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Thread: Acoustic nitrox analyser

  1. #1
    RBW Member pegcjs will become famous soon enough pegcjs will become famous soon enough pegcjs will become famous soon enough pegcjs's Avatar
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    Drager Ray

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    Acoustic nitrox analyser

    Hi folks,
    just got a few minutes to start this off. I've recently been playing with using gas thermometry as a method for gas analysis - basically the method currently used for helium sensing in trimix, but in this case to measure oxygen content in a nitrox rebreather. Built and tested a sensor which I've published in IEEE sensors (link to journal). Abstract replicated here:

    "Current practice in both sport and commercial diving promotes the use of oxygen enriched air 'Nitrox' to reduce risk of decompression injuries. Most gas analyzers used to prepare and monitor these gas concentrations utilize electrogalvanic (EG) cells which require repeated calibration, are highly moisture sensitive and have a limited lifespan. Here we report the design, construction, and evaluation of a new sensor based on the acoustic properties of different Nitrox mixtures. This sensor has no consumable chemicals, works well in all humidity levels and responds more quickly than EG cells. "

    I'm thinking of building a couple of test boxes that would allow these sensors to be incorporated in an RB loop to get some data on their behaviour in a actual diving. So far I've only tested in a semi closed unit in the lab (not underwater). Anyone interested in taking one of these for a few dives in a CCR to see if it works ? If there's any interest I'll try to get a couple of units built to lend out for testing.
    If anyone would like a copy of the paper PM me.

  2. #2
    RBW Member Danseur is an unknown quantity at this point Danseur's Avatar
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    Re: Acoustic nitrox analyser

    I was wondering why someone hadn't done exactly this!

    Well Done and Thanks!

  3. #3
    RBW Member ericr is an unknown quantity at this point ericr's Avatar
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    Re: Acoustic nitrox analyser

    Hi - I'd be very interested in a copy of the paper.
    Best Regards
    Eric

    Quote Originally Posted by pegcjs  View Original Post
    Hi folks,
    just got a few minutes to start this off. I've recently been playing with using gas thermometry as a method for gas analysis - basically the method currently used for helium sensing in trimix, but in this case to measure oxygen content in a nitrox rebreather. Built and tested a sensor which I've published in IEEE sensors (link to journal). Abstract replicated here:

    "Current practice in both sport and commercial diving promotes the use of oxygen enriched air 'Nitrox' to reduce risk of decompression injuries. Most gas analyzers used to prepare and monitor these gas concentrations utilize electrogalvanic (EG) cells which require repeated calibration, are highly moisture sensitive and have a limited lifespan. Here we report the design, construction, and evaluation of a new sensor based on the acoustic properties of different Nitrox mixtures. This sensor has no consumable chemicals, works well in all humidity levels and responds more quickly than EG cells. "

    I'm thinking of building a couple of test boxes that would allow these sensors to be incorporated in an RB loop to get some data on their behaviour in a actual diving. So far I've only tested in a semi closed unit in the lab (not underwater). Anyone interested in taking one of these for a few dives in a CCR to see if it works ? If there's any interest I'll try to get a couple of units built to lend out for testing.
    If anyone would like a copy of the paper PM me.

  4. #4
    RBW Member RZEP17 will become famous soon enough RZEP17 will become famous soon enough RZEP17 will become famous soon enough RZEP17 will become famous soon enough RZEP17's Avatar
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    Re: Acoustic nitrox analyser

    Hi,
    Very intresting!
    I'd love to have a copy of the paper.
    rgrds
    Tomek

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    RBW Member anarchista is an unknown quantity at this point anarchista's Avatar
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    Re: Acoustic nitrox analyser

    Quote Originally Posted by pegcjs  View Original Post
    I'm thinking of building a couple of test boxes that would allow these sensors to be incorporated in an Rebreather loop to get some data on their behaviour in a actual diving. So far I've only tested in a semi closed unit in the lab (not underwater). Anyone interested in taking one of these for a few dives in a CCR to see if it works ? If there's any interest I'll try to get a couple of units built to lend out for testing.
    If anyone would like a copy of the paper PM me.
    I have the material evaluation measurements of oxygen in the Nitrox, the oxