Page 1 of 3 123 LastLast
Results 1 to 15 of 36
  1. #1
    BobL is offline Member: Blue and white apron brigade
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    7,182

    Default Humidity measurements in compressed air

    Warning - Larger Volume of Nerdy science/engineering content.

    I’m posting this here because it's more of a measurement/electonics project than metal work and may interest a (very) small group of like minded folks.
    My health and sleep have been poor lately so I'm restricted in what I can do anywhere including in the shed, Typically I do something for 1/2 an hour in the shed and then have to rest for about the same length of time so I have been undertaking some soft distracting projects and things just take a lot longer than usual.


    Recently I started upgrading my 8 year old 1/2” galv pipe reticulated compressed air system.

    The potential flow restrictions starting at the compressor where the 1/2” BSP tank outlet was connected to the retic line via a 1/4” ID 4 way, brass junction, and several short (300mm) long segments of 1/4” ID SS braided gas hose in line with the 1/2” galv pipe, all have ben replaced with 1/2” BSP fittings and hoses.

    About half of the 20 odd 1/4” outlets and quick release hose Nikko style fittings inside the shed that were corroded/damaged/leaking have being replaced with genuine 1/2” Nitto, “One Touch" fittings.
    A 1/4” hose and a damaged 3/8” hose inside the shed have been replaced with 1/2” hose so I have a couple of hoses with good flow.

    The water/corrosion issue had been mostly eliminated a few years back when I installed an auto vent valve that opens the tank drain under the tank for 1/2s every 45 minutes - I leave my compressor and auto vent valve on most of the time so the vent valve will release any water condensed inside the tank. The effect of the vent valve is easily observable from the amount of water released from the reticulated piping system "low point" where another drain/tap can be opened to release water out from that point. Depending on compressor use and how often that drain was opened, prior to the installation of the auto tank vent valve the amount of water coming from the low point drain would be as much 100 mL whereas after the auto tank vent valve was installed it was rarely more than a few mL. However, when I run the compressor continuously for more than a few minutes the compressor tank does not get a chance to cool down and condense the water and so the auto tank vent valve has very little effect. I had a small inline water trap/filter at the end of one of the piping arms inside the shed and could easily see water appearing in the sight glass. I thought about installing that in the main trunk line but the small size of the inlets and outlets of the small water trap/filter would almost certainly stymie the air flow so what I wanted was something larger.

    While i was thinking about this, literally at my feet was a box of assorted plumbing stuff I had scavenged from a skip at work many moons ago, and the box contained a standard CUNO SS water filter, and it dawned on me that a few $ worth of brass plumbing fittings would enabled me to install it into the compressed air trunk line as shown below.
    filterhousing.jpg

    Instead of hard plumbing the filter/water trap inline I decided to instal it using 1/2” compressor hose and 1/2” BSP and Nitto One Touch fittings so it can be easily installed and removed.
    All up the 4 quick release fittings are not cheap (~$60) but I figured if it all fails I can recover and reuse these fittings elsewhere in the system.

    I’m figuring the CUNO will initially will act more as a water trap than a filter but I though I would insert a standard $4 , 0.5 micron water sediment filter, which I already have a stash of for use on my under sink water filtration system for my coffee machine, into the CUNO. The sediment filter should also prevent pretty well any crap coming from the compressor to the outlets. The intention is to eventually surround the the sediment filter with desiccant to see how much water in can pull from the air.

    Then I thought I would like to know how efficient any of these measures are at removing water from air so I knocked up a measuring device incorporating an in-line humidity sensor.
    SensoronFHousing.jpg

    The sensor (S) is the very widely used relative humidity and temperature sensor known as a DHT22, and is located inside the black plastic 32mm T-piece fitting (SC).
    Using Nitto fittings on the sensor housing means it can be placed anywhere in line although the primary positions would be in the pre and post filter locations.

