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  1. #1
    Join Date
    Jan 2005

    Default DRAFT: FAQ - Dust Extraction (Practical Aspects)

    Last edited by BobL; 5th March 2020 at 01:39 PM.

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  3. #2
    Join Date
    Feb 2006


    Noise measurements of dust collectors.

    With an increasing number of woodworkers locating their DCs inside enclosures I thought it would be useful to establish some sort of basic noise measurement protocol so that we can more accurately compare the noise output (sound pressure level - SPL) of our DCs and the noise reduction performance of our enclosures.

    1) How to perform the measurement.
    To get any sort of reliable comparison the same distance from the source and height above the ground should be used.
    I recommend measuring SPLs 1m from the DC and 1m above the ground.
    You may wish to take a number of readings at different points (but still at 1m away and 1m above the ground) around the DC to see if there are any variations. You can average these if you wish.
    Ideally you should not be standing near the meter so if you have some sort of way of running the meter with you standing even a few metres out of the way that would be better.
    Make sure there is minimal background noise when making the measurements - either wait until the unwanted noise ceases, or drops to greater than 10 dB below that of the SPL being measured.
    It is possible to remove the back ground mathematically but it's not a straight forward subtraction of SPLs.
    If there significant background noise I suggest you report it.

    2) Measuring the SPL of a DC.
    The best way to do this would be to place the DC inside an anechoic chamber but I suspect few of us have one of these in the shed so the next best place to do this is in as open a space as possible.
    If it is done inside a shed or an enclosure then reflection and absorption off surrounding surfaces may dramatically effect the results.
    This means dragging the DC outside onto the middle of a lawn or similar.
    There may still be some effects due to reflections especially from hard surroundings like brick walls or concrete driveways but this is probably the best that can be done in most cases.

    3) Now the SPL reduction performance of the enclosure
    With the DC inside the enclosure and the door closed, measure the SPL using the same 1m away and 1m above the ground.
    Once again you may should measure the SPL at different locations around the enclosure BUT this time look for the highest result. This is likely to be nearest the escape route or vent. This is the result you should report.
    The difference between the SPL the DC makes in the open and the SPL outside the enclosure is the effect of the enclosure. This includes both its reflective and absorbing ability and the effect of reflecting surfaces around the enclosure.
    What you should be looking for is the

    4) Now the SPL reduction performance of the door.
    Enclosure doors are the weakest link of most setups and it is often these that need attention by better sealing etc.
    Now open the enclosure door and measure the SPL in the same locations as you did for 3)
    The difference between the results obtained with the door open and 3) is an approximate guide to the performance of the door.
    Do not be surprised if the SPL with the door open is greater or lower than that measured at 2)
    A greater SPL means that the enclosure is reflecting too much sound internally so it may be worthwhile investing in more exposed sound absorbing surfaces inside ether enclosure. If the SPL is lower - great.

    5) SPL measuring devices
    Accurate SPL measurement requires a calibrated device which few DIY will have access to, but increasing numbers of Smart phones have SPL apps.
    For those using SPL apps you may wish to read this thread Noise monitoring using smart phone Apps
    Even for less accurate apps the difference between SPLs e.g. 2) and 3) [ie overall enclosure performance], and the difference between 3) and 4) [i.e. door performance] are likely to be more reliable than any single SPL level.

    It would also be helpful to post what device/app you used for your measurements.

    6) SPL at the neighbours fences.
    This is for your benefit because this is what matters to your neighbours.
    This is where you may need to take some background readings (i.e. SPL with and without DC running.
    At the mens shed the noise from an afternoon sea breeze and local traffic was often greater than the SPLs measured at the boundaries.

    Please provide any feed back on the above suggestions so I can edit it before it goes up to the sticky.

    Back to TOC.
    Last edited by BobL; 5th March 2020 at 01:23 PM.

  4. #3
    Join Date
    Feb 2006


    Measuring air flows in workshops.

    1) Units

    Air flow is a general term for “air volume moved per unit time” (eg cubic metres/second or m3/s). Air flow has also been used to describe “air speed” (eg metres/second or m/s)”. In this post, “air flow” will only refer to the former. Because many DC systems and other devices still use imperial units especially for flow (usually cubic feet per minute or CFM) I will where possible also quote these.

    There are no instruments that measure air flow directly in DC ducting in any sort of an accurate manner. There are plenty of instruments that measure air speed (m/s) enable you to enter the cross sectional area (m2) of the flow, and then display a flow in m3/s but this does not apply to DC ducting as will be explained below.

    2) Air speed measuring Instruments.

    Air speeds are measured by devices called anemometers.
    The main types available are A) Propeller, B) Hot Wire and C) Pitot (pee-toh) Tubes

    a) Propeller based systems.
    An example of a propeller anemometer is shown below


    These are available on Ebay for as little as $10 and are designed for use in large to very large ducts (as used in airconditioing systems) or in the open air.
    This means they can be used to measure air speeds through doors and windows, air conditioners, and room air filter systems.

