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How to use pitot tube and manometer to measure air flow in duct

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Forum topic by Sark posted 07-27-2020 08:52 PM 1028 views 1 time favorited 20 replies Add to Favorites Watch
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Sark

353 posts in 1328 days


07-27-2020 08:52 PM

Topic tags/keywords: manometer cfm static pressure sp dust collector duct

I just finished the first step in my dust collection system which is getting the blowers installed with 240v and shock absorbers and about 10’ of 6” duct. More on the dust collector design when I’ve got a bit more to show.

But before moving on with connecting the duct to the cyclone, I’d like to know what system is delivering in SP and CFM.

I have a pitot tube and a digital manometer. The manometer measures both absolute pressure and differential pressure (has two ports). The pitot also has two ports. But I can’t find any directions on proper use. Lots of videos on fluid manometers, flow meters,,,etc… But I can’t find any explanation on how to use a simple digital manometer and pitot tube.

My questions are: what is the absolute pressure measuring? What is the differential reading measuring?
For example: In the duct, the absolute reading is 8.6” H2O and the differential is 6.2” H2O. Should I disconnect one of the tubes before taking the absolute reading? I’m confused. Anyone can point me in the direction? I’ve looked at lots of web pages without getting any clarity. Thanks for your help.


20 replies so far

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hairy

3191 posts in 4500 days


#1 posted 07-27-2020 09:23 PM

Maybe this will help.

https://www.youtube.com/watch?v=f92mDnLpWFU

-- You can lead a horse to water, but you can't tie his shoes. Blaze Foley

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clagwell

340 posts in 760 days


#2 posted 07-27-2020 10:15 PM

Assuming your pitot tube looks like this:

The port pointing to the left gives the total pressure, that is, the sum of static pressure and velocity pressure. The port pointing up is the static port. You measure static pressure here.

The pressure difference between the two ports is the velocity pressure. Take the square root of velocity pressure in inches of water and multiply it by 4005 and you have velocity in feet per minute. Measure the velocity at the appropriate traverse points across the duct and average those readings. Multiply the average by the duct area to get CFM. The absolute pressure at the static port is the static pressure.

Do that for various restrictions at the test pipe inlet. Plot the CFM and pressure values to get the performance curve for your DC.

If you don’t want to do a traverse then just measure the centerline velocity and multiply it by a fudge factor of 0.9 or so to estimate the average.

Again, the velocity pressure is the difference between the two ports. The total pressure port should read positive with respect to the static port. The static port will be negative with respect to ambient pressure. You should connect that to the negative input on your meter to reduce the effect of asymmetry in the sensor. That is, plus to the total port and minus to the static port for velocity. Remove the total connection for static reading.

Doing that gives you a positive number for what is actually a negative value for static pressure but that’s the way fan curves are plotted.

Oh, one more thing. The usual practice is to locate the the pitot tube 1.5 pipe diameters from the fan inlet with a 10D pipe length. Your 10’ pipe is 20D so long enough to move the measurement point a bit farther away from the fan. A foot or two would be fine.

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

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Sark

353 posts in 1328 days


#3 posted 07-28-2020 12:58 AM

Clagwell: Thanks for the response!
Here’s a picture of my pitot tube, let’s see if this makes sense:

This pitot tube has two concentric tubes.
At the end of the inner tube, there is a port which is presumably measuring the total pressure.
The outer tube ends in a series of concentric holes about an inch away from the end of the tube. I’m assuming that this must be measuring the static pressure?

The difference in pressures between these two points is the velocity pressure, and is measured when I use the differential button on the manometer. At the center of the duct (the highest reading) I measured 6.1” . Apply about an .9 factor to get an average flow at all points in the duct, and I get a velocity of 9,382 fpm, (which seems high). Which equates to 1,842 CFM and that is pretty much exactly what I would expect from this 4 HP system.

So how do I measure the static pressure, just use the inner tube with the holes in it, and disconnect the other port from the instrument?

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Sark

353 posts in 1328 days


#4 posted 07-28-2020 01:36 AM

I think I just answered my own question. The reading on just the outer section of the pitot tube (the part with small concentric holes) is about 8.5” H2O and this is about what I’d expect from the system. Now I as I connect up the rest of the dust collection system, we’ll be able to see what a difference each of the components make. Next on the list is ducting the exhaust to the outside. No filters on this system, so hopefully not much loss.

