Air flow calibration measurements

[BobL]

This is a technical post about measuring air speed/flow, which I know won't interest many woodies but it does highlight the many difficulties involved in making reliable air flow measurements.
It involves a cross comparison of 3 different air flow meters. This is just part one of a multipart post which will evolve in this thread over the next few weeks

I recently purchased a Testo hot wire anemometer (air speed meter) and was interested to see how well it agrees with my 2 other hot wire anemometers.
Lappa has posted in this forum on using a Testo to measure airflows in his DC system.

The Testo is a very handy product since it can be left operating in a remote location in a ducting system while recording of data takes place on a tablet or mobile phone communicating with the probe via Blue tooth.
It comes with a one point calibration certificate which states at 8.0 m/s it was measuring 8.2 m/s and has an absolute uncertainty of +/- 8.8%.
Knowledge of this absolute uncertainty is critical in comparing readings obtained by different sensors.

The two other meters I am comparing the Testo to are

1) Kurz meter.
This is an old analog meter with a zero to 30 m/s range and comes with a calibration certificate stating it has been compared to a NIST standard and has an uncertainty of +/-4%, but given how old it is I really doubt it's that good.
It jumps around quite a bit while measurements are made and you will see how I got around this problem below.

2) TSI meter
This is a much newer digital meter with a zero to 20 m/s range and comes with a 10 point calibration certificate traceable to NIST standards. It does have a short term integration feature which smooths the readings. The quoted uncertainty is +/- 5% but the calibration curve shows that the accuracy is > +/-2.5% for all readings above 60 FPM. Because of the quality of this calibration I decide to reference the Testo and the Kurz against the TSI meter.

Simple comparative tests have been done before by me between the TSI and the Kurz many years ago when I convinced myself they agreed within spec.

A real world test would be to use each of the meters to perform a full air flow (CFM) test in the same duct at the same air speed. Each measurement involves the systematic measurement of the air speed (m/s of FPM) at different radial points across duct and then performing calculations to get the flow (CFM). I will eventually do this but for the moment I wanted to see if I could get a comparison of just the air speed using another method.

To do this I needed to setup a test duct and locate the sensor in the same relative point in the air stream so the 3 probes measure the same air speed. This is not as easy as it sounds as the air inside a normal duct varies speed with distance from the walls of the duct and also (especially at higher speeds) preferentially weaves and wanders around in the the duct. If the sensors are placed serially along the duct they cannot be too close together as they will interfere with each other. If serial placement was used the would need to be placed meters apart and given the test duct for one sensor needs to be around 10x as long as the duct is wide, this would require a very long test duct.

The other arrangement is with all 3 sensors at the same position along a duct but distributed evenly (120º apart) at the same radius so the sensor elements are all the same distance from the wall of the duct and this is what I did. .

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A complication arises in that the Testo probe has a 9.1mm diameter while the TSI and Kurz are 1/4" diameter probes.
This alone could change things significantly so I turned up some 9.1 mm diameter Al sleeves that were placed over the TSI and Kurz sensor shafts but I had to leave the ends of the sensor uncovered others I would disturb the calibration of the TSI and Kurz. This is major unknown effect #1.

The duct itself consists of two lengths of 9" (240mm) diameter PVC stormwater pipe.
The reason I used two pieces was because the sensors had to be located in the middle of the length of the duct and I needed to be able to reach in to make sure they were the same distance away from the wall of the duct. This I was able to do with a tolerance of <1%. I also had to be able to see if the sensor heads were oriented correctly. Being able to break the duct and get up close to the sensors was the only way this was easy to do.

Despite the great pains taken to get the probes at the same distance from the walls of the duct (113 mm) this is no guarantee they will experience the same air speed - see below,

Here is what the test duct looks like
It's around 4m long with the probes placed at the 2m mark.
For accuracy the test duct length should be ~10x the duct diameter. Even better would to have a length 10x the duct diameter before the sensors and another length 10X the duct diameter after the sensors.
This would require ~2400 mm before the seniors and 2400 mm after the sensors. My setup has 2000 mm before and 2000 mm after the sensors, so technically I am a little short but I am deliberately working at slow air speeds for this preliminary test so it should be OK, although it remains as a minor unknown. A good test is how the air speed varies in time which I will discuss later.

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The piece of duct with the probes in it can be disconnected and rotated to see if there is a positional effect. This is necessary to check because the flow can differ inside the duct due to back reflections from junctions etc down the line. This involves lengthy systematic testing that I will do next.

Another view showing the pot plant reducer that attaches to the ducting for my lathe
In the middle you can see an MDF ring which is where the sensors attach to from the outside.
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Here is a close up of the MDF ring with the Testo probe (Orange and black) on top and the Kurz probe at the 4 o'clock position.
The TSI is at the 8 o'clock position on the other side of the duct.
The small screw in the middle of the Al strip straddling the probe, locks the probe in place.
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Now you have 3 meters jumping around like 3 frogs in a sock so how the hell do you record each reading at exactly the same time.
Ideally you plug all 3 into a computer and even the old Kurz has an analog signal out cable that was used to connect it to a computer but I have lost the calibration data for this and I did not want to go through that calibration again so I had to find another way.
I've done this a few times before and all I do is get a digital camera and take a photo of all 3 of the displays.
I just click the camera 10-15 times at the same air speed, open or close the gate valve to the lathe to vary the air speed and repeat the measurements.
The air speeds I used did not matter as long as a range of speeds was used during the test.

