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  1. #1
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    Default DC systems pressure testing

    Some of you may have seen my thread on building a sensitive pressure/Temperature meter for measuring pressures on a DC system. The technical WIP thread is show here DC temperature/pressure meter

    Apart from a couple of small things I have just about finished it and have started measuring a few of useful pressures with it.

    But firstly a few pics of the finished setup.
    OverallSetup.jpg
    BPS= Li-ion battery power supply, provides power to run the meter
    2 = calibration plumbing and manual pressure/vacuum piston - use to calibrate the pressure sensor in the meter. It's not essential it be there but I thought it looks cool and there's space for it so I just attached it there.
    3 = USB adapter to charge battery
    4 = Positive pressure manifold inlets
    5 = Negative pressure Manifold inlets
    6 = Arduino micro controller measures Pressure and temperatures (DC enclosure air temperature and DC motor Temperature)
    7 = Shed earth.

    In the thread listed above above I describe the problems I had with interference with measuring the temperatures. After a fair but of mucking around I tracked the problem to interference coming in via the power supply (A standard 12V DC adapter). The meter runs fine when using a battery but any AC adapter seems to allow the interference in. I'm working on filtering the DC output of an adapter but in the meantime I set up a battery powered supply as shown below.
    PowerSupply.jpg
    Electrical interference still gets through to teh meter and screws with the temperature readouts if the the battery is permanently on charge via a the USB adapter as well as being connected to the meter, so its either "battery on" and "charing off" or VV. This is what the two toggle switches do on the side of the box. I though switches would be better than fiddling around with repeatedly plugging adapters in and out.

    The meter only draws about 35mA so the battery lasts for days but I leave it on charge when not using the meter. When I want to use the meter I just flip the battery charging switching off and switch the battery through to the meter. The Green spade terminal is connects the USB shielding so can earth that if I need to.

    The taps on the manifolds are used to select from various pressure points connected back into the DC ducting system via black poly drip irrigation tubing. This saves a lot of swapping of the black tubing on and off the meter itself.

    It depends if you want to measure positive or negative pressure. For example, pressures upstream from the impeller will all be negative so if you want to measure at points in these locations then they will need to be connected to a negative pressure manifold tap and a tap open to atmosphere opened up on the positive pressure manifold.

    Pressures after the impeller, eg the filter bag housing relative to atmosphere, are positive so need to be connected to a tap on the positive manifold and a tap open to atmosphere on the p pressure manifold. This pressure is a useful guide to the extent to which the filters are blocking up and the first pressures I have been measuring.

    As a starter I should say my bags have not been emptied or even shaken for more than 3 months and then they are less than 1/3rd full. Most of the sawdust in the bags comes from the docking of a heap of 1.2m long twisted pine pieces into firewood size bits. There rest is mainly sanding so there is a fair amount fine dust there.

    The filter back pressure readings do jump around a fair bit - this is probably related to my choose of location of the pressure pick up point - but most of the readings are in the 0.09 to 0.12 kPa which equals 0.36 to 0.48" WC. This is consistent with BPs recommendation that 1" WC be used for the nominal pressure losses associated with a large <5 micron filters. The reason this is consistent is that my DC has two bags and thus BP's recommendation should be halved. So 0.5" compared to between 0.36 to 0.48" WC I think that is reasonable. BP does not say if that is for dirty or clean filters so I just gave my filters a bit of a shake and a lot of "stuff" fell off into the and that pressure dropped to between 0.2 and 0.25" . Now if I run the DC I can see the filter bag back pressure slowing creeping back - I will eventually make a more systematic study of this.

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  3. #2
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    Feb 2017
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    Hi Bob,

    Great work, I like how you develop, test and improve your workshop and then pass on your findings. Great work and well done!

    Regards
    Stephen

  4. #3
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    It will be interesting to read how you use the information to understand and improve your system.

    I read in another post some comments about measuring pressure in the pipes. What is the best probe and orientation for it. I want to measure in a 150 mm pipe and use a digital meter. Thanks

  5. #4
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    Quote Originally Posted by Maple71 View Post
    It will be interesting to read how you use the information to understand and improve your system.

    I read in another post some comments about measuring pressure in the pipes. What is the best probe and orientation for it. I want to measure in a 150 mm pipe and use a digital meter. Thanks
    The first question is do you intend to convert the pressure readings to air flow and this requires a special deal tube, multi holed probe called a "pitot tube" - this is separate from the meter

    If not then there are digital meters on ebay ranging from $30 to around US$400 for something like the pro Dwyer models.
    I would be wary about buy a used unit.
    Testo industrial pressure meters Like this one are around $750
    https://www.testo.com/en-AU/testo-512/p/0560-5126

    I use one of these for general work.
    https://www.ebay.com.au/itm/Manomete...kAAOSwMM9bBiSK

    You have to be very careful and select a manometer with the right pressure range or it may be damaged or not be sensitive enough to measure small differences.

    What sort of pipes/pressure are you talking about.
    Probes for just measuring pressure can be made from drip retic fittings and tubing.

  6. #5
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    I have a UEi manometer with the proper range. I just want pressure and will use a performance curve to estimate flow rate.

  7. #6
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    What’s wrong with using the UEi as the digital manometer? It should measure pressure directly.
    If you want measure flow you have to use a pitot probe like the ones from Dwyer. Flows calculated from a performance are not very accurate.

  8. #7
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    I can understand that calculating flow from the pressure and a performance curve lacks some level of accuracy. You have pointed out repeatedly how difficult it is to measure flow properly. Based on your posts, it is beyond the capability of almost anyone. So......one can measure pressure fairly accurately and estimate flow. One way or another, people want to measure flow. Which is the worst or best of the choices.

  9. #8
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    Quote Originally Posted by Maple71 View Post
    I can understand that calculating flow from the pressure and a performance curve lacks some level of accuracy. You have pointed out repeatedly how difficult it is to measure flow properly. Based on your posts, it is beyond the capability of almost anyone. So......one can measure pressure fairly accurately and estimate flow. One way or another, people want to measure flow. Which is the worst or best of the choices.
    It depends what you want to achieve.

    The main issue is turbulence

    If you want to accurately measure the flow in m^3/hr or CFM or whatever, then that is indeed a time consuming business that requires using a test duct, a suitable pitot tube or hot wire anemometer, systematically measuring air speed at positions across the duct across the duct and integrating the flows into volumes/time taking the annular cross sections into account. By making sufficient readings over time this should take turbulence into account.

    Converting a single point pressure reading into a flow using a performance curve is limited mainly by turbulence, probe orientation and position usually because of the Bernoulli effect
    Probes with openings close to duct walls may have different pressures than those in the middle of the flow
    A probe with an opening different angles relative to the flow will generate a different pressures.

    Obviously you can pick a specific probe orientation and position and keep that constant but even then turbulence will still be a problem unless you go to relatively low flows as used in AC systems

    In the example shown below I'm trying to illustrate the effects of measuring the difference in flow between a straight length of duct and the same duct with a blanked of Y at the end.
    The bottom diagram represents typical flow in a short length of straight duct. The naked opening will generate turbulence that will propagate down the duct and it will take a length of about 10 duct diameters to minimize the turbulence - in practice it is never removed at typical DC duct speeds .

    Screen Shot 2019-03-12 at 8.53.59 am.png

    The upper diagram shows the turbulence generated by adding the Y. Locating a probe at the same location and orientation may or may not give a reliable pressure because teh flow around it changes when the Y is added.

    If you intend using single point pressure changes to make improvements to improve flow by say port design or ducting changes, here are a few things you can do to minimise the turbulence issue .

    1) Maybe forget about measuring and calculating the flow - just work with the pressure reading relative to atmosphere. Higher pressure differences means lower flow.
    2) Locate the probe as far away down stream from the modification point as possible - the turbulences generated by the modification will increasingly be dominated by local ducting and junction structures and should remain relatively constant
    3) Use a test duct.
    4) If working with naked openings, add a BMH as this helps reduce the turbulence generated by naked openings.

    I will be exploring these things with my new pressure meter.

  10. #9
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    I have an electronic manometer the same as BobL's. I recently bought a calibrated pitot tube (important) at a reasonable price. I watched some YouTube videos on Pitot tube use and measuring. One guy who is an "AC EXPERT" had a long perspex tube and was going through a demo on measuring. While he spoke about the correct way to measure across the tube he stated that you could do it at five places and it was good enough as there was only a tiny difference between the reading even traversing across the entire diameter. He ended up putting 4 evenly divided marks on the tube and away he went. He was holding the pitot tube by hand an moving it in and out and then would do a close up of the meter showing a nice steady reading.

    So away I went. Its not that easy!!

    Any deviation of angle no matter in what direction or no matter how small will cause a changing in reading so, by hand ,its very very hard to get an accurate, steady reading IMHO.

    I ended up making a fitting to hold it steady so it was at 90o to the pipe and facing directly into the airflow (not at an angle). Even then I could not get the nice steady reading the guy on youtube was getting.

    The other point is what measurements setting do you use on the manometer? I measured using all of them and as you can see from the excel spreadsheet, some give much better resolution than others. Overall, I prefer the hotwire anemometer over the pitot tube.

    IMG_0188.jpg

  11. #10
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    Quote Originally Posted by Lappa View Post
    . . .. he stated that you could do it at five places and it was good enough as there was only a tiny difference between the reading even traversing across the entire diameter. He ended up putting 4 evenly divided marks on the tube and away he went. He was holding the pitot tube by hand an moving it in and out and then would do a close up of the meter showing a nice steady reading. . . . . .
    Spot on, that's one of the main differences between AC and DC systems air flow. AC is low speed, lower turbulence, flow whereas DC is always higher speed and hence greater turbulence. AC systems have small variations across a duct both spatially and temporally (space and time) so a couple of quick readings and you are done. DC measurements hav etc be done on a finer spatial scale and over a number of readings (time) to get a representative result.

    The Pitot tubes also responds almost instantaneously where as the hot wire anemometers have a bit more lag in the them. However, in both cases results from attached meters should be integrated over time to get reliable results. On my new pressure sensor I have it set up so that it integrates anywhere from 1 to 100 readings before spitting out an average result.

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