    A DHT22 clone costs ~$5 and connected and read by a "~$20 all up" box of Arduino based electronics that took me (even at my slow working speed) about a day to assemble and program
    For comparison/reference data I added second sensor and its located outside the shed next to the compressor intake air filters.
    Arduino.jpg



    Because the DHT22 outputs relative humidity (RH), in the case of how much water is in compressed air, rather than RH it is more useful to know the Absolute Humidity (AH) ie mass of water per unit volume of air. It turns out that if you know the temperature of the air then AH can be calculated using a couple of longish formulas which the Arduino micro controller handles easily.

    The screen output shows the Saturation Water Pressure (S) in kPa and the Absolute Humidity (H) in mg/L for both of the DHT22s (I = inline sensor and O = sensor near compressor air intake outside shed)
    Flipping the switch on the top of the box toggles the display to reveal the RHs and Temps for the same sensors.

    The H readings (typically 6-10 mg/L) obtained for outside the shed made sense although they were at times considerably higher than that reported by the BOM - more about that later.
    For the in-line sensor (I) the readings I got were somewhat puzzling (ie much higher) and after a bit of reading I found out that the in line pressure (90 - 135 psi) could screw up the operation of these sensors.
    These sensors operate by measuring the change in the capacitance of a capacitor which absorbs and releases water between the capacitor plates depending on the humidity and the pressure.
    It is possible to make corrections for pressure but it also appears that the high pressures can damage the sensors permanently.

    All this means this sensor cannot operate reliably inline under pressure and so the compressed air has to be be somehow released into a sampling chamber without changing its humidity. The big problem is that decompressing air cools it and as it mixes with air from the atmosphere this all change its humidity.

    The "proper"way to assess the dryness of compressed air is to measure its "Dew Point" ie the temperature at which the water in air starts to condense, the lower the temperature required the less water there is in the air. Half decent Dew point instruments are not cheap and even then the most accurate measurements are not usually done inline but by bleeding a small amount of air (~1 l/min) from a compressed air line using a needle valve and passing that air through a thin warming Cu coil into a small chamber which is effectively at atmospheric pressure but has not mixed with air from the atmosphere.


    I figure if this approach works for dew point it might work for the DHT22 so this is what I am in the middle of setting up. I will post more of this as I build this system up. With the assistance of a mate, except the olives for the coil compression fittings, I have cobbled together all of the parts at minimal cost. It may well all come to naught but it's a low key project that keeps me suitably entertained while I feel as I do.


    Back to the environment of the compressor located in that enclosure in a ~1m wide gap in between the shed and the back fence. The enclosure sits on top of a 600mm high limestone retaining wall up against the shed. Also In that gap is a narrow garden bed with a rampant vine that I hack back using electric hedge trimmers approx every 4 weeks just so I can walk through that gap. The vine grows about 1m above the fence line providing good screening against the difficult neighbour on that side otherwise I would have removed it long ago. It's watered via regular garden retic which soaks pretty well everything in that gap including the limestone retaining wall and because its all so confined there's not much ventilation so it's a more humid microclimate environment than elsewhere in the back yard. This probably explains the higher than usual humidity levels I'm seeing in outside air especially on watering days. To cool the compressor when its running, a small 240V fan in the false ceiling of the enclosure draws cool air from outside the enclosure through a 5mm wide gap under the bottom of the enclosure doors, and pulls the warm air out of the enclosure through the false ceiling. This means the fan is constantly sucking moist air into the enclosure, so all in all not that good an air intake setup for a compressor to begin with.

    However, I thought I might close the gap underneath the doors and install a 100mm PVC snorkel such that the fan will suck drier air from up near the shed roof through the snorkel and into the enclosure. As I already have the PVC and Junctions to do this its not going to cost me more than a bit of time. Even using the current sensor setup I can assess if its worth doing.

    Sorry about the long post. Hope it was not too boring.



  2. #2
    Join Date
    Jul 2011
    Location
    Adelaide
    Posts
    837

    Default

    BobL

    Interested to see where you go with this.

    Eric

  3. #3
    Join Date
    Aug 2008
    Location
    Perth
    Posts
    26

    Default

    Bob, I built with Terry a few years back a similar setup. Except is was measuring the humidity in the gas flow at 3000 psi (207 bar) and 82 deg C. I ran into the same problem of the capacities sensor's characteristics changing due to the 200 x pressure increase that it was designed to operate at. We operated at the full P and T, just had to come up with our own calibration curve with know humidity at P and T steps. On speaking with the swiss manufacturer of the sensor they were impressed that the sensor operated at those conditions and wanted our calibrations.

  4. #4
    BobL is offline Member: Blue and white apron brigade
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    7,182

    Default

    Thanks Ben.
    Yeah sometimes you just have to try things to see if they work.
    Don't happen to have a spare sensor laying around ?

  5. #5
    Join Date
    Aug 2008
    Location
    Perth
    Posts
    26

    Default

    I should have, let me check on Tuesday. Glad to see someone else trying to measure the water content of compressed gasses while still at P

  6. #6
    BobL is offline Member: Blue and white apron brigade
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    7,182

    Default

    Quote Originally Posted by benhoskin View Post
    I should have, let me check on Tuesday. Glad to see someone else trying to measure the water content of compressed gasses while still at P
    Wow! - I was kidding, but it would be interesting to have a crack at it.

  7. #7
    Join Date
    Nov 2017
    Location
    Geelong, Australia
    Age
    57
    Posts
    2,651

    Default

    Interesting project Bob. Thanks for posting and I hope your health is on the improve.

    Steve

  8. #8
    BobL is offline Member: Blue and white apron brigade
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    7,182

    Default

    Thanks Guys,

    Despite my ongoing health and sleep issues I was able to knock up a couple of olives this afternoon and assemble and quickly test and almost complete absolute humidity measuring system for compressed air.

    The olives are about 6mm long and have a max OD of 6.4mm, and while they are a tad on the rough side but I only had ~20mm of stock to work with so not much room to manoeuvre.

    Those on the left (~3.5mm ID) are the ones I already had, whereas mine are the slightly longer pair on the right and fit my 1/8" Cu tubing.
    Olives.jpg
    It turned out that a 1/8" drill bit produced a bore in the olive with a very nice tube fit.
    I then annealed the the olives in a flame and tightened the cap nuts up lightly, then with a spanner until the sound of escaping comp air was below my hearing threshold and then used the detergent and water leak bubble test and about a 1/2 turn was enough to stop the bubbles.

    So here below is the almost complete test cell setup.
    Completesetup.jpg

    Air is currently sampled from either the filter inlet (I) or outlet (O) by switching the braided SS hose to I or O.
    Next week I will purchase some "quick fit" 1/4" plastic pressure tubing fittings and permanently set up a T-piece and pair of small plastic taps in place of the braided hose. Switching between I and O will then just involve switching the plastic taps.

    The sampled air flow is controlled by needle valve 1 (NV1) which leaks a small amount of air (~1L/min) into the ambient temp coil (ATC) which expands and warms the air to ambient T before entering the black plastic-T sensor chamber where the HDT22 sensor makes continuous readings every 2 seconds. Preliminary testing shows the readings take about 1-2 minutes to stabilise. This is because the fresh air has to flush out the old air.

    When NV1 is fully open Needle Valve 2 (NV2) can be used control the flow and allows for humidity measurements to be made under compressed air pressure. A calibration curve could be then constructed for a sensor that is not damaged by high pressures.

    Still to add is a small gas flow meter so that consistent escape flows can be established. The small plastic taps mentioned above will be used to isolate the test cell when it is not being used - ie not using the compressor so the needle valves can be left in the same position.

    Close up of just the test cell/chamber
    ExpansionChamber.jpg
    Apart from the black plastic fittings and the sensor, all other fittings, valves, the hose, coil etc for the test cell have been scrounged so it has been a low cost project.
    The most expensive single items are going to be those little 1/4" plastic taps I mentioned above so the gas can be sampled alternately the I and O sides of the filter.
    It would have been cheaper but way more clunky to use 1/2" BSP brass taps.

    With a flow rate of 1L/min this setup only uses 60L/hr which is not a lot but I will only operate it when I want to check the moisture levels to see if the filter needs attention.

    I have not taken a lot of measurements but below (I) is sample reading from the outlet of the filter 3mg/L and O was the current reading inside the compressor enclosure 8.3 mg/L.
    It is reassuring that the moisture content of air in the lines is lower than the outside air.
    This is probably just the effect of the compressor tank auto vent valve as I don't expect the large filter to contribute much at this stage. I also expect the results to improve as the insides of the test cell dry out. A SS test cell would have been better and I may make that one day, I just wanted to test out if it works..

    Display.jpg

    More testing over the next week or so.

  9. #9
    BobL is offline Member: Blue and white apron brigade
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    7,182

    Default

    WARNING: More nerdy stuff.

    I have been thinking some more about this and would like to add some comments/info about the following.

    1) Calibration under pressure

    As I said above the twin needle valve setup I show above enables measurements to be done under pressure. This (as Benhoskin states) assumes the sensor will operate under pressure as it may not.

    There are two possibilities for measurement under pressure,
    a) sample the compressed air by venting a small amount to atmosphere but measure on the pressure side of a fully open NV1, and control flow with NV2,
    and
    b) close the sample loop and return the gas to the pressure side.

    So, if you can do this measurement at atmosphere why would you bother with measurements under pressure? There are many reasons for doing this including the value and toxicity of the gas, or as maybe in Ben's case the environmental measurement conditions are restrictive. None of these really apply to compressed air but if the measurements were performed under pressure it would be possible to eventually not lose the sample compressed air from the system ie make it a close loop system.

    To do this the sensor has to first be calibrated under pressure, so the first thing to do is make a series of Absolute Humidity (AH) measurements at the same compressed air line pressure. ie measure AH at atmosphere and then AH under pressure. These are recorded along with the compressed air line pressure. The air line pressure is then systemically changed (ie start at full compressor pressure and release some air from the tank) and further calibration readings taken. For a compressor with a switching range of say 90 to 120 psi you might calibrate from 90 to 120 psi in 5 psi steps. From these readings, calibration function/curves or lookup tables showing "atmospheric AH" values for given "pressurised AH readings" at a specific air line pressure can be generated.

    Then all subsequent readings can be performed under pressure and the function can be programmed direct into a micro controller and pressurised readings corrected and displayed. To do this automatically would require using a pressure sensor hooked up to the microcontroller. Such sensors can be had for ~$10 and I have ordered a couple to have a play with.

    A much simpler but laborious way would be to take any pressurised AH reading and lookup the corresponding atmospheric AH values for a given air line pressure from the graph or table described above. The latter will not be very fine grained but they might not need to be especially give the humidity readout tolerance of the HDT22 is ~+/-2%.

    Then the small volume of sampled air under pressure can be draw from a T-piece in the line, run past the sensor and return it to the main line. there are a few tricks needed to get the air flow happening properly but it's not that hard to do. The sensor could then be left on all the time and not lose any compressed. Whether I go this way or not will depend on how bored I get not being able to do much in the coming months.

    2) Measurements using a consumer level weather station style sensor (ie no micro controller)

    What about if you don't want to or unable to deal with micro controllers? There are plenty of inexpensive consumer level wether stations around that measure relative humidity and temperature that might be able to be used. You would need to be able to maybe removed the sensor from the weather station so that the sensor can be encloses inside the sampling chamber and in the first instance operate the sample chamber only at atmosphere. Assuming this is all possible, how to you convert an RH reading into to AH.

    Well here is a conversion graph and table you could use.
    Screen Shot 2019-01-28 at 6.13.55 am.png

    Screen Shot 2019-01-28 at 6.19.40 am.png

    The way to use the graph is to select the closest RH curve eg 50% (pale blue) and then proceed along that curve until you are above the temperature the RH reading was taken (eg 25ΊC) and then scoot across to the Y axis and and read about 10 mg/L - the table says 11.5 mg/L - I can provide the equations or finer grained charts/tables/graphs if you need them.
    Remember this is for atmospheric pressure readings. If you want to do this under pressure calibrating it will be a PITA but it is possible but you will need a number of conversion charts/tables/graphs to cover all the air line pressures.

    3) Wifi/remote connects
    If you decide to use multiple sensors and want to avoid cables between theHDT sensors and the micro controller from cluttering up your workshop then the HDT22's + a mini-micro controller plus a wifi module are cheap, play nicely together and are not that hard to setup so that several could be used feeding back to a master micro controller. The units will still need power so being close to a GPO will be handy.

    Something else to waste a shed lot of time on when I get bored, ie unlikely.

  10. #10
    BobL is offline Member: Blue and white apron brigade
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    7,182

    Default

    Quote Originally Posted by BobL View Post
    Back to the environment of the compressor located in an enclosure with a small 240V fan in the false ceiling of the enclosure draws cool air from outside the enclosure through a 5mm wide gap under the bottom of the enclosure doors, and also pulls the warm air out of the enclosure through the false ceiling. This means the fan is constantly sucking most air into the chamber, so all in all not that good an air intake setup for a compressor to begin with.

    However I thought I might close the gap underneath the doors and install a 100mm PVC snorkel such that the fan will suck drier air from up near the shed roof through the snorkel and into the enclosure. As I already have the PVC and Junctions to do this its not going to cost me more than a bit of time. Even using the current sensor setup I can assess if its worth doing.
    This morning I did some measurements of the absolute humidity of the air on the limestone floor of the enclosure where the air enters through a gap under the enclosure door and compared those measurements to some taken at a height of ~ 1.5m above the ground outside the enclosure. The "floor air" was consistently between 20 and 25% moister than outside air suggesting a snorkel could be worth considering - I'll make a some more measurements before I decide if it's worth doing.

  11. #11
    BobL is offline Member: Blue and white apron brigade
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    7,182

    Default

    I made some more air moisture measurements during the day (which almost reached 40Ί) inside and outside the compressor enclosure and at compressor floor level and 1.5m above the enclosure.
    Why I found was that apart from the early mornings, the air inside the compressor enclosure consistently contained more water than the air outside the enclosure. Then it dawned on me, of course it does because that's where the EFFIN compressor tank auto drain valve dumps its water. So it appears I have been repeatedly partially recycling water through the compressor with this setup. IE, compressor sucks in air, vents tank water onto the limestone floor of the container, and then water evaporates and fills up the enclosure with moist air, so that it can go through the compressor again.
    DANG!


    What I need to do is add a short length of tubing to the auto vent drain so that it vents outside the enclosure, but then also don't draw air into the enclosure from down at ground level but through a snorkel with its external opening well above the ground. Finally I need to seal the limestone floor of the enclosure to prevent moisture wicking back inside the enclosure from rain and garden retic.

    Then to see what would happen I ran the garden retic n for the usual 10 minutes waited for another 10 minutes and got the enclosure vent fan running. As to be expected the effect was quite dramatic as this raised the air moisture content inside the compressor enclosure to about 50% above what was already in the enclosure. Even at 1.5m above the ground there was easily a small but detectable increase in the MC - might have to use a taller snorkel.

  12. #12
    BobL is offline Member: Blue and white apron brigade
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    7,182

    Default

    Fixed up a few compressor enclosure things.

    Added a drain hose so the compressor Auto Drain Valve (ADV) now vents outside the compressor enclosure.
    The water droplets in the hose is the amount of water that stays behind after about 15 discharges.
    It discharges for 0.5s every 45 minutes.
    Arrow "A" shows how the air got into the enclosure - this gap has been sealed up with some foam rubber.
    Drain.jpg

    Then I added a snorkel.