    However, propeller devices are not suitable for measuring air speeds in ducting where to obtain a meaningful result, air speed measurements have to take place inside a duct.
    The duct diameter must be at least 10 times larger than the longest dimension of the propeller device (this includes the non-propeller component).
    These devices cannot even be used reliably for relative or comparative measurements.

    b) Hot wire Anemometers
    These anemometers start at around $120 and can be used for both ducting and open air measurements.

    Here is an Amalog Anemometer that can read up to 30 m/s

    Above is an example of a digital hot wire anemometer. This one is limited to 20 m/s.
    It also incorporates a built in thermometer and has a fast and slow response rate readouts.

    Note that both of these anemometers come valid calibration certificates, which the cheaper models don’t have , or the certificates provided are of dubious quality.

    Their mode of operation uses a sensor heated by an electrical current, which is cooled by moving air. The temperature of the sensor is thus an indirect measure of the air speed.

    Here is a close up of the sensor the whole probe is ~6mm in diameter.

    The sensor is small white ceramic cylinder on the end of a probe, which can be inserted into a sealed hole in the side of a duct.
    Orientation of the probe is important and care must be taken with positioning to obtain accurate results.
    Provided the probe is perpendicular to the air flow, usually the probe is rotated to obtain a maximum value as the true air speed value..

    The typical dimensions of the probe tip is ~15 mm long x 6mm in diameter. The smallest duct these can be used is thus ~100 mm in diameter, however there ways of getting around this described below.

    One disadvantage of the cheaper anemometers is that they are limited to 20 to 30 m/s and some air speeds in DC systems may be greater than this. Higher air speed units are available but cost correspondingly more. Once again there are ways around the low air speeds also described below.

    EDIT[10-11-18]: Recently there have become available some very useful blue tooth hot wire anemometer systems more details here Air flow calibration measurements

    c) Pitot tubes

    Eg Pitot Tubes | Dwyer Instruments
    A calibrated tube starts at ~ $60 (the telescope unit shown above is lot more) for the tube itself to which a pressure measuring system (manometer) must be added.
    This can be as simple as a U-tube manometer or a digital manometer costs about $60.

    Pitot tubes establish an air pressure difference generated between a moving air stream pushing directly into a hole at the tip of the Pitot tube and at 90º to a series of holes around the end of the tube. This pressure difference has to be measured and converted into an air speed via a manometer and calibrated mathematical expression making them more awkward to use and because of these issues I do not recommend these devices for beginners. Pitot tubes can be made quite small so they can be used inside smaller ducting , used closer to duct walls and they can measure higher velocities accurately than hot wire anemometers but they have to be carefully positioned to obtain an accurate and reproducible result.

    3) Measuring air speeds in ducts

    If air speed through a duct, junction or port needs to be measured, irrespective of what instruments are used these measurement MUST be performed inside a length of duct.
    Using any instrument at the opening of a duct.junction or port will not give a correct result. This is because the airflow into a these opening is not smooth or even and a substantial amount of air is drawn from the back, and the sides of the opening. These openings are also very turbulent where various air streams fight each other to enter the opening. This can lead to results that are above or below the real flow.

    Because of the above measuring these connected to a test duct and performing the measurement inside the test duct is best done using a hot wire anemometer probe or a pitot tube.

    The test duct should be a straight, smooth walled length of ducting at least 5 and preferably 10 test ducting diameters long.

    A hole large enough to pass the air speed measurement probe into the air flow is drilled halfway along and into the side of the test duct and preferably a sealable port added. The port can be a piece of plastic or ally tubing threaded into the test duct such that it holds the probe at 90º to the air flow and something like a rubber O-ring added to the probe so that it seals the gap between the port and the probe.

    Here is a close up of the port into which the probe is inserted.
    The inside of the port is made flush with the inside of the duct

    The test pipe does not have to be the same diameter as the diameter of the ducting being assessed. If anything it’s better to use larger ducting so the air speed is lower and thus less turbulent so can be more accurately measured. This is especially the case for high-speed air movement produced in narrow ducts by vacuum cleaners. A larger test dust also more easily enables a physical larger size anemometer probe to be used.

    In the picture below are the test ducts I use for my measurements. The larger diameter duct is 225mm in diameter use to measure flows through 150 and 100 mm diameter ducting. The smaller duct is 150 mm and I use that for 100 and smaller ducting. The red markings represent where the test ports are located.


    The 150 mm test duct is around 1.8m long so its length easily meets the criteria for 10x the diameter.
    The 225 mm duct should be 2.25 m long but is only 1.4m long but, a) beggars can’t be choosers, b) it already takes up a lot of room in my shed, and c) the 10x criteria is less important if the test duct is of a much larger diameter pipe than the duct being tested.

