small square of plywood. This I placed over the end of the ducting, motors running, and (hooray!) measured 10” of pressure. Which I take to be static pressure, since no air was flowing. Is this a true assumption?

Yes. You should get the same reading with your digital manometer.

Next I moved this beautiful instrument to a hole on the side of the duct and measured 6”. Is this the static pressure? It’s 4” less than what was measured with the tube at the end of the duct with no air flowing.

Roughly, yes. If you just poked the tube into the hole the protruding length caused the air to speed up going over it. This drops the pressure to somewhat below the static pressure. Since static pressure is negative to begin with your reading likely overestimates the actual static pressure. For accuracy it should be tiny hole and flush with the inside of the duct.

Again, if you substitute the digital manometer you should get the same reading.

- Sark


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

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Sark

316 posts in 1208 days


#4 posted 08-10-2020 03:12 PM

Clagwell, my confusion comes from an inability to understand or believe the results I’m getting from the pitot tube/digital manometer. Also I have no instructions on how to use the manometer and couldn’t find any. Two very simple devices, why can’t I understand it?

To simplify: The manometer has two ports and one button. The button toggles from reading absolute pressure to differential pressure. Question: When reading absolute pressure, which port does it read from, the left or right or does it make a difference? Question: When reading absolute pressure, must you disconnect the other port or can you leave it connected to the pitot tube? (I think the answer is you must disconnect one tube to read absolute pressure)

Differential pressure seems obvious: the manometer subtracts the reading from one port from the other.

Here’s the results I get:

One tube connected to central part of pitot tube (total pressure)
Total pressure: 4.4” H2O (Way too low, lower than the SP)

One tube connected to SP part of pitot tube:
SP: 6.77” (Should be about 10”)

Differential pressure (both tubes connected)
Diff: .13” (too small)

Absolute pressure (both tubes connected)
Total Pressure= -2.4”. (Makes no sense)

None of this seems right to me. A standard 2HP blower should develop about 10” of SP. When I slap a piece of plywood over the duct (stopping the airflow) and measure the pressure, I do indeed measure 10” of pressure. Which I assume to be static pressure. And in this case, both water and digital readings agree. So giving up on any other measurement ( or discussion), is this a valid way to measure the static pressure?

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clagwell

293 posts in 640 days


#5 posted 08-10-2020 04:00 PM



None of this seems right to me. A standard 2HP blower should develop about 10” of SP. When I slap a piece of plywood over the duct (stopping the airflow) and measure the pressure, I do indeed measure 10” of pressure. Which I assume to be static pressure. And in this case, both water and digital readings agree. So giving up on any other measurement ( or discussion), is this a valid way to measure the static pressure?

- Sark

Yes, that is the way to measure the MAXIMUM static pressure of the blower. Keep in mind that the static pressure at the blower inlet decreases with increasing air flow. That’s what a fan curve is all about.

One common source of confusion is the static pressure in a duct on the inlet side of a fan is NEGATIVE. The static pressure in an airstream is a measure of the potential energy per unit volume. Velocity pressure measures kinetic energy per unit volume. The two add up to total pressure. That is, the total energy is the sum of kinetic and potential energies. For example, 800 CFM in a 6” duct is about 1” velocity pressure. If the static pressure is say, -2.5” the total pressure is -1.5”. Polarity is important.

You have two manometers, one analog the other digital. They should give the same readings in the same conditions. Play around with the two until you understand what the buttons on the digital do.

If you really want to give up on the digital manometer then just use the analog one you built with the pitot tube.

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

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Sark

316 posts in 1208 days


#6 posted 08-10-2020 05:27 PM

Thanks!! OK, I’ll keep working at it, and there is a least some hope for understanding. I can relate to the difference between potential and kinetic energy (degree in physics an eon ago). Much appreciate your help.

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pintodeluxe

6204 posts in 3661 days


#7 posted 08-10-2020 07:37 PM

This is a cool investigation. I tried to measure air velocity with an anemometer, and nearly lost my grip on the thing. I could just imagine that electronic device getting sucked up into the dust collector, and thrashed around the cyclone.

I figured my seat-of-the-pants test proved there was probably enough air velocity in my system.

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

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Sark

316 posts in 1208 days


#8 posted 08-13-2020 05:45 AM

Success! Thanks to clagwell and others.

The digital anemometer has two ports and a button labeled ‘Dif.’ When you connect both ports the reading is the pressure difference between the two ports. Simple. The ‘Dif’ button allows you to compare two different readings that are separated in time. It is like a tare weight on a scale. You can zero out a reading and compare it to your next reading. Two separate readings compared, that’s what the Dif button allows (and I had no use for this feature).

A pitot tube has two ports, one to measure total pressure (TP), and the other to measure static pressure (SP). The difference between these two measurements is the velocity pressure (VP). From the VP, the air speed in the duct can be calculated, or as I did, just looked up on a chart. For my 2 fan system, here’s the numbers:

Total Pressure: -4.2”
Static Pressure: -6.7”
Velocity Pressure: 2.5”

2.5” of VP= 6,500 ft/min, and in a 6” duct = 1,276 cubic feet/min (cfm)

With one fan running:
VP=.95” which is 3,800 ft/min or 744 cfm.

Confusion arose because I didn’t understand the anemometer especially the Dif button.
Second confusion was I didn’t understand that the pressure is negative, hence minus signs for total pressure and static pressure…but a plus sign for the velocity pressure.

Thanks for all your help!

For those learning, (as I have been) about air duct measurement, I’m including a picture of the pitot tube and the manometer. The part of the pitot tube pointing left towards the manometer is for measuring the static pressure (SP). The part of the tube that goes straight up in the picture is to measure the total pressure (TP). As shown in the picture, the manometer is connected to both ports on the pitot tube, and that measurement is the difference between the TP and SP, or is the velocity pressure (VP).

The pitot tube and manometer each cost about $40, in case you want to play with the instrumentation.

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Sylvain

1074 posts in 3347 days


#9 posted 08-13-2020 08:11 AM

6500ft/min in a 6” tube will not give 1276 cfm.

The air speed is null against the tube-wall and maximum in the center of it.
If the air flow is laminar, the velocity profile is parabolic; if the air flow is turbulent, the air flow has another velocity profile.
look here for example:
http://www.learneasy.info/MDME/MEMmods/MEM23006A/fluid_mech/lab-windtunnel.html

To establish the speed profile, one has to make multiple measures along a diameter.
One has to make an integration to obtain the number of CFM.

interesting link:
https://www.dwyer-inst.com/ApplicationGuides/?ID=16

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn

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clagwell

293 posts in 640 days


#10 posted 08-13-2020 10:22 AM



...
The digital anemometer has two ports and a button labeled ‘Dif.’ When you connect both ports the reading is the pressure difference between the two ports. Simple. The ‘Dif’ button allows you to compare two different readings that are separated in time. It is like a tare weight on a scale. You can zero out a reading and compare it to your next reading. Two separate readings compared, that’s what the Dif button allows (and I had no use for this feature).
...


Yeah, sometimes you have to play with a new tool a bit to figure out which is the pointy end. Glad you got it working!

After looking closer at your manometer pic I have to apologize. Except for color and brand name it looks identical to one of mine. At some point in the past I must have known what the Dif button was for because I never use it now. What I remember when I occasionally use the instrument is to only use the power button and the average button on the side – don’t touch anything else!

Speaking of the average button, be sure you know how to use it. Much easier than trying to estimate an eyeball average on a couple of digits or a water column height bouncing around.


6500ft/min in a 6” tube will not give 1276 cfm.