Then I review these on the computer and type them into a spreadsheet - it sounds long winded but its actually quite fast and it is a good chance to get up close and personal with the data rather than blindly using what comes out of the computer.
It's also a chance to reject wild outliers and start to make sense of the data

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I took 10 sets of readings between 1 and 9 m/s (with 2 sets at ~6 m/s)
The 10-15 data points for each set are averaged and age results for each meter compared to each other.
The "error bars" are a measure of the overall tolerance/uncertainty in the comparison for each point.
These are the simple sum of the standard statistical error for the TSI meter measurement and the standard statistical error for the other meter .
These should really be added in quadrature (square root of the sum of the squares but I was being conservative in my uncertainty assessment.

The air speed variation in time differs significantly between the 3 meters.
The Kurz reports instantaneous speed with about a 1/4 second response rate so that had the greatest variability, about 50% more than the variability of the Testo.
The TSI reported the least variability between readings, (about half that of the Testo) but that could be because the TSI does have a weak smoothing function built in.
This should not affect the final results.

The results are plotted below as % differences of the Testo and Kurz from the TSI readings.


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What does it all mean?
A simple conclusion is, if the error bars for a point on the graph cross the zero vertical axis the meter agrees within spec with the TSI meter for that air speed.
For the Testo it appears to consistently read on the low side compared to the TSI meter, with all bar one of the readings below that of the TSI.
However, 6 out of 10 of the Testo readings (1, 2.7, 3, 5 and two 6 m/s) are within uncertainty which shows they are not far from agreement.
The average deviation for the Testo from the TDI is -8.5% (the "minus" means its reading too low) and given the absolute uncertainty of the Testo is +/- 8.8%, from that point of view it could be said the two meters agree within spec.

The Kurz has an average deviation of -6.5% which while slightly closer than the Testo to the TSI is outside the Kurz quoted spec of 4%, but given how old this meter is, this does not surprise me.

Overall, given the gotchas, I am amazed these results got anywhere near agreeing to this degree, and there is much more testing to do especially at higher speeds.
The kind of test described in this post cannot be done at higher air speed because to achieve this will require the use of 6" duct (to get the air speeds up) and the smaller cut will put the probes closer together which is likely to create a mess of the air flow.
The only way to do sensor comparisons at these speeds is to perform full airflow measurements.

One possible outcome of this experiment is that if the Testo is indeed reading on the low side it could be that Lappa's system is better than he thinks.
And who's to say my Testo is the same as Lappa's in the first place? :oo:
Conversely the TSI could also be reading too high and I could have a slightly worse system than I think I do. :D

Now let me reiterate that the above conclusions could still be in doubt because I have not really assessed the chances of non-radial variation of air speed within the duct. This could easily result in the deviations observed.
To test this I will be rotating the section of duct that holds the sensors into a different positions with different sensors at the top each time and repeating these measurements.
Even this might not show up everything but it will be a start.
The real test will be a full volume measurement for which I will have to make up shorter Al sleeves for the TSI and Kurz meters.

What it does show is how different meters can give different results and how messy in general air flow measurements are.
If this doesn't put you off air flow measurements I think by the end of this thread you may convinced.

[QC Inspector]

I'm looking forward to seeing what the results are overall when you are done. Given any thought to see how much of a difference to the reedings when you put a bell mouth on the duct?

Pete

[BobL]

I'm looking forward to seeing what the results are overall when you are done. Given any thought to see how much of a difference to the reedings when you put a bell mouth on the duct?

Pete

Ah ha - I have been waiting to do that for a while.
I did some tests with the first MDF turned BMH I made a few years ago with a jerry rigged testing setup and the results showed a small modest increase, but while I have this testing setup in place it's one thing I think I can test a bit more reliably than last time. This test only requires a relative comparison, i.e. with and without the BMH, so absolute accuracy is not required.

[BobL]

OK I have rotated the test duct , so the 3 sensors alternately sit in the 12, 4 and 8 o'clock positions, and repeated the calibration.
This is being done to see if there are preferential airflow paths in the test duct.

I'll show only the Testo calibrations against the TSI meter otherwise the graphs gets very crowded.
The blue Testo12 line and points are for the Test at the 12 o'clock position , Testo4 is when the Testo is at the 4 O'clock position etc.

The XTesto12 point is the average Deviation across the range for all the 12O'clock deviations, etc.

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Although the data is somewhat scattered, the results for the 4 and 8 O'clock positions appear a little closer to zero deviation line than the 12 O'clock position, but otherwise the averages appear to overlap so much that one cannot really tell the difference.

My summary is-
There are no discernible preferential air flow paths in the duct
My Testo, between air speeds of 1 and 9 m/s, measures on average about 5% lower than the TSI meter.
OR
The TSO measures 5% higher than than Testo meter.

[BobL]

I did a quick and dirty BMH measurement - not a full airflow (CFM) measurement - just what the difference in air speed is

without (just a short stub of 150mm duct)

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and with the BMH in place
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20 readings on each were averaged.

The TSI meter gave a 15.8% increase while the Testo gave a 11.7% increase
The uncertainties were +/- 1.0 and +/- 2.1 % respectively.
Note the result means they don't agree but it was a quick and dirty measurement only of the air speed in one place in the duct.
A full air flow (CFM) measurement would be more definitive.

