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  1. #1
    BobL is offline Member: Blue and white apron brigade
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    Default Lowish pressure measurements

    One of the things I have been working on over the last week is a Press/Temp measuring gizmo for my dust extraction system.

    Its an Arduino based system and the WIP and very grimy techno details have all been posted in the Woodies Electronics section, see DC temperature/pressure meter

    Temperature wise, the idea is to monitor the temperature of the impeller motor. The Dust extractor is located in a noise reducing enclosure outside my shed and I cannot see the motor or get to it easily to see if it is running warm.The reason I want to know if it is running warm is that originally the DC had a 3HP SP motor on it and this was replaced last year (with the good help of AB) to a 4HP 3P motor running on a VFD. This allows me to run the impeller faster and get more suck. Now while the VFD is current limited I wonder how hot the motor gets at higher Hertz. The motor of course is fan cooled and there's a lot of air flushing from the dust collector flushing past the motor itself siI doubt there's a problem but I just want to put my mind at ease about it. There are 2 K type thermocouples one for the motor and one for the temp of the air inside the enclosure connected back to the Arduino Micro controller inside the shed.

    Pressure wise, the idea was to setup a sensitive air pressure sensor that can resolve pressure differences generated by experimental changes to a DC system. By changes I mean different ducting junction sizes, junction arrangements, location of air breathing holes on machinery etc. Many of these changes produce pressure changes that are just too small an effect to be measured reliably using budget level manometers or U-Tube manometers.

    The sorts of pressures I am talking about are in the <3.5 kPa or 14" of Water Column("WC) range and the differences being measured may be ~1/100 smaller ie 35Pa or 0.15"WC which is larger than the uncertainties resulting from using conventional budget level digital, and U Tube manometers.

    After some searching I found a sweet pressure sensor that measures up to 6kPa with a sensitivity as precisely as you you can the output Voltage or up to 5V. This is perfect micro controller territory and is what I have done as described in the link above.

    Then I wanted to calibrate my sensor. My digital manometer is a cheapie and only good to 10Pa for a single reading but as multiple reads are needed for a relative measurement the final uncertainty is +/-40Pa. However I can still use the pressure meter to calibrate my meter because I use multiple readings across the pressure range to get me into the ball park and then I rely on the reproducibility and linearity of the new sensor to tell me if its better or worse. For relative measurements it is possible to perform a "better than /worsethan" working comparison just using the V/out of the sensor or the Analog to Digital output of micro controller but I wanted to see how well I can calibrate it with what i have and also how accurate I could get it.

    To do this I needed a way to generate and hold a range of pressures so that I could read the A/D output of the pressure sensor and the reading of my digital manometer at the same time.

    Below was my first attempt at a calibration rig using a syringe (S), small PVC pipe test chamber (C) and drip irrigation tubing and fittings. The sensor and Digital manometer are connected up to the two open ends of irrigation tube. Tap T is opened and syringe plunger is depressed to create a small pressure change.
    Without the chamber even small changes in the position of the syringe plunger sent the sensor off scale.
    Even when I could get the signal on scale it drifted around and the setup leaked badly so the signal decayed rapidly so I use gas thread sealant liquid but that stuff is awful and spreads everywhere like "grey goo". The rig was also very temperature sensitive - just putting two fingers on the chamber set the pressure wandering.
    Ocalib.jpg

    Eventually I gave up the syringe and used a clamp to squish a section of black irrigation tubing with one end close which easily generate enough of a pressure change. This sort of worked to get things on scale but the other problems remained like constant leaks which meant I was not sure what I was recording. I even got a half decent calibration curve out of it but eventually realised the sensor was reading about 3% too high.

    At work when we did this sort of thing for vacuum work we used SS fittings because they can be cleaned and well sealed, so off to Blackwood and for $50 I got two real nice Tee-Pieces, 2 x SS taps, and 4 x SS nipples all in 1/4" BSP (I already had the compression fittings other bobs and bobs and put this together.

    Ball valves T1 and T2 are opened to equilibrate pressures. Then T2 is closed and Screw piston (SP) is screwed into position to generate the required pressure.
    Actually I just realised T1 is not correctly located - it should be between the two Tees - this is so the SP can be locked out while taking that pressures calibration measurements - more about that later.
    Calibsetup1.jpg

    The metal working bit of this post is the screw piston.
    I made the piston and brass bits (these are outside the pressure zone) from a 6 x 60 mm SS socket head screw.
    The cylinder is a bored out (~9mm) slightly over length 1/4" BSP nipple
    I turned an O ring groove in the head of the screw and then added 4 other o-rings behind the piston
    Piston.jpg

    It goes on the piston like this.

    Setup2.jpg

    And then this - the blue line shows the range of action.
    Piston2.jpg

    The first thing this does is provide "not quite exquisite" but very fine input pressure adjustment - however, unfortunately it still leaked - ever tried to find a leak at low pressure - small bubbles take ages to develop under water, in fact at really low pressure they don't even develop because even shallow water prevents leaking! Initially I suspected the Poly pressure fittings as the tubing is only drip irrigation poly but I added an extra PVC olive (3mm long stub of 5mm ID clear PVC tubing) and this works fine. Then I suspected my screw valve so thats when I added the extra O-rings behind the piston

    After phaffing about a bit and getting totally bamboozled I gave up testing at low pressure and removed all the SS parts and tested them under water at 40 PSI! air pressure. I figured if I could seal it at 40 PSI it would stay sealed at 0.3PSI!. I did indeed find 2 leaks (see orange arrows two photos back). I was really pleased my home made piston did not leak even at those pressures.

    So I can now generate any pressure between 0 and ~3.5kPa and while it drifts slowly up and down it will stay steady for long enough at that pressure for me to get reliable readings from the sensor and digital manometer provided the temperature does not change. If anything the temperature effects are now more obvious, Just handling the valves and taps for too long will increase the pressure. Being all metal fittings teh pressure increases rapidly but also equilibrate faster than plastic. There's also long term drift, eg like when I going in and out of the room, or sudden steps: AC kicking in! Fortunately the software records everything so I can dial up a pressure and go away and come back in a few minutes and take a fair reading.

    Here is an example of what I can see on the laptop while its running.
    Presscurve.jpg
    A is the max value the sensor can read/output in bits (4096)
    B (purple line) is the sensor output in bits (0 - 4095). At around "B" you can see the slight rise over about 2 minutes - that was from me just entering the room!
    I then opened up T2 and let it show the sensor baseline, then at "O" I reapplied the pressure one turn at a time and left it stabilise about 20 seconds at each step - then let it sit for a while close to max output.
    At E2 I touched one of the Tee pieces with 2 fingers for about 3 seconds which sent it over the max limit. so I reduced the pressure and then did the 2 finger heat application again at E2. you can see how long it takes to stabilise. So "No touchy while works" - I might have to wear cotton gloves!

    This thing is sensitive enough to measure the atmospheric pressure difference over about a vertical height 1m - see link above for that experiment.

    Now I need to repeat the calibration and see how it goes.
    Attached Images Attached Images

  2. #2
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    Default

    At 6kPa, you are certainly talking lower than usual pressures. I wonder how much of the variation you are seeing is barometric...

    For doing really (accurate) low stuff, we used to use 'inclined manometers' - basically a tube on a slight slope (a few degrees) so that a slight change in pressure level would show as a significant horizontal movement. I think we were limited to around 4" water gauge, but picking a difference of say 0.1" was pretty easy.

    Michael

  3. #3
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by Michael G View Post
    At 6kPa, you are certainly talking lower than usual pressures. I wonder how much of the variation you are seeing is barometric..
    I'm measuring that as I type this.

    For doing really (accurate) low stuff, we used to use 'inclined manometers' - basically a tube on a slight slope (a few degrees) so that a slight change in pressure level would show as a significant horizontal movement. I think we were limited to around 4" water gauge, but picking a difference of say 0.1" was pretty easy.

    Michael
    Thanks Michael - yes I have used those and we had a number at work that we used to measure clean room over pressures to about +/- 1mm. The slope needs to be shallow to obtain a significant gain but then the meniscus gets spread over a longer distance so it's a bit more difficult to read the horizontal length accurately meaning the gain in resolution is not as great as one might think.

    If the angle is really shallow then they need to be very long to measure small differences at any signifiant height and my other issue is I need to be able to measure differences up to about 14". Even using just a 30º slope, to measure 14" of WC it needs to be at least 28" long. Using a really narrow tube helps constrain the meniscus to assist reading but then adhesive forces become significant and affect accuracy although accuracy is not needed for my application although I would like to retain some level of accuracy where possible.

    At work we used several commercial versions that used a constant slope of 30 degrees which only increased the resolution by a factor 2 over verticals and several with variable slope but I did not trust the accuracy of these at low pressures. I made one that measured up to 6"WC and had a slope of 15º that gave a factor ~3x increase in resolution over a vertical but it was 2ft long. Lower angles than this became problematic. I am still considering making a long one to use as a calibrating device because it would be about 3x better than my hand held digital manometer.

  4. #4
    BobL is offline Member: Blue and white apron brigade
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    I finally got to use the low pressure meter to measure something.

    My dust collector (DC) is housed in a sound reducing enclosure (a shed lean to) outside my shed.
    It has 3 doors about 2m x 475 mm in size air and a 475 x 150 mm S shaped baffle exit to help muffle the sound.
    When the doors are reasonably well sealed and when DC is running and I close the last open door I can sense some resistance which means some pressure is lost on the sucking side of the impeller. While it feels significant I could not measure this pressure on my 0.1" resolution hand held pressure meter.

    Both sides of the meter are connected to 4mm ID poly irrigation tubes and both tubes are hung outside the enclosure in the same spot to get a working zero (green line on graph ~60Pa)
    The first thing I notice is the new meter is very sensitive and light breezes scatter the measurements much more than I thought it would.
    To cater for any changes in atmospheric pressure I also bracket the measurement by repeatedly re-measuring the zero notice the second green/zero line onto RHS is now ~65 Pa.

    Then I start the DC with the door open and put the positive pressure tube inside the enclosure - this generates the blue lines.
    The I close the door and this generates the red line data ~80Pa. There is clear difference in positive pressure of the red line compared to the blue and the green lines. The pressure difference averages about 14Pa which is about 0.06" or 1.4 mm of WC which explains why I could not see this with the hand held meter. So while it feels like a lot of back pressure when I go to close the enclosure door it's actually less than 1% of the total static pressure generated by the DC at 50Hz.


    Enclosuretests.jpg

    There is even a small consistent statistical difference between the blue (P measured inside the enclosure with door open) and the green /zero line data.
    The effect is very small, ~ 3.5 Pa ( 0.015" or 0.035mm of WC) remember this is with the doorway is wide open. This is useful because it tells me I would have to increase the cross section of the S baffle air escape path by a long way to gain much of a benefit. In other words live with the escape path I have got.

    These were quick and dirty measurements - I'm sure if I chose a still day and collected more data I could get a clearer result. I am really pleased how the pressure sensors working.

  5. #5
    BobL is offline Member: Blue and white apron brigade
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    Setting up this pressure meter has involved lots of connecting and disconnecting of 4mm ID tubes and hoses that push fit on barbed fittings.

    At a certain point in the development/calibration cycle I had to move from testing and calibrating the meter itself on the electronics bench in the house to the shed, but then I have had to several times take the meter back to the electronics bench. Pushing pulling small tubes on/off barbed fittings is a quick way to over stretch hoses and some of then are so "grippy" they have to be cut off. Compression fittings can be used in a few situations but even those don't like repeated connects and disconnects without damaging the tubing.

    Somewhere under the house some 4 years ago I remember stashing away a 20L tub full of small tubing fittings during a men's shed raid on a local large hospital that was being decommissioned. The tub had half a dozen old school sphygmomanometers in it plus a bunch of tubing fittings and sphygmomanometer parts. At the time I left the 4 working sphygmomanometers at the mens shed and took the two two broken ones plus all the fittings home to sort through and never got around to it.

    Well I found the tub this morning and amongst the parts there was a heap of metal tube push fittings, tubing, seals and gaskets, and about a dozen lovely little chrome plated taps I could have used when I set my system up. But the most useful things were a handful of these Al tapered fit-quick connect fittings used to connect the sphygmomanometer bulb to the cuff. These are just perfect as they easily hold the pressures I am working at and are very quick to connect and disconnect.


    Quickfitstapers.jpg

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