PS, it’s a really strong suction, and sucked my hand into the duct with some force…

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Andre

4101 posts in 2774 days


#5 posted 07-28-2020 04:25 AM

When I put mine together ended up buying an anemometer, no more guessing:) First time I tested the system found out that a heavy duty garbage can was no match, collapsed it like a beer can:) This was under a Veritas Cyclone lid.

-- Lifting one end of the plank.

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clagwell

340 posts in 760 days


#6 posted 07-28-2020 09:34 AM

Those numbers sound reasonable for a 4 HP blower. Do you mind a few questions?

1) Where is your measurement port? Pic?
2) What does the intake end of the duct look like? Pic?
3) What’s the fan diameter?
4) Can you post a pic of your blower?

And a suggestion – record the motor current every time you test airflow.

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

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Fred Hargis

6727 posts in 3461 days


#7 posted 07-28-2020 10:45 AM

Dave did a great job with his explanation. I would add you wnant to measure int in a few different spots across the pipe diameter and average it out. It may not make much difference, it didn’t change much on my system, but it’s the proper way. At least do it once to see how much difference there is.

-- Our village hasn't lost it's idiot, he was elected to congress.

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Redoak49

5022 posts in 2957 days


#8 posted 07-28-2020 11:00 AM

How about some pictures of your dust collector and testing set up?

Can you provide the specs on your dust collector?

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Sark

353 posts in 1328 days


#9 posted 07-28-2020 04:03 PM

Following are a couple of pictures of the dust-collection system as a work in process. It is located on a mezzanine storage area in my garage shop.

Blowers: Grizzly 2 HP blowers. Yes, two of them. Each about 20 years old, but dust collectors seem to run forever and I got them for not much money. I’m quite certain that a single 4HP blower with a bigger impeller would be superior, but then how much dust collector can you buy for $300? I gave away/discarded all but the motors/fans which are mounted on shock absorbers.

Ducts & Fittings: The main duct is 6” diameter #28 gauge. The ports on the Grizzly are a bit under 5” (140mm) so I fabricated a 6,6,5 Y for the first blower and a 6-5 taper piece and 45 degree angle for the second piece. The 5” duct fits nicely over the blower intake ports. I’m fabricating most of the fittings, partly for the experience and partly because the layout was so complicated that I couldn’t figure out in advance what I would need. Long story, and if you’re interested, I’ll post as a separate topic.

Dimensions (approx): From the end of the duct to the Y is 50”. From the Y to the 45 deg angle is 20”. Connection to the blowers are each about 15” .

Location of measurement port: 9” from end of duct. In picture, shown with marking pens. Picture of the pitot and Pyle manometer is above. Advantage of this type of pitot tube, is that it can be easily inserted into a small hole anywhere you wish.

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clagwell

340 posts in 760 days


#10 posted 07-28-2020 06:20 PM

Nicely done! I’m looking forward to to seeing real world results from a compound blower setup. How did you choose the parallel connection over series? It will be interesting to see what loads you connect

You might consider moving the test port farther from the entrance. That should help reduce the effect of the entrance on your measurements giving you a better comparison when you start adding duct.

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

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Sark

353 posts in 1328 days


#11 posted 07-28-2020 08:40 PM

I researched the issue of connecting two blowers, either in serial or parallel. It’s easy to imagine doubling the SP of the system by a series connection where one blower sucks the air from the blower in front of it. That’s really what I first wanted. But alas, practical engineering problems intervene in such a scheme.

Blowers are designed to create vacuums. When air is blown into a blower, that blower will be running essentially without load. The first blower in the series does all the work, so there isn’t much of an increase in the overall static pressure available. A more technical analysis can be found having to do with impeller design, balance between the two blowers, pressure differentials, CFM and relative air velocities but the net result is that small dust collector system-designers are warned away from attempting series-connected blowers. Also, a lot of work done on low pressure, low velocity, high volume systems as are used in HVAC systems just don’t apply to dust collection in a workshop.

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clagwell

340 posts in 760 days


#12 posted 07-29-2020 09:35 AM



I researched the issue of connecting two blowers, either in serial or parallel. It’s easy to imagine doubling the SP of the system by a series connection where one blower sucks the air from the blower in front of it. That’s really what I first wanted. But alas, practical engineering problems intervene in such a scheme.

Actually, it really is almost that simple. A blower’s performance curve is a complete description of the flow vs pressure performance. That is, for a given flow rate there’s a unique pressure at that flow. For blowers connected in series the flow rate is the same in each blower and the pressures add. So to get the fan curve of the compound fan you really do just add the two pressures for each flow rate. If the fans are identical then the pressure simply doubles.

For a parallel connection add the flows at the same pressure.

Of course, if the fans are not similar to one another then one of them will tend to dominate. For example, a shop vac and furnace blower in series is still essentially a shop vac. If you put them in parallel then the compound is not much different from the HVAC blower alone

Blowers are designed to create vacuums.

No, not even close to a vacuum. A centrifugal fan produces a pressure difference. Compared to a vacuum it’s a very small difference. Your blowers are likely capable of 10 to 11 inches of water. A vacuum is 400 inches of water.

When air is blown into a blower, that blower will be running essentially without load.

The flow is the load on a blower. No load means no flow. It doesn’t matter how the flow gets there. It’s only the amount of flow that determines the load.

The first blower in the series does all the work, so there isn’t much of an increase in the overall static pressure available.

There is no first blower in the series from the viewpoint of the blowers. That’s a concept that’s only meaningful to the observer. The airflow path is ultimately a loop, what goes in one fan’s inlet must have come out of the other fan’s outlet. There is no first or last in the loop, it’s a loop.

A more technical analysis can be found having to do with impeller design, balance between the two blowers, pressure differentials, CFM and relative air velocities but the net result is that small dust collector system-designers are warned away from attempting series-connected blowers.

That makes no sense. Where did you find that?

Also, a lot of work done on low pressure, low velocity, high volume systems as are used in HVAC systems just don’t apply to dust collection in a workshop.

True. Hobby dust collection is much simpler than HVAC.

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

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Sark

353 posts in 1328 days


#13 posted 07-30-2020 02:08 PM

Clagwell thanks for the info! As regards your question, “where did you find that?” the sad fact is that this was speculation on my part on why blowers shouldn’t be connected in series. So I’m rather chagrined and embarrassed. Can’t delete my remarks, but I’d at least like to retract them.

So where did the idea that blowers shouldn’t be connected in series come from? I think I may be able to find the source of that advice. I started on my dust collection design maybe 3 years ago, and had (at that time) a 1 HP blower and thought I could just add a 2 HP blower and create a 3 HP system on the cheap. So I searched on the web, and if memory serves me, even posted the question on this forum. The response was negative, that much is certain.

Ironically, the negative feedback is one of the reasons that I wanted to try it. Plus I lucked into some pretty cheap 2 HP blowers. Now with the quarantine creating extra stay-at-home time, I’m actually implementing this long awaited dust collection system. And the choice is to go with parallel rather than series connected blowers.

Question: Would you advise a serial or parallel configuration (or would it make a practical difference) with the following setup? Typical garage shop with the usual equipment, two 2HP blowers, 6” ducting for all long runs, cyclone separator with 6” inlet and outlet.

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AlanWS

110 posts in 4526 days


#14 posted 07-30-2020 02:49 PM

I would expect you would be better off with the blowers in parallel. Here’s my reasoning:

Your goal is to get as much flow as possible at the tool. That means to the right on Clagwell’s graph. If you have a lot of flow resistance to overcome, (high on the graph) putting the blowers in series would be better, but the maximum flow you could get is the same as one blower with low resistance. If the flow resistance is low, the parallel setup can give you up to twice the flow as a single blower. Using 6” diameter duct with those blowers and a reasonable layout (mostly straight and with gently curves) I’d expect low enough flow resistance to make parallel a better configuration. It also has a better shot of keeping the linear flow rate in a 6” duct fast enough for the dust to remain suspended and not accumulate anywhere.

-- Alan in Wisconsin

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Redoak49

5022 posts in 2957 days


#15 posted 07-30-2020 08:19 PM

I would be interested to know the model number and impeller size of the 2 hp Dust collectors.

I can barely get 1800 cfm with an 8” duct, 5 hp and 1 5” impeller . I measured mine with a hot wire anemometer and did a traverse with at least 5 points. Getting that high a flow thru a 6” duct and smaller impeller ( lower static pressure) seems difficult. The highest flow I can get thru a 6” duct is around 1 300 cfm.

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