    The transition from the test duct to the duct being tested should be smoothly ramped rather than stepped. Neither of the above (the grey plastic “pot plant pot” in the case of the 225mm test duct, and a Level Invert coupling for the 150 mm duct) are ideal. The transition should where possible be at least 5x as long as the diameter of the duct being tested.

    If the test duct is large enough in cross sectional area then even a propeller anemometer can be used.
    If a propeller type anemometer casing is say 75 mm long then the test dust that is 750 mm diameter should be used.
    But it does not have to be a pipe and could be a rectangular (750 x 750 mm) duct, although a 500 x 500 mm duct would still give a reasonable result.
    The transition from the rectangle to the test duct would need to be ramped using some sort of pyramidal hopper to conical to circular transition.

    So finally you are able to measure air speeds and it’s here where major mistakes are made because it is rarely possible to just multiply a measured air speed by the cross sectional area of the duct especially where high air speeds are involved.

    4) Performing the air speed measurements

    A single point measurement inside the middle of a duct will give you an idea of the air speed but simply converting that to an air flow will in most cases produce a result that much too high. This is how manufacturers of DCs measure their DC flows and is why their flow rates are as much as 50% too high.

    In the diagram below are the air speeds (black squares) obtained by placing a pitot tube at different radial positions across a 154 mm duct. A pitot tube was used because some of the air speeds were >40m/s and the hot wire anemometers available could only mead up to 30 m/s.

    This shows the air in the centre of the duct is moving at more than twice the speed of the air near the sides of the duct. Even though the air nearer the walls moves slowly, (apart from the air speed and flow closest to the wall), because of the larger volumes, more air is still carried by the slower moving air nearer the wall than the smaller faster moving areas in very middle of the pipe. See red squares on the diagram.

    BTW I don’t show a value for the speed on the duct wall, itself because I cannot place the probe right on the wall, but it is accepted that it will be close to zero. A small Pitot tube can get much closer to the wall than a hot wire anemometer fo if you want to measure wall effects the Pitot is a little better.


    The above graph shows measurements for the radii on one side of the pipe (0 to 77mm).
    I did measure the other side and the results above are the average of the two sides.
    It is really important to check both sides of the duct because they might not be the same.

    A Pitot tube is used in exactly the same way except that the pitot tube measures pressure differences and these must be converted to air speeds and then the rest is the same as using the hot wire anemometer.

    Sometimes there may be dips or peaks in the air speed outside a smooth curve – in which case you may have turbulence.
    This may be cause by such subtle things as a crooked exit or there may have some projecting into the air stream.
    Sometimes the measurements wander up and down over a period of a few seconds or as much as 30 seconds, which is a right royal PITA.
    This could be a clue that your test duct is too short, your probe is too big for the size of the duct, the test duct is not straight or the transition between the test duct and the duct being tested is not smooth.

    Sometimes you simply cannot do anything about this and you have to average a series of air speeds as best you can. Sometimes I record every 2nd or3rd air speed reading of the meter and stop and average 10. Why every 2nd or 3rd – because that is about as fast as I can write them down J An integrating air speed meter can be useful although these also have problems.

    Reliable measurements can take a lot of time setting up and performing so a significant amount of patience is required.

    5) Calculating the air flow

    There are several ways to turn the above air speed data into a single figure flow rate.

    Using just the mid-point air speed times cross sectional area of a 154 mm duct (42 m/s x 0.0186m2) will give a flow of 0.782 m3/s (1657 CFM)
    Using an average air speed (34.2 m/s) gives a flow rate of 0.635 m3/s (1347 CFM)

    The most accurate method is to determine a mathematical function for that air speed versus radius curve and integrate the function across the radius. Unless you already know how to do this I don’t recommend it.

    A close approximation of the integration method is what I call the summing method.
    · Divide the inside of the cross section of the duct up into a series of discs 1cm wide, and calculate the areas represented by these discs.
    · Then multiply each disc area by the air speed inside each disc to determine the air-flow through each disc. These are the red squares in the diagram above
    · The sum of all the air-flows will give the total air flow.

    This method applied to above data gives an air-flow of 0.585 m3/s or 1239 (CFM) which is consistent with the approximate maximum air flow possible through a ~150mm duct at the usual DC pressure.

    From the above you can see that mid-point flow is over estimating the flow by about 34% while the average flow is over estimating by about 9%.
    These over estimations are MUCH greater for 100mm ducting.

    Fudge method A
    It would be useful to be able to just use the mid-point air speed, then multiply that by the cross sectional area, and then a fudge factor.
    Bill Pentz suggests a 20% fudge factor but you can see from the above that in this case that is not enough and still leave the result 14% too high - more for 100 mm pipe.
    Unfortunately the fudge factor is also not constant but depends on a number of things like duct wall roughness, and even the air speed itself, I.e. the fudge factor is not even the same for the same pipe if widely different air speeds are involved.
    Unfortunately the single point air speed measurement fudge factor also does not take into account any unusual variation across the duct so measuring air speed systematically across the radius is still needed.