The air speed is null against the tube-wall and maximum in the center of it.
If the air flow is laminar, the velocity profile is parabolic; if the air flow is turbulent, the air flow has another velocity profile.
look here for example:
http://www.learneasy.info/MDME/MEMmods/MEM23006A/fluid_mech/lab-windtunnel.html

To establish the speed profile, one has to make multiple measures along a diameter.
One has to make an integration to obtain the number of CFM.

interesting link:
https://www.dwyer-inst.com/ApplicationGuides/?ID=16

- Sylvain

All good points, and will certainly be addressed in the future, along with some more error sources that you didn’t bring up. For right now, though, we’re just trying to get his instrument skills squared away. The numbers he has are still more accurate than a vane type anemometer would provide and should be useful going forward.

I appreciate though that you brought up the fact that there’s more to it than what we’ve seen here so far.

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

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Robert

3932 posts in 2328 days


#11 posted 08-13-2020 11:21 AM

Probably interesting and a lot of work to get the numbers, but the bottom line is reality IOW is it collecting well or not?

I have a set up that by everything I know is not supposed to work, but it does. Only because I couldn’t find a 3hp blower and used a 1 1/2 “temporarily” 4 yrs ago. Cyclone/exhaust outside/6” ducts/floor sweep 25’ away will suck up a screw.

-- Everything is a prototype thats why its one of a kind!!

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Sark

316 posts in 1208 days


#12 posted 08-13-2020 07:08 PM

Sylvain, thanks for the links, good information. Now to get accurate according to the links, takes something like 12-20 measurements (equal area) which are then averaged. Not sure I have patience for this much data collection. From what I’ve read, applying a factor of .9 or .8 to the central reading will get me close enough to satisfy my curiosity about how the system is working. “Useful numbers going forward” as clagwell said.

Also, thanks Clagwell for pointing out the average button, which I didn’t use in my data report. Next time…

Still working on running the ducting, so Robert, nothing to report on how it works. Since I have no dust collection right now, a system with two 2-hp blowers, Super Dust Deputy and 6” ducting is bound to be an enormous improvement. Which according to the corollary of Beranek’s Law, I am certain to admire greatly.

Finally, the first dust collection I set up was for my cabinet shop. I was scrambling to make money in the business, and didn’t have time to fuss with airflow measurements, theory and other niceties. We installed a ClearVu system with a 5HP blower and ran 6” PVC ducting everywhere. Worked amazingly well without any analysis and almost no design. Just too big, too expensive, and too noisy for my little garage shop.

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Sylvain

1074 posts in 3347 days


#13 posted 08-14-2020 08:15 AM

Of course, in real life one doesn’t need a precise measurements and applying a 0.9 factor to your calculus will be good enough.

And anyway, it will vary with the atmospheric pressure, humidity and temperature of the day, not to mention what one will connect to the system.

Precise measurements are only needed for the manufacturer who wants to verify his design and publish specifications.
But if one likes experimenting, why not.

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn

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kroginold

57 posts in 1896 days


#14 posted 08-14-2020 09:16 AM

I think the most accurate way to measure air movement in a duct is with a hot wire anemometer. Take readings at multiple points in the opening and average the readings. Does not depend on movement of tiny fans and works on any size ducting

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clagwell

293 posts in 640 days


#15 posted 08-14-2020 02:04 PM



I think the most accurate way to measure air movement in a duct is with a hot wire anemometer. Take readings at multiple points in the opening and average the readings. Does not depend on movement of tiny fans and works on any size ducting

- kroginold

Yes, a HWA is certainly a good tool for measuring flow in HVAC where the ducts are large and the velocities low. At speeds below 1000 fpm or so a Pitot tube puts big demands on manometer resolution. That’s where a HWA really is useful.

OP’s 6500 fpm is well outside the range of most HWAs and a 6” duct is too small for a proper traverse – you just can’t get close enough to the wall with any HWA I’ve seen.

Even with a HWA and an oversize test duct you still need a manometer to fully characterize the fan so in this case it’s actually less costly to use the proper tool for the job.

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

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