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Can an Ultrasonic wave penetrate a LPG cylinder

  1. Aug 13, 2015 #1
    Hey guys, First time poster here.
    Presently I'm doing a project to measure the levels of LPG remaining in an LPG cylinder. I'm currently trying out ultrasonic sensor that primarily works as a distance sensor(HCSR04). As far as I'm aware, the lower the frequency higher the penetration of the wave hence I'm using the 40KHz sensor. But I'm not getting any results and I'm not technically adept enough to measure the second echo either. It would be helpful if you guys could help me out with the necessary details regarding my problem. Alternate suggestions to my problem are also welcome
     
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  3. Aug 13, 2015 #2

    NascentOxygen

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    Hi darklord27. :welcome:

    "not getting any results" is a rather sweeping statement. Can you elaborate, and describe what you have been attempting?
     
  4. Aug 13, 2015 #3
    Okay, The sensor I'm using is an HCSR04 ultrasonic distance sensor that uses a 40KHz wave. I've been trying to get it penetrate the Cylinder by placing it on its surface first. The echo returns back in no time. I've been thinking of using an impedance matching agent such as a fine gel but there is a good possible chance of damaging the sensor. I'm having doubts on using the sensor as such since it is made to measure distance i.e detects only the first echo. The things I need to know are.

    1. Is it possible to penetrate the Cylinder using ultrasound sensors?
    2. If so, what frequency and bandwidth should I use?
    3. Brief Idea about the complexity of the circuitry required.
    4.Any alternate methods that i could refer to for my purpose if this isn't the way to go forward.

    The questions are many and my knowledge about what I'm into is low. Please bare with me .

    Regards.
     
  5. Aug 13, 2015 #4

    davenn

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  6. Aug 13, 2015 #5

    davenn

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    This isn't my field of expertise,( I come to it via my activities in the past with seismic surveying using sound)

    I suspect that the majority of the sound energy will be reflected back to the sensor from the inner metal surface as shown in the first 2 images on that page and any energy that does actually penetrate into the LPG gas/air space possibly wont have have enough energy to re-penetrate the gas metal boundary and make it out bto the sensor ?

    definitely worthy of some experimentation

    just my initial thoughts :smile:

    Dave
     
    Last edited: Aug 13, 2015
  7. Aug 13, 2015 #6

    anorlunda

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    Have you considered measuring the change of sonic reflectivity of the tank wall above the liquid level compared to below liquid level?

    I tap the side of my LPG tank to make it ring like a bell. The quality of the bell sound changes when I tap below the liquid level.
     
  8. Aug 13, 2015 #7

    rbelli1

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    Or you could try from the bottom of the tank. You would still have to detect multiple reflections as you get a reflection from each surface the wave hits but you don't have to deal with a gas.

    BoB
     
  9. Aug 13, 2015 #8

    anorlunda

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    Or you could weigh the tank.
     
  10. Aug 14, 2015 #9
    I've tinkered with those modules...

    I think it's unlikely that you'll be successful using that sensor. It's strictly for non-contact applications in air. You've correctly surmised that that sharp impedance change between the air and steel is reflecting effectively all of the ultrasonic energy and preventing the sensor to "see" the liquid level inside.

    But it might be fun to try to modify it. I actually have one of those (about 10ft away from where I'm typing this). I don't see any obstacles to careful cutting away the protective cowling and sticking the transducer faces directly onto the tank.

    However, the speed of sound through steel is so much faster than it would be through liquid or gaseous propane that the sound wave might not exit the steel into the inside of the cavity or re-enter the steel sidewall after bouncing around. For that matter, the return signal may be dominated by the original wave propagating around the circumference of the tank and back to the sensor.

    I did a google search for "ultrasonic tank level measurement" and found that all of the commercial offerings (on the first page, I admit I didn't look hard). are meant to be installed on the insides of tanks. There is probably a good reason why this is.
     
  11. Aug 14, 2015 #10
    Weight is the easiest method I agree, but I was looking for a non invasive tech that doesn't use weight as the principal measuring meter.
     