    Fudge method B
    This uses the average air speed at the different radii and then applying a fudge factor (usually around 10%) this works pretty well as it takes unusual variations of the air speed into account. Sometimes when performing repeated measurements in the same test duct over a limited air speed range I do use this fudge method but first I work out what the fudge factor is.

    Changes in ducting or junctions or machine ports often produce changes air flow of only a few % so if reliable comparisons are required the summing of individual air flows method is the best to use. Often it is impossible to detect any clear differences in the air flow.

    6) Measuring air flows through large areas and devices

    When the air speed is relatively low and the area through which the air is moving is large then any three type of anemometer can be used.
    This includes doors windows, most air conditioners and room air filters but not fans

    Simply measuring the air speed systematically across the area and averaging the measured air speeds will give a reliable result.
    If the air speed is high and the area is small then edge effects can start to affect the results and precautions such as those mentioned for ducting should be used.
    Fans such as axial exhaust fans generate quite variable air speeds so should be attached to a test duct.

    Provided there are no other openings or gaps involved, sometimes it is easier to measure the incoming air speed through a door or window than to try and measure the outgoing air speed through an exhaust fan. The air speed through the larger area will be slower and much less turbulent than the fan If the air speed through the door is too low construct the space with a piece of cardboard or timber. For a roller door try reducing the open area of the door. Try several door positions so you get different speeds but you should always get the same flow provided you don’t constrict the air flow too much. This will tell you if you are measuring the air flow consistently. You may also need to watch out where you are standing as you may be restricting the flow thereby creating a higher than otherwise air speed.


    · Hot wire anemometers are the easiest way of reliably measuring air ducting speeds for wood workers.
    · Propeller type anemometer are useful for measuring the air speed through large area openings but not suitable for duct measurements.
    · Duct flow measurements MUST be performed inside a duct
    · The use of large test ducts connected to ducts being tested usually makes it easier to perform air speed measurements.
    · The air speed must be measured systematically at various points across the test duct diameter
    · The final calculation of the flow rate should take the variation of the air speed with position inside the duct into account.
    · Just like woodwork, considerable patience and care are needed for good results.
    Last edited by BobL; 10th November 2018 at 12:18 PM.

  5. #4
    Join Date
    Jan 2015
    Latrobe Valley


    And I was going to use the wet finger test Bob...... feels cold in font of duct = 1000CFM
    Looks like I have to buy another toy.


  6. #5
    Join Date
    Feb 2006


    Bell Mouth Hoods.

    I prepared a presentation for Mandurah Woodturners that included a bunch of images for bell mouth hoods so I thought I would post an FAQ for these hoods here.

    Bell mouth hood are supposed to be the most efficient way to get air into and out of a duct within a short space. A long smoothly tapered hood will have a similar efficiency but take up a lot more space.

    A bell mouth hood (BMH)is a bell shaped like shape placed on the end of a duct.
    In theory a flaring trumpet shape gives the optimum efficiency but also takes up room compared the a simple curved edge.
    There has been considerable research done on this related to Motor vehicle carby air intakes and loudspeaker design cabinets and in practice a simple curved edge with a radius of curvature half the radius of the duct being used delivers within 1% of the maximum possible flow obtained with a trumpet shape.

    Here is an example of a BMH for use on a wood lathe
    The good is mounted on an articulated arm so it can be moved around to get the hood as close as possible to the work
    2hp dust extractor mod  - build-bellmouth5-jpg
    The ducting in use is 150 mm in diameter so the radius of curvature of the edge is 37.5 mm
    The radius is not that critical so +/- a few mm won't make a lot of difference

    Here is what the cross section looks like

    The one above is made out of a sandwich of 3 x 18mm thick MDF sheets and turned on a lathe.

    Here is an example of one being turned on a lathe.
    This one is not really a BMH but more of a radiused edge for an impeller intake but the principle of turning a BMH is the same.
    Mens Shed Dust Collection-cvmaxbnh-jpg

    The following images shows why BMHs are effective compared to a naked duct.
    The red lines represent stream flow lines and show the approximate directions that the air is draw from and the paths followed.
    A naked duct draws a substantial amount of its air from behind the opening which means it draws less air from the front of the duct.
    Assuming the dust is being made at the front of the duct a naked duct won't draws as much air or dust from the source.
    The second issue with a naked duct is the path taken by the air from behind the duct has to make a 180º turn to enter the duct.
    In doing this it collides with the other air entering the duct and causes turbulence slowing the total amount of air drawn into the duct.
    A well made BMH will draw about 10% more air, from as much as twice as far in front of the duct as a naked duct.