None the less it's clear there is a difference - it's the easiest and cheapest 10% improvement in flow you'll get.

The change in motor current is only 0.1A and as the built in ammeter on the DC remote switch box only reads to 0.1A its rather surprising.

I don't know how much longer I can keep the test ducting in place in my shed as it takes up a lot of room and divides the shed in half.
It's going to be used at the mens shed in the near future.

[BobL]

I did an even quicker and dirtier change in air speed with/without an oversized 4" BMH.

Now things are really hanging out in the breeze 1) 9" duct, 2) 9 - 6" reducer, 3) 6" duct, 4) 6 - 4" reducer, and then 5) 4" BMH
Its all just "push fit" but things are starting to get "wobbly".

And . . . . . .

I noticed that even gently adding and removing the BMHs to the end of the other bits and pieces on the end if the test pipe changes the air speed/direction by a visible amount.
This highlights the problems of measuring air speed and the need to measure total air flow.

To make sure I was not changing the air speed/direction, I went back and forth (BMH/no BMH/BMH) a few times to check I had not changed things
The result was 17% improvement!!! - the flow looks pretty good too.

Tomorrow I am going to clamp and screw the bits and piece on the end of the flange down to make sure I'm getting reproducible results.

[Enfield Guy]

Thank you for doing this work.

Cheers

[QC Inspector]

Interesting to see the difference a smooth intake makes. Actually when I suggested a bell mouth, I was thinking a 9" one. I thought it would reduce turbulence of the air at the probes?

Pete

[BobL]

Interesting to see the difference a smooth intake makes. Actually when I suggested a bell mouth, I was thinking a 9" one. I thought it would reduce turbulence of the air at the probes?

Pete

Yes it should.
Making one is not going to be easy.
A 250 mm BMH will need to be 375 mm in diameter - I will need a bigger lathe to make up the former :D
Anyone have a lathe with a 16" swing I could borrow for a couple of hours. I can be an outboard swing because a tail stock is needed to drive the 250mm pipe onto the former

[QC Inspector]

Time for a bigger flower pot. :D

Pete

[BobL]

I spent some of today rejigging the test pipe so I could make volume flow (up until now I have just been doing air speed) measurements using any of my meters.
I used small tek screws and pipe clamps to lock everything in place - even so one has to be careful not to knock the set-up otherwise the results can change quite dramatically.

The first test I did was on the 6" BMH with the Testo meter and I got an increase in flow of 10.4 to 10.8% which agrees with the quick and dirty "air speed only measurement".

To minimise the number of changes and to get more comparable results I'd set the probe to a prescribed radial position and then measure the air speed "with" and then "without" the hood; then move the sensor position and then measure without and with the hood. This reduces the number of times the hood is removed and replaced by 50%. I did this all the way across the test duct.

The Testo is real nice to use - I can use the meter in my shed via my iPad mini, and after each measurement email the data to my PC up in the house.
Even so the full comparative volume/flow test takes about 45 minutes to collect the data, and then it takes about a hour to process all the data.

It was interesting to see the air speed difference between the "BMH" and "no BMH" consistently between 8 about and 10% across the middle of the pipe.

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At about 1/4 of the way across the pipe (3cm mark) the air speed difference increased to about 20% and then dropped to near zero when the meter probe was close to the internal wall of the duct where the air speed is quite low anyway.

Because of the air speed differences being so consistent in the middle of this test duct I think I can test things out in terms of % difference just using a couple of air speed measurements. This will make testing a lot quicker than doing a complete volume/flow measurements. if things appear too close to distinguish I may still need to use volume/flow measurements.

[BobL]

This norming I did a couple of interesting tesst.

I did not realise that when I did the above test yesterday it was being done with a second 6" port wide open at the same time, so the volume/flow in the test duct was around 500 CFM.

This morning I checked it again with only the one 6" port open and got a 12.5% improvement with the BMH.

I also measured the effect of the BMH at around 1/4 flow and got an improvement of 3.1 + 1.7%

This makes sense as the greater the flow the more effective you would expect the BMH to be. Or maybe this only works up to a certain point?

Also I tested the effect of the 1" mesh guard at full flow and the resultant loss in flow is 1.14 +/- 0.84% , so just discernible, but more importantly it does not substantially negate the effect of the BMH.


http://www.woodworkforums.com/attachment.php?attachmentid=408600&d=1489814719

Now I'm going to try some 100 mm BMHs.

[MandJ]

I'm wondering if some users reading the above may think that 10% to 12% is not a great enough improvement to justify making buying or fitting a bell mouth. However what I find with the Bell mouth inlets (as Bob has mentioned before) is a greatly enhanced (larger) area of pickup forward of the bell mouth, way more dust capture area, now add that 20% increase in flow and the use of a bell mouth intake is, IMHO, absolutely worth the effort. I've converted 3 x 150mm intakes with the Bell mouths I got from Bob, and have just fitted the 100mm unit to the above table pickup on the Bandsaw, once again, a noticeable improvement, even on the tiny bits of white laminate that occasionally pop off.

Look forward to seeing how the 100mm BM shapes up in the tests.

Forgot to add: Really great to have the small loss with the intake screen verified at last.

[BobL]

I'm wondering if some users reading the above may think that 10% to 12% is not a great enough improvement to justify making buying or fitting a bell mouth. However what I find with the Bell mouth inlets (as Bob has mentioned before) is a greatly enhanced (larger) area of pickup forward of the bell mouth, way more dust capture area, now add that 20% increase in flow and the use of a bell mouth intake is, IMHO, absolutely worth the effort. I've converted 3 x 150mm intakes with the Bell mouths I got from Bob, and have just fitted the 100mm unit to the above table pickup on the Bandsaw, once again, a noticeable improvement, even on the tiny bits of white laminate that occasionally pop off.

Look forward to seeing how the 100mm BM shapes up in the tests..

100mm BM measurements have been done - just taking a break from processing the data results coming soon. You are right about the advantage being much more than just the actual flow increase. It captures much more dust from the front of the hood so if you point it at a dust source it picks up more of that dust and not just air with little or no dust in it from the sides and behind the hood, which is what a naked duct doe.

[BobL]

Some more tests.

This (the yellow one) was the first 150 mm BMH I made from 3 x 18 mm thick layers of MDF. It was not a neat fit on the elbow and I only every did a basic test on its performance and I can't even remember why I go for it - somewhere between 5 and 10%

The improvement if flow for this one at full flow is 8 +/- 1%
It has been replaced by a PVC type.

http://www.woodworkforums.com/attachment.php?attachmentid=403624&d=1483591744

This was the first of the 100 mm MDF BMHs I made.
Its a bit more trumpet shaped than the PVC ones and gave a 20.4 +/- 1.4 % improvement in flow


http://www.woodworkforums.com/attachment.php?attachmentid=381650&d=1464688699


This standard PVC 100 mm BMH gave an 18.3 +/- 1.2% improvement.
http://www.woodworkforums.com/attachment.php?attachmentid=404513&d=1484814582
These of course are flow dependent. If you use it on a 1HP DC with clogged bags you won't see that much of an improvement

Now - what else can I test?

[BobL]

More tests.

This was my old 9" to 6" reducer I used on my old 9" test duct, until I dropped it and split the rim.
It never fit properly anyway and it was about to go in the bin so I thought I'd see how it performs as a BMH.
Note, it's not exactly a smooth transition at the base but it is nicely tapered and has a rounded rim.

Improvement in flow is 7.9 +/- 0.9 % , so not bad considering what it is.
The downside of using such a hood is that it is quite long.

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This is a Standard "Big Gulp" hood. I had this on my TS as the hood for a while, both in its original form like this.
It gives a 5,1 +/- 1.4% increase in flow - compare that to the +18% for a standard 100 mm PVC BMH.
One of the reasons the hood is not as good as it should be is its opening is around 94mm

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A question has been posed by a mate of mine (as well as myself) as to what might happen if the hood sides are made extra long.
Inadvertently I had already done this to the big gulp when I added polycarbonate sides to the hood so I could see more easily and used that as the hood on my TS for about 8 years.

Like this
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Well, there appears to be no improvement (0.5 +/- 1.5%) over a naked 4" duct end, so it is worse than the Big Gulp by itself.
It appears you can go to far with extending the side.

As you can see, almost anything and everything is getting shoved onto the end of the test pipe.

[BobL]

I have gone back over all the data comparing BMH to no-BMH measurements and realised there is on average ~25% smaller variability in the measure air speeds in the test duct when using a BMH.

This is highlighted by this graph where the error bars for the BMH results show up "on average" as smaller than for the naked duct.
This means the BMH must be making the air inside the duct at the measuring point less turbulent.
I knew this was the case at the actual air inlet itself but it's interesting to see that this transfers to the air down inside the duct and would explain the improved performance of the hood.


http://www.woodworkforums.com/attachment.php?attachmentid=411064&d=1493124534

[rob streeper]

Hi Bob,

Interesting work. Are you going to do particle density (#/M3) and or particle removal efficiency studies as well?

Thanks,
Rob

[BobL]

Hi Bob,

Interesting work. Are you going to do particle density (#/M3) and or particle removal efficiency studies as well? I've done quite a lot of this already but I'm open to suggestions for more experiments. Just bear in mind I cannot place the particle detector inside a duct containing sawdust or it will die. I can only simulate %dust removal/efficiencies using the lower amounts of dust in regular air.

[BobL]

I've been looking for a way to independently calibrate the air speed measurements of these meters and I have and think I have come up with something that might work using a motor vehicle.

The speeds required are between zero and 30 m/s = 108 km/hr.

The plan is as follows
1) find a quite piece of straight road with a 110 lm/hr speed limit on a calm day

2) Mount the sensors outside the vehicle away from the effect of the car pushing air out of the way. The meter component can sit inside the car easily enough.

3) Have a driver operate the vehicle at a specific speed as shown by the GPS using cruise control.

4) As a passenger I will operate the mobile phone that communicates with the Testo meter and have it relay the data to the phone using blue tooth. The other meters will have to be read and recorded manual

The faster the air speed the greater the accuracy will be in theory although the effect of the car pushing air out of the way becomes greater and may affect the results.

Target calibration speeds of 18, 36, 54, 72, 90 and 108 km/hr should cover it.

If there is a slight breeze I could measure it and I will need to know the direction as well to allow for it.
Or if the breeze is stead enough do a reverse run and average the two measurements?

Anything else I haven't thought about or any other suggestions?

[QC Inspector]

Many years ago Burt Rutan tested his aircraft designs with scale models mounted above a pickup truck. He stayed in the back to monitor the instrumentation. You're in good company. ;)

[Bohdan]

The probe would have to be mounted a long way from the vehicle to not be affected by the air flow around the vehicle.

Maybe out on a long pole in front.

[BobL]

The probe would have to be mounted a long way from the vehicle to not be affected by the air flow around the vehicle.

Maybe out on a long pole in front.

Yep I guessed that - I will have to experiment.

At slow speed this will be less of an effect so I will start at slow speed and vary the distance away from the vehicle until it plateaus out so I know far I will have to go.

[BobL]

I used a length of 12mm diameter galv tube to make a mast that clamps onto a roof rack.
The mast holds the Testo sensor 1.2 m above the car and convinced SWMBO to spare me ~30 minutes drive she drove and I operated the iPad that drive the meter.

First promising thing was it all stayed in place - nothing fell off or broke.

Not ideal measuring conditions.
Midday on Kwinana freeway, moderate traffic - enough to make running under cruise control away from other vehicles difficult.
Ambient breeze was variable around 9 kph coming from a ~45º direction
Blue tooth drops out unless I have the window open and hold the iPad next to the window

We did a run back and forth between the Narrows and Canning bridge and got some reasonably stead sections of 80 and 90 kph.
Then on the way home we did a 50 kph run in a side street .

Doing a run back and forth enables the ambient breeze effect to be minimised but it does not take into account gusts or changes in breeze speed or direction.

The 90 kph setting on the GPS gave 82.9 kph (22.4 m/s or 10% low), 80 kph on the GPS gave 75.5 kph (20.4 m/s, 5.6 % low), and 50 kph gave 46.5 kph (13.3 m/s or 7.1% Low).
These lower differences are consistent with the difference between the Testo and the TSI meter I have seen inside my test duct at lower speeds.

We will take it out for more testing on a calmer and quieter day.

[BobL]

More testing this arvo.

I've tested flexy before but its always been a simple - "this is better than this" sort of test without trying to quantify the losses involved.

Part of the problem is the losses from ducting and flexy are not linear e.g. if you double the length of the flexy it won't reduce the flow by two and you will always get some flow no matter how long the flexy is.

The losses are know to be logarithmic so a KPI in terms of a "%loss per metre" is not valid. Instead one has to think of the "length of pipe to produce a 50% loss in flow" or the "half loss of flow length"
e.g. if a 5m length of flexy reduces the flow to 50% then another 3 m (i.e. 6m in total) will reduce the flow to 25% etc.

I disconnected some of my bits of flexy and measure the flow for each length relative to a length of straight ducting.
Then i used the changes in flow data to compute the the "length of pipe to produce a 50% loss in flow"
Note these numbers are for my 3HP DC - the actual numbers will change for other DCs but the order should remain the same.

I had enough pieces to test 4 types of flex.
150 mm CV is the highly flexible stuff from ClearVue .
T150 mm HT is some stiff quite corrugated High Temperature flex I picked up through Gumtree.
It looks like the green and black flex in this photo I use it on my lathe, sander and combo jointer/thicky
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100 mm CT is the standard soft transparent stuff from Carbatech
100 mm TC is the grey stuff from Timbecon with the plastic as opposed to wire helical support. This is a length I had behind the shed and is pretty old and beat up with several feet having being crushed and squeeze open again.
I don't think it's quite the same as what they sell lately.

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However the CT flex has corrugations that are much more pronounced that the TC stuff which is relatively smooth bothe
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On the graph you can see the 150 CV is about twice as "lossy" as the HT flex. I was quite surprised by this as I had purchased the CV stuff to replace the short lengths of the HT I am currently using on my machines but now I will keep the HT stuff on my machines and give the CV flex to the mens shed.

Remember, the higher the number on the Y axis the better the flex (longer length needed to reduce the flow).
The tolerance of the measurements are not shown but are about twice the size of the actual blue diamond points shown.

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I was surprised to see that the CT 100mm flex is only about 2/3 as effective as the TC flex.

It also highlights the fact that only about 4 m of CT flex will knock the stuffing out of the flow.
OK we knew this already but it highlights why we should keep the flexy as short as possible.

[MandJ]

Looks like I'll be keeping my 100mm and 150mm TC type flex, only have short lengths of around 600mm of the 150mm, was going to get something more flexible but now thinking I should just put up with it.

[BobL]

Looks like I'll be keeping my 100mm and 150mm TC type flex, only have short lengths of around 600mm of the 150mm, was going to get something more flexible but now thinking I should just put up with it.

My understanding is there are 2 types of TC flex, their older stuff with the round white plastic supporting ribs while the new one uses get square cross section supporting ribs. I have not tested the newer stuff. If anyone in perth has a piece around 2m long that I could borrow for 1/2 hour that is enough for me to test.

Next week I will also go to the Mens shed and borrow a length of the current Grey 150 mm CT flex and bring it home and test that.

[MandJ]

Both of mine have round white plastic ribs. Look forward to the 150mm testing, all of this is really good info and thanks for all the testing.

[Lappa]

How was the change in pipe diam accounted for in the calcs?
Cheers

[BobL]

How was the change in pipe diam accounted for in the calcs?
Cheers

The measurements are relative to a similar length of PVC ducting of the same diameter - then all I had to take into account was the length.

So order of measurement for the 150 mm size was

~3m long piece of 150mm PVC ducting
Then various piece of 150 mm flexy
Then back to the 150mm PVC ducting

Same thing was then done with the 100 mm ducting and piece of flex.

I have organised a setup to directly compare the GPS with the Testo using my van.
Although the van doesn't have cruise control I will do it using the camera method.

I have transferred the mast that holds the test probe from SWMBO car to mt van roof rack - this will hold the Testo probe
The mobile phone running the Testo software sits in a cradle next to my GPS on the van dash.
In the back of the van poking through the open cargo barrier door I have set up a camera on a tripod clamped to the door frame.
This will enable me to photograph the GPS and the Mobile phone at the same time.
The camera focusses and is activated by a small remote I can easily operate while driving.
That way whatever the speeds are on the mobile and the GPS, the camera will record both speeds simultaneously.

Just waiting for a relatively calm day.

[BobL]

This morning at ~5:30am (the normal time I get up anyway) I did a test run with the Testo air flow meter and the GPS.
The BOM was registering the wind speed as calm (<1 kph) and when the vehicle was stationary the Testo was registering <0.1 m/s which is <0.36 kph.
As well as being calm there was very little traffic around and lights were all green on major streets.

This is a shot of the inside of the van showing where the mobile with the Testo App running and the GPS are located.
411395

The camera is zoomed in on the Mobile/GPS and this is what the view looks like.

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As I said above just drove around and occasionally would hit the camera remote to take a picture.

Here is a summary of the results.
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The correlation coefficient of 0.9918 is surprisingly good.
The slope of the line from the 0.2079x on the equation of best fit means there is a consistent 21% difference, i.e. Testo is measuring 21% too low.
Given I got such smaller differences than this using SWMBO's car I reckon the van's lack of aerodynamics must be affecting the result.

I will switch back to SWMBO's car - it's going to be harder to mount the camera in that vehicle and I will have to get SWMBO to drive and I will operate the gear.
Despite the problems and the fact that the data has to be entered manually into Excel, the camera method is much easier to execute on the road.
The driver just drives smoothly and as they should pays attention to the traffic and the speed limit, the camera records both speeds simultaneously. I imagine there may be some unwarranted differences if the measurements were taken during a period of rapid speed changes.

[BobL]

I should be working on something more urgent but I dropped into Pipe-on-line this morning to pick up a few junctions for some mods I'm doing to my DC system, and also picked up some 100 mm bends to test.

This test was primarily done to test the "2 x 45º are better than 1 x 90º bend bends" hypothesis. I did measure this back in 2008 or thereabouts and because I could not detect any difference (using a fairly ordinary air flow meter I borrowed from work) I assumed it was just my technique. Since then a few things have changed, the main one is I tested this out using a very clogged 1HP DC with a sort length of 100 mm test duct. Now I have proper test duct so I can perform more reliable tests, and the new TESTO meter enables me to quickly collect a heap more data and place it straight into a spreadsheet for calculation.

So the setup was similar to the flexy testing using the big (240 mm diameter) test duct.
On the end of that I attached two pieces of 100 mm duct connected by a straight coupler and measured the flow.
Then in between those two pieces I inserted a bend and measure the flow again.
Everything was measured several to ensure some sort of reproducibility.

Three types of 100 mm 90º bends were tested
1 was a standard Female - Female 90º bend This had a midline radius of curvature of 1.4R (1.4x the internal radius of the pipe) = 90FF on the graph
2. was a Male- Female 90º bend with a 1.8R curvature = 90MF on the graph
3 was 2 x 45º bends joint with minimal amount of 100 ducting. = 45-45 on the graph.

Age graph below shows the percentage flow loss for each bend when compered to straight duct flow.
The high the bar the worse the performance of the bend.
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The little bars on top of the blues bars are the measurement uncertainty and does not include the uncertainty of the flow in the straight duct.

The first thing that stands out is the 2 x 45º bends don't appear to be doing that well.

Although there's not much in it there also appears to be difference between the FF and MF bends probably due to their slightly different radii of curvature with the MF bench having the larger radii and then smaller losses.

Please note that this applies only to these 100 mm ducts on my system - results on your system may vary but I would be surprised if the order varied. The slower the flow the less difference there will be

I will test some flexy bends next.

[MandJ]

Thanks for these ongoing tests Bob.

What about 2 x 45 with around 600mm between them compared to a single 90? I wonder if the 100mm results are indicative of 150mm 90 V extended 45's?

[BobL]

Thanks for these ongoing tests Bob.

What about 2 x 45 with around 600mm between them compared to a single 90? I wonder if the 100mm results are indicative of 150mm 90 V extended 45's?

OK - will give it a go.

[BobL]

Here's a few more

Experiment performed as in post #32
This is all compared to an equivalent length of 100 mm ducting

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45-45 no spacer = are two 45 degree bends with minimal ducting in between
45-45 spacer = are two 45 degree bends with about 250 mm of 100 mm ducting in between
45 bend only = a single 45º bend , interesting that the loss is less than half of 2 x 45º ?
90 deg CT flex = a minimal (i.e. tight) 90 degree bend made out of CT flex
90 deg TC flex = a minimal (i.e. tight) 90 degree bend made out of TC flex
90 deg bed = 90 degree MF bend same as tested in post 32.

Because it was so stiff it was very difficult to force the TC flex into a tight radius so the radius for the RC would have been slightly(~20%) larger than for the CT flex.

[PS]
A couple of observations I forgot to mention yesterday,
- on touching the CT flexy bend I notice it was vibrating quite significantly, this is a strong indicator that it is not working efficiently. The TC flexy bend also vibrated but nowhere near as much probably because it is a much stiffer flex.
- the 45º bends are 1R bends which probably explains why 2 of them are not as efficient as the wider (1.8R) Radius 90º bend.

[MandJ]

If 150mm results are similar, it would appear that the effort put in eliminating the dreaded 90 bend with variations of 45 bends is a wasted effort, and actually results in more loss?

[MandJ]

It's almost like once the air has started to change direction through a bend, that any straight transition in a bend (as in 2 x 45) is seen as another change in direction and results in even more loss. As a single smooth 90 has only one direction change it has less loss, with the exception of a single 45 bend, again only one smaller change in direction.

[BobL]

If 150mm results are similar, it would appear that the effort put in eliminating the dreaded 90 bend with variations of 45 bends is a wasted effort, and actually results in more loss?
Correct, but before anyone goes ripping their150 mm 45º bends out let me test the 150's so see how significant the effect is with them.
I have one spare 150 mm 45º bend under the house and I hope there's a spare at the men's shed.
Also I want to test the 150 mm 90º bends as well.


It's almost like once the air has started to change direction through a bend, that any straight transition in a bend (as in 2 x 45) is seen as another change in direction and results in even more loss. As a single smooth 90 has only one direction change it has less loss, with the exception of a single 45 bend, again only one smaller change in direction.

Sounds like an explanation, and couple that with all the stormwater AND DVW 45º bends that I have are 1R bends.

A Y and single 45º probably should be OK.

[MandJ]

Thanks again Bob, yes the 45 Y and single 45 look good, I actually hope the results for 100 x 90 bends are similar for 150mm, it sure make it easy in some places, like my Router table and the Lathe pickup - both are 150mm 90's.

Mike.

EDIT: What I was trying to say is - it would be nice to know that I'm not really loosing as much flow as I initially thought by using one or two 150mm 90 deg connections :)

[Colin62]

- the 45º bends are 1R bends which probably explains why 2 of them are not as efficient as the wider (1.8R) Radius 90º bend.

That's quite a big difference in radius. I agree that is going to explain at least part of the increased losses measured for the two 45s.

[BobL]

OK did the 150 mm tests this morning and here are the results once again relative to a straight piece of 150mm duct.

90deg2RTx =represents a DWV 90deg bend with a radius of curvature of 2 (actually 2.2), Tx = test #, 90deg2RAve is the average of the 5 separate tests
412034

45Tx are tests x done for for a single 45deg bend - two tests done on this bend.

45-45Tx are for two 45º bends joined by minimal duct. 3 tests done on this one

90deg1RTx, are the tests done on a stormwater 90º bend with a 1R radius of curvature. 3 tests done on this as well.

As you can see its a MUCH tighter radius of curvature compared to the other 90º bend.
412035

I'm showing most of the data so you see how tricky the results are.

Observations
1) These are amongst the trickiest measurements I've made and I had to go back and increase the sample size and number of repeat trials to get enough data to see if there is any difference between these bends. This sort of test really pushes the whole measurement system/process.

2) Apart from the 1R 90º bend, the differences in flow rates from the straight duct are are around 2% or less, so in practice it doesn't really matter what you use.

3) Looking just at the averages, the 90º 2R bend and the single 45 are indistinguishable @ around 1% +/- 0.4%.

4) The 45-45º bend is around 2.3% +/- 0.3% so reasonable well resolved as being slightly worse than the 2R 90º bend but being only 2.3% its no biggie if you have them already installed.

4) The worst performer by far is the 90º 1R stormwater bend at around 7.4 +/-0.8 % loss - so avoid using these unless you have insufficient room to manoeuvre.



412036


So 2 x 45º is better than the 1R 90º but just worse than the 90º 2R bend - I don't reckon its worth changing any 2x45º bends for the 2R.
probably worth changing any 1Rs if you have them.

I hope this is of some use.

[mannum3]

Well worth knowing, Mannum Men's Shed is completing the 150mm ducting, there are a few more bends to go so we will make good use of this information.

Cheers Barry

[MandJ]

Bob, I know it's been said before, but another big thank you for the time and effort you put into this.

It feels to me like the last of the guess work has finally been removed when it comes to ducting our home workshops. Combine all the work and testing you did with bell mouth intakes, machine ports, DC modifications and limitations, filter media, room ventilation, fine dust warnings, and more that I have likely left out. Those of us interested in our heath and our enjoyment of WW now have a greater understanding and a better chance of installing and modifying a DE system that is more affordable and efficient than would have been possible without the effort and information you have supplied.

Mike.

[BobL]

Bob, I know it's been said before, but another big thank you for the time and effort you put into this.

It feels to me like the last of the guess work has finally been removed when it comes to ducting our home workshops. Combine all the work and testing you did with bell mouth intakes, machine ports, DC modifications and limitations, filter media, room ventilation, fine dust warnings, and more that I have likely left out. Those of use interested in our heath and our enjoyment of WW now have a greater understanding and a better chance of installing and modifying a DE system that is more affordable and efficient than would have been possible without the effort and information you have supplied..

Thanks Mike. I enjoy the challenge of these types of measurements especially determining the measurement uncertainty or tolerance side of things. This is something I received a lot of training in as a student and used every day at work. When undertaking research I spent a lot of time measuring small difference between quantities (often at the <0.1% level) so I had to spend much more time on the uncertainties than the actual quantity being measured. I ended up teaching undergraduate experimental physics for 20 odd years where the methods for teaching uncertainties etc are hammered into students. It was interesting to see new students coming it with a very limited understanding of uncertainties and by the time they left most of them had a really good grounding in the area. Several decades later a former student said to me at a reunion "uncertainty management" was one of the most useful things he had learned at uni.

Something I forgot to mention in the 150 mm bend test post was the results I obtained (the actual percentages) apply only to my system and your system may have different percentages but the order (e.g. this is better or worse than this) should still apply.

This is reflected in the % differences in flow seen with the 100mm bends compared to the 150 mm bends. The greater %differences measured in the 100 mm bends is because the same static pressure pulling the air into a duct will cause proportionately greater resistance and hence loss of flow in a smaller compared to a larger diameter bend. If I was to do the same tests on 225mm bends on my system I doubt I would see any differences in the flow loss of the bends.
-

[MandJ]

Something I forgot to mention in the 150 mm bend test post was the results I obtained (the actual percentages) apply only to my system and your system may have different percentages but the order (e.g. this is better or worse than this) should still apply.

This is reflected in the % differences in flow seen with the 100mm bends compared to the 150 mm bends. The greater %differences measured in the 100 mm bends is because the same static pressure pulling the air into a duct will cause proportionately greater resistance and hence loss of flow in a smaller compared to a larger diameter bend. If I was to do the same tests on 225mm bends on my system I doubt I would see any differences in the flow loss of the bends.
-

Yes I noticed that trend in other tests you have carried out, it's why I found the 100mm and 150mm tests results so valuable as these are the two most often used sizes in home workshop extraction, along with some of the flex you tested the results are really helpful.

Cheers.

[BobL]

Because the CT flex 90º bend was so inefficient (33% loss) I went back and explored it further.

It turns out the losses relative (to straight duct) for a 90º bend vary between 20 to 35% - this is the benefit of doing multiple trials.
After a bit of fiddling about it was clear that small changes in the radius of curvature of the bend was having an effect but not in the way one might expect.

Remember I only have a fixed length of flex (~300mm) so to generate the close to 1R radius of curvature I had to squeeze the flex together, for ~1.5R I let it relax, and for ~2R radius I wrapped the flex around a 100 mm diam pipe - this stretched the flex and put it under tension
What I found was the the radius of curvature close to 1R was more efficient than around 1.5R, while the 2R was more efficient than both

This result is unusual because it should be more efficient at 1.5R than at 1R.
Perhaps it is because when the flex is tight (compressed or stretched) the flex vibrates less than when it is more relaxed?

One needs to bear in minds that,10 of the 20 to 35 % losses comes from the the fact that the flex itself even when straight is relatively lossy.

This is consistent with the better efficiency of the TC flex which is quite stiff so it doesn't vibrate as much and comparing the flexible CV 150mm ducting with the stiffer 150 HT flex discussed in post #25.

What this tells me is that the more flexible flexy that folks seem to gravitate towards is far from ideal and should be used only in very short lengths.

[Lappa]

I noticed in the test with the 150mm bend tests, the pipes are facing the ground. Would it make sny difference if they where facing to the side or upwards to eliminate any chance of return turbulence from the ground?

I know with certain testing we do (not air flow but sound) we need a minimum of 3m clearance.

[BobL]

I noticed in the test with the 150mm bend tests, the pipes are facing the ground. Would it make sny difference if they where facing to the side or upwards to eliminate any chance of return turbulence from the ground?
I know with certain testing we do (not air flow but sound) we need a minimum of 3m clearance.

Good question and I did notice reductions in flow if things got too close even along side the openings.

When testing the lengths of flexy I could not just let it lay on the ground as that did affect the flow by about 15% plus it sucked up a heap of sand and dirt!

I tested the flow through a length of flexy with the opening at 1.5 m above the ground and then ~500 mm above the ground (laying on those chairs in post #25) with the openings elevated slightly upwards and they were the same.

The 2R bend (highest air flow) was tested with the right angle duct straight up/down and sideways and it made no measurable differences.
I would have done all the tests with the duct point upwards but this was tricky because it was raining and the air intake was outside the shed
The bends were all tested with the opening the same height above the ground so that effect is sort of cancelled out.

[Lappa]

Question BobL regarding the Testo.
How long are you finding the batteries last? I wouldn't have done as much measuring as you and I just had to fit the 3rd set!

Cheers

[BobL]

Question BobL regarding the Testo.
How long are you finding the batteries last? I wouldn't have done as much measuring as you and I just had to fit the 3rd set!

Cheers

I just moved onto my 3rd set.
Claimed battery life is 15 hours and i reckon I would get close to that.