  12. Aug 14, 2015 #11
    I will try that out. Thanks. Do you know about any sensor or application that could help measure sonic reflectivity?
     
  13. Aug 14, 2015 #12

    Nidum

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    Use a transmission method rather than a reflection method .

    Introduce ultrasound on one end of a diametral line and sense signal at other end .

    Signal detected after passing through liquid will be stronger than signal after passing through gas .May also be detectable different phase shifts compared to energising signal .

    Several ways of making a practical version . For a portable device possibly a caliper with source and detector at ends of arms . Used by manually applying device at incremental heights from bottom of cylinder .
     
  14. Aug 14, 2015 #13

    Baluncore

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    You need to measure acoustic reflectivity from inner surface of the cylinder wall. That will require a contact sensor that you can slide up the wall.

    Lower technology is usually better. I pour a litre of boiling water over the side of the cylinder. As the water dries, the internal liquid level becomes visible as a distinct line. That is due to the difference in heat transfer at the liquid level inside the cylinder.
     
  15. Aug 14, 2015 #14
    What about a thermal sensor?

    Apply a little heat. The temperature of the steel should change differently depending on what's behind it. You might use a thermal camera?
     
  16. Aug 14, 2015 #15
    Power constrains are low. I did think about a thermal principles and tried to apply the hot water through the sides in an engineering perspective. Thermal sensors and heating etc would require good amounts of power and it seems to be impractical at this point.
     
  17. Aug 14, 2015 #16

    anorlunda

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    Are we talking about a little tank, 10 kg? or a big one 1000 kg?

    If it is big, then I return to the simple idea of weight. You could install a strain guage (also called load cell) on one of the supporting legs of the tank, To calibrate, you only need to know the sensor reading tank full, and again tank empty, then do a linear interpolation.

    I'm a big fan of KISS in engineering. As Baluncore said, low tech is better.
     
  18. Aug 14, 2015 #17

    SteamKing

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    It sounds like (sorry for the pun) all you've been doing is measuring the wall thickness of the gas cylinder with your sensor. The sound travels from the transducer into the metal and is reflected immediately back to the sensor from the interface (back wall) between the cylinder wall and the interior of the cylinder, which is why you get the instantaneous return.

    Ultrasonic thickness gaging is an NDT way of measuring metal thickness for a number of different applications. I've used it, for example, to gauge the plating thickness on floating vessels, to tell how thick the plating is. It's used all the time in the marine industry to determine which plates need to be replaced because their thickness has been reduced below safe levels by corrosion. I'm pretty sure this method is also used for shore-side applications.

    It's possible your particular sensor is calibrated to pick up the echo from the back wall and to ignore any other sound reflections. You might have to hook up the receiver to an oscilloscope to see if you are obtaining multiple reflection returns from inside the cylinder which would be spaced out and return at different times.

    In any event, you'll need some advice from a true expert, someone who knows NDT and ultrasonics.
     
  19. Aug 14, 2015 #18
    I was thinking more of a 5W strip up the side, then viewing the way the heat dispersed. You shouldn't need a lot of power, but you will need an expensive camera.

    Applying a lot of heat all over (like a water bath) would make it difficult to watch the heat flow.
     
  20. Aug 14, 2015 #19
    If you fed a single sync pulse from the bottom you could range gate away the first transition reflection. That signal would continue to reverberate (reflect back and forth off both sides of the steel) with the strength falling off with time. This might swamp the signal from the liquid/gas interface, but it might not. I've used this technique successfully with UWB radar, but that doesn't work well through metal.
     
  21. Aug 14, 2015 #20

    sophiecentaur

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    Going in from the floor of the tank would allow better matching and coupling into the liquid than into gas through the steel. I don't know how directional the beam that gets into the liquid would be and you could expect significant echos from the sides of the tank unless the liquid level is less than the radius of the tank (i.e. near empty`)
    Perhaps a system using transmission rather than reflection would work. The shortest delay would correspond to the direct path from bottom to top and the delay value would be least with a full tank and most with an empty tank (relative speeds in gas and liquid).
     
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