    BMHs can also be used in reverse - i.e. to transfer air from a cabinet or chamber to a duct or from a duct to a chamber like an impeller - like this.
    The same geometry applies.

    BMH work at any size and just applying the principle of a curved edge to any air flow entrance or exit is better than leaving a hard 90º edge.
    Even just rounding over an edge with a router would help.

    BMHs require more space than conventional connections and hod and sometimes it is not possible to fit BMH to machines cabinets but it is still worth using half a BMH is you can.
    There is more discussion on BMH here Improving machine cabinet dust ports

    Back to TOC.
    Last edited by BobL; 5th March 2020 at 01:28 PM.

  7. #6
    Join Date
    Feb 2006

    Default Dust Extractor Enclosures

    Enclosures for dust extractors are increasingly being used by wood workers to remove dust and noise from their sheds.This post is not about the "why", but rather the “how" , to reduce noise aspects of these enclosures.Apart from weatherproofing external enclosures, and making internal enclosures air tight (otherwise dust leaks back into the shed) the principles of construction are the same for both types of enclosures.

    On cyclones most of the noise comes out of the impeller outlet so if this is muffled using a free flowing Muffler most of the noise can be reduced to the point where the enclosure only needs to be a weather proof structure of some kind.

    Further noise reduction may be needed in high density living situations. if this is you then just follow the same pathway as shown below.

    On a DC which uses filters the noise also comes out of the impeller outlet but the filters get in the way so this is where a noise reducing enclosure is required.

    The Enclosure Frame.
    The frame and roof of the enclosure needs to be stiff.
    To assist with this decreasing the spacing between the framing components and cladding the frame with a sheet of wood or metal will help reduce the amount by which the walls and roof can flex.
    A standard timber stud wall covered with MDF should be sufficient but if you are at all concerned, then further reducing the spacing between the studs and adding extra noggins will help.

    To reduce the sound escaping the enclosure at least two layers of materials are needed.
    One layer needs to be made of a thick/dense/heavy material that reflects most of the sound back into the enclosure.
    The other layer needs to be a thick layer of air trapping material that will absorb some of the sound as it passes through it.
    Remember no single layer will reflect or absorb ALL the sound that strikes it or tries to pass through it - Egg cartons alone will not work

    Dense Layer (DL)
    The material for the DL can be, Lead, concrete, brick, wood, or plaster,
    The denser it is, the thinner it can be, so a lead sheet 5mm thick will be equivalent to wood around 60 mm thick.
    However it cannot be too thin or it may act like a drum and transmit the sound out of the enclosure.
    This means the thicker this layer is the better.
    As a guide, using something like 16 mm chipboard will produce a useful effect.

    Air trapping Layer (ATL)
    The air trapping layer ATL can be,
    - multiple laters of cardboard
    - foam rubber (old mattresses are excellent),
    - acoustic insulation material (expensive)
    - even plain old thermal insulation will do something.
    The thicker this this the better.
    As a guide using something like 50 mm of thermal Rockwool insulation or foam rubber will produce a useful effect.
    Carpet also has to be quite thick - enough to make a 50 mm thick layer to be effective.

    The order of layers is not that important in this situation and ease of construction and type of material used usually determines what is done.
    For example, if the wall structure itself is not thick enough to contain the ATL then sticking it to the inside wall works for foam rubber, but no so for fibreglass.
    The more layers of both the better they work and they don’t have to be sandwiched between each other.

    External cladding
    Flat Sheet metal is suitable as an external weatherproof cladding but not as a sound reflector so if you use use sheet metal as an external cladding the framing structure will need additional stiffening to reduce any drumming effect.
    Ribbed sheet metal and corrugated iron are suitable for external cladding as would be marine ply. Less desirable would be weatherboards.

    Some examples of wall/roof structures
    35 x 45 mm pine stud wall clad in Colorbond, in between the pine studs fill with 50 mm of insulation fibre and clad with 16 mm chipboard or MDF or plaster
    90 x 45 mm pine stud wall stiffened on the outside with 12mm chipboard covered with Colorbond.
    Infill with 90 or 100 mm of insulation and cover with 16 mm chipboard or MDF
    Better still
    Use 2 layers of the 16 mm MDF or plaster, and add 50 foam rubber on top of that.
    Top of the wazza
    If you want to get really fancy you can use a concrete wall, or a double stud wall setup with a 100 mm gap between the walls full of acoustic insulation.

    The limit to all this is the sound escaping from the air escape path whereby it’s a waste of time making the walls more sound absorbing if the sound coming out of the air escape path is dominating the noise.

    Location of DC inside enclosure
    While it may be tempting to hang the DC on the walls of the enclosure this can lead to problems with the sound from the DC transmitted direct to the walls. It better to place the DC direct onto the ground or floor of the enclosure and if you enclosure has a floor consider separating that from the walls. Carpet on the floor will also help

    These represent a significant pathway for noise to escape. They need to be covered in the same material as the walls and be well sealed.
    Foam rubber edge seals and positive closing mechanisms will help.
    Remember to make the doors big enough to get the DC in and the sawdust out

    Air escape path.
    The air has to be able to escape freely from the enclosure otherwise it will add resistance and reduce the extent of to the air/dust collection.
    If you have a lot of spare height space then a muffler or straight tunnel is one option.

    Tunnels can be
    - collapsible insulated air con ducting plus some addition foam or rock wool type insulation if needed
    - An MFD box lined with rockwool that is held in place with some chicken wire. This is what we're using at the mens shed for the ClearVue cyclone.
    The box is 400 x 400 x 1.8m x 12 mm thick MDF and lined with 90 mm of rockwool which is held in pace by an 8" tube of chickenwire.
    For a smaller DC a 350 x 350 x 2m MDF box lined with 50 mm of insulation would be OK

    Baffle boxes are the same as tunnels except the air does multiple U turns to escape.
    More U turns - better sound absorbance, there should be at least 2 U turns in the box, air entry and exit to the box counts as 1/2 a U turn.
    Some insulation inside the turns helps.
    Because multiple U turns generate slightly more resistance a large cross sectional area is needed. I recommend more than 2X the cross sectional area e.g. I use 4 for my setup at home
    See BobL's shed fit.

    Straight tunnels have to be longer and baffle boxes have to be fatter (Wider - taller)
    The longer the travel path the more sound will be absorbed but the larger the cross sectional area of the escape path needs to be to prevent too much resistance building up.

    I would recommend at least 1 and preferably 2m long for a muffler. A baffle box can be shorter but in practice you end up using the same path length because the party od more convoluted.

    Here are some schematic cross section examples.
    1: This is a basic muffler and insulted pipe of some kind
    2: same as 1 only thicker insulation means better sound absorbance
    3: Is a baffle box. This one has 3 U turns 1/2 for each of the exit and entrance, and two internal turns.
    4) is a sort of combo of a muffler and baffle box. This is like a car muffler. It works great at reducing noise but requires D be larger than the other examples because it generate significant turbulence.

    Outside shed for Dust Collector-mufflersand-bb-jpg

    Back to TOC.
    Last edited by BobL; 5th March 2020 at 01:30 PM.

  8. #7
    Join Date
    Feb 2006



    DomAu reminded me that on the Cincinnati website (Cincinnati Fan - Industrial Fans and Blowers) they provide a lot of useful info on fans and ducting.

    This 16 page PDF ( has a lot of useful info.

    To save you the trouble of wading through this document I have transferred some of the info here where I will also give my opinion about this info.

    The first one is a general comment on fan and ducting installation.
    This has some good info about how close turns and discharge points should be placed to a fan - impeller.


    The next one shows in particular how much resistance in terms of ft of straight pipe each configuration represents.

    EG for 6" ducting a 90º elbow with a radius 1.5 x the radius of the duct(R) i.e. 9" elbow radius, will have the same resistance as a 12 ft of straight pipe whereas a 2.5R elbow will have half that resistance.
    These figures are approximate for the sorts of flow rates that are normally used in dust extractors - the effects are less for slower speeds, and greater for higher speeds.


    This one is interesting as it shows the effect of type of orifice.
    I'm not sure I agree with the numbers but I agree with the order.
    A proper Bell Mouth should have <2% losses and an unflanged pipe should only lose 90% for very high flow rates on narrow pipes
    The effects are far less for slower speeds.
    This one is pretty obvious but in case you are not aware here they are.
    Note in particular the 3 way in the last line.

    Back to TOC.
    Last edited by BobL; 5th March 2020 at 01:31 PM.

  9. #8
    Join Date
    Feb 2006


    Understanding and using consumer level PM2.5 dust counters.

    This is a preliminary discussion about using PM2.5 dust counters because I know some folks are interested in getting something along these lines. I am still testing the counter I have purchased and the following may be revised so as for all stuff on these forums, use at your own rick

    The recent increasing concern over exposure to PM2.5 (particles of 2.5microns and smaller) has seen the production of low cost particle counters/sensor and more recently consumer type products with usable displayed values starting to be sold for <US$100. e.g. Developments in Dust Sensor tech

    Up until this year PM2.5 dust was considered the 8th most common cause for deaths world wide but was recently upgraded the be the 5th most common cause.
    PM2.5 dust has been impilcated for some time with causing many respiratory problems, but the latest studies has a strong link to other problems the main one being vascular (Strokes/heart) issues.
    It also applies to ANY particles natural or man made.

    The typical dust values displayed by these new sensors are?

    1) PM1 concentrations.
    These are concentrations in micrograms of 1 micron and smaller dust per cubic metre of air (ug/m^3).

    2) PM2.5 concentrations
    These are concentrations in micrograms of 2.5 micron and smaller dust, per cubic metre of air (ug/m^3)

    This should be of concern particularly to older woodworkers or any woodworker with a preexisting health condition.
    The WHO recommended limit for a 24/7 exposure is only 25 ug/m^3 - remember this is 24/7!
    But this does not mean if you spend 40 hours a week above this level and the other 128 hours well below this level you will always be OK.

    If you constantly are exposed to >25 ug/m^3 of PM2.5 dust in your shed you probably should do something about it.

    These counters are built to specifically address this range of dust particle sizes so the accuracy of these devices has been optimised for this range and this seems to agree with the limited testing I have done.

    3) PM10 concentrations
    These are concentrations in micrograms of 10 micron and smaller dust per cubic metre of air (ug/m^3)

    For woodworkers the PM10 output is the most appropriate since it covers the widest range of larger dust particles including those known to cause nose and throat cancers.
    The WHO recommended limit for a 24/7 exposure is only 50 ug/m^3 - remember this is 24/7!
    Once, like PM2.5 dust, this does not mean if you spend 40 hours a week above this level and the other 128 hours well below this level you will always be OK

    The PM10 output of these counters is achieved by a a bit of a fudge and its not as reliable as the PM2.5 output especially when the dust count goes over 1000 ug/m^3
    Any readings above
    1000 ug/m^3 are not likely to be accurate,

    4) Air Quality index (AQI)
    Some counters/Sensors also display an Air Quality index (AQI)- some with a colour dashboard style health level indicator for level of safety.
    This can be converted into dust concentrations (ug/m^3) but threshold levels for the AQI vary from country to country so sensors that display this colour dashboard style health level indicators may not be all that useful.

    You need to bear in mind that you cannot see any of this dust in air as it is too small and it is only after sufficient of it has deposited out of the air that on surfaces that you know it has been present in the air.

    The now almost outdated OHS standards for wood workers are as follows and are based on European woods like beech, oak and ask and northern hemisphere conifers.
    Apart from MDF and a few woods like WRC these standards do not take into account the unknown toxicity of Aussie timber or particle size ranges, and are based on a maximum for an 8 hour working week for all airborne dust - these values are effectively - PM10 levels..

    1) Softwoods - 5000 ug/m^3
    2) Hardwoods - 1000 ug/m^3
    3) MDF and woods like WRC - 500 ug/m^3

    5000 ug/m^3 average across a 40 hour working week and then zero exposure for the other 128 hours of the week works out to an average 1200 ug/m^3 week
    1000 ug/m^3 average across a 40 hour working week and then zero exposure for the other 128 hours of the week works out to an average 240 ug/m^3
    500 ug/m^3 average across a 40 hour working week and then zero exposure for the other 128 hours of the week works out to an average 120 ug/m^3

    This assumes you get zero for the rest of the week which won’t be case as you will be exposed to other dust when not in a shed.
    If you drive on a freeway for 2 hours a day or live near a freeway you could well exceed the lower levels for wood dust on some days

    So the question is, what values should we use.
    If you are as (older) person with preexisting health issues and you already live in a dusty environment I would err towards the side of the WHO values for both PM10 and PM2.5

    If you are young & fit and live in a low dust area e.g. west coat Tassie you can get away with a bit more so you may tend towards the old Aus OHS standards

    Given the unknown toxicity of Aussie timber, my recommendation would be to set up a shed (even a DIY shed) that contains PM10 levels to at least that for the MDF and WRC OHS standards i.e. no more than 500 ug/m^3 .

    This should then cover all timber in the Australian standard - and remember according to the latest studies it should not matter what the particles are from.

    cover the lack of accuracy that the PM10 readings output by these meters generate

    If you get one of these sensors and your readings are constantly over 500 ug/m^3 then It’s time to do more than you are doing in terms of dust control.

    Bear in mind that these sensors are not designed to be showered in a stream of sawdust direct from a machine and doing this will kill the sensor in short order.
    These sensor are designed to be placed on a wall at head height, nearby to but not right in the middle of any dust making activity.
    The current expected lifetime of the tiny laser inside the sensor is >3yrs - this can be extended by only running the detector when its its needed and not over exposing it to too much sawdust.
    The detector can also be cleaned by running it for a few hours each month in clean air (i.e. air coming direct out of a room air filter). This is then the time to see if the sensor is reading properly close to zero.

    Back to TOC

    Last edited by BobL; 5th March 2020 at 01:34 PM.

  10. #9
    Join Date
    Feb 2006


    Been meaning to post this for a while here as Info for newbies and others posting in this forum.

    Requests for help and questions about dust extraction can lead to a lot of guesses and PPP (Posting ping pong) while we work out what your situation is.
    To help answer your questions about dust extraction the following information is useful and should be included in initial posts for assistance with dust extraction.

    1) The number of hours you spend a week actually making dust in your shed - not just sitting or doing non-dust making activities in your shed.
    2) The size of the shed you work in, not the size of the area in the shed you utilise, but the total size of the shed.
    3) The type and size/power of dust making machinery you use in the shed.
    4) The types of wood you work with
    5) Size and location of openings like doors and windows that you can keep open without annoying the neighbours and irrespective of the weather.

  11. #10
    Join Date
    Feb 2006


    Abbreviations used in dust extraction:

    BMH: Bell mouth Hood

    BP: Bill Pentz - wood dust guru

    CFM: Cubic feet per minute - used to describe air flow in ducting

    DC : Duct collector, dust collection
    DE: Dust extractor, dust extraction

    DWV: Drain Waste and Vent A type of PVC duct used for sewage connections and often used in DE systems.

    FPM: Feet per minute - used to describe air speed in ducting

    HEPA: High Efficiency Particulate Arrestance - a type of absolute filter that comes in various efficiencies.

    PM10: Particulate matter (dust) of 10 microns and smaller
    PM2.5: Particulate matter (dust) of 2.5 microns and smaller
    PM1: Particulate matter (dust) of 1 micron and smaller

    SPL: Sound pressure level in decibels (dB)

    PM me if you want any others included.

    Back to TOC.
    Last edited by BobL; 5th March 2020 at 01:35 PM.

  12. #11
    Join Date
    Feb 2006


    Testo have released a "“Practical guide air flow measurements in ducts according to DIN EN 12599”

    It describes two ways of measuring air flow
    a) Trivial method
    b) Centroid method - this is a simplified version of the method I recommend and I suspect it has been developed for people with limited maths skills.
    Provided sufficient measurements are taken (Id take maybe 1.5 - 2 times more than they recommend) it should still generate reliable values.

    They even discuss turbulence and the need to measure multiple times the same location and different radial positions inside the pipe to take this into account.

  13. #12
    Join Date
    Feb 2006


    Quote Originally Posted by BobL View Post
    Testo have released a "“Practical guide air flow measurements in ducts according to DIN EN 12599”

    It describes two ways of measuring air flow
    a) Trivial method
    b) Centroid method - this is a simplified version of the method I recommend and I suspect it has been developed for people with limited maths skills.
    Provided sufficient measurements are taken (I'd take maybe 1.5 - 2 times more than they recommend) it should still generate reliable values.
    I worked out the measurement positions to use the centroid method on a 6" (154mm ID) PVC duct.

    The method involves calculating equal area concentric annuli (rings )inside a duct.
    The resulting annuli then have radii that are not evenly spaced as shown in the diagram below
    An anemometer is then used to measure the air speed at the midpoint inside those annuli (as marked by the red arrows) and a simple average of these will give a more reliable result than an average at equidistant points across the duct.
    The higher the air speed the more reliable this method will be
    The numbers alongside the arrows are the distances in mm the measuring tip of the anemometer should be located from the inside wall of a duct.
    So 3mm is just inside the pipe and 77 mm is in the middle of the pipe

    Screen Shot 2019-06-08 at 5.57.19 am.png

    Because the air flow is rarely even inside a duct I would also recommend measuring using the same method across the duct in at least 3 other places such as is shown by the grey, green and brown lines.

    Each individual point needs to be sampled long enough to take into account turbulence, hence an averaging hot wire anemometer or one that can store say 1s readings over a 20-30 second period would be desirable.

    Back to TOC.
    Last edited by BobL; 5th March 2020 at 01:38 PM.

  14. #13
    Join Date
    Jan 2014
    Sydney Upper North Shore


    I presume the measurements you have shown are the location of the small measuring element in the sampling port?
    The sampling port on the Testo is 8mm and the measuring element on mine seems to be right in the centre ie. 4 mm

  15. #14
    Join Date
    Feb 2006


    Quote Originally Posted by Lappa View Post
    I presume the measurements you have shown are the location of the small measuring element in the sampling port?
    The sampling port on the Testo is 8mm and the measuring element on mine seems to be right in the centre ie. 4 mm
    Just have to try and get the centre of measuring element as close as possible to the positions mentioned in the drawing.

    I do most of my measurements in the middle of a 4m long section of 225 mm (240 mm ID) PVC so in that case it would be easier to locate the element although I probably won't change the way I do it - I measure every 10mm across the radius (so it works out to about 11 measurements) and then do the integration maths.

    If anyone wants me to work out the concentric area spacings for a 100 mm duct I am happy to do that.

  16. #15
    Join Date
    Feb 2006


    I have added a linked Table of Contents (TOC) to the first post in this thread so you don't need to scroll all the way through the thread to see the topics discussed. I will progressively do this to some of the other stickies as well.

    Have also added a short TOC to the "Modifying the 2HP DC "thread.

    If anyone spots a potential entry to either of these TOCs then shoot me a PM and I'll see what I can do.

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