    This shows the amount of space between the shed and back neighbours fence and the compressor enclosure and the rampant vine that grows up the back fence.
    A 100mm 240V fan located in the false roof pulls air into the enclosure through the snorkel which is just visible above the enclosure roof.
    The venting fan runs continuously when the compressor is running and for a programmable time eg 10 minutes minutes after the compressor turns off to help the compressor and motor cool off.
    This compressor has no fan or air flow across the cylinders but with the new arrangement it has.
    IMG_3535.jpg

    View from other side of the enclosure
    It's just some 100mm PVC DWV pipe offcuts and 90Ί bends I had laying around under the house.
    The flared end is a "seconds" from the ones I make for woody dust collection.
    Snorkel1.jpg

    And heres what it looks like inside the enclosure the inlet comes out quite close to one of compressor filter banks.
    "S" contains a DHT22 sensor permanently located inside the enclosure.
    inlet2.jpg

    Right now I've left the vent fan running to dry out the damp floor of the enclosure.

    I was a bit concerned that the snorkel might release too much noise from the enclosure but it is a comparatively quiet (twin belt driven) compressor anyway.
    Sound pressure level wise, with a basic urban background of 54dB (ie compressor off) the SPL with the compressor running is 56dB at the top of fence line.
    Immediately in front of the flared air intake inlet above the enclosure roof it is 75dB but it rapidly subsides as the meter is moved away from the opening.

    It will take some time to dry that enclosure floor out so until that happens I can't really tell if this has been worth the effort.

  13. #13
    BobL is offline Member: Blue and white apron brigade
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    7,182

    Default

    Just picked up 2.5kg of free Silica gel crystals from a mate.
    Sigel.jpg
    They're a mix of orange and dark red. The orange are indicator crystals and turn green when saturated. Guess what's going into the filter

  14. #14
    BobL is offline Member: Blue and white apron brigade
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    7,182

    Default

    As I said post #8 to assess how effective any filter is I need to be able to compare the AH pre and post the filter so I need a way to sample a small amounts of air pre and post the filer and divert the these to the sensor.
    I ordered some 1/4" quick fit pressure connections online but decided not to wait so had a bit of a look locally and found that Blackwoods prices were almost comparable so went and picked them up this morning.

    So this is what it currently looks like.
    The quick fit setup enables me to easily divert either the inlet or outlet sides of the filter to the sensor.
    It also means I do not need to use the needle valves to shut off the air flow.
    The needle valves can can say in the same position so the I or O always see the same flow.
    Those few quick fit fittings used (3 BSP adapters, two elbows, 2 taps and a T-piece) cost almost as much as the Nitto fittings
    IOsystem.jpg

    By switching the taps back and forth at about 2 minute interval and watching the AH readout on the microcontroller I have not been able to tell if the water sediment filter removes any water. If it does, it is less than ~5%. Tomorrow I might try adding the desiccant.

  15. #15
    Join Date
    Apr 2018
    Location
    Lismore Northern NSW
    Age
    50
    Posts
    81

    Default

    I bet the desiccant makes a big difference

Page 1 of 3 123 LastLast

Similar Threads

  1. Nylon Compressed Air tube
    By glivo in forum METALWORK - Machinery, Equipment, MARKET
    Replies: 0
    Last Post: 2nd Jul 2018, 09:41 AM
  2. Compressed air for swarf/chip clean up
    By jack620 in forum METALWORK GENERAL
    Replies: 34
    Last Post: 6th Aug 2017, 04:30 PM
  3. Replies: 30
    Last Post: 20th Jun 2017, 12:11 PM
  4. Workshop Humidity Control
    By RayG in forum METALWORK GENERAL
    Replies: 10
    Last Post: 12th May 2017, 03:00 PM
  5. compressed aiir powered grease gun
    By BobL in forum METALWORK GENERAL
    Replies: 5
    Last Post: 21st May 2015, 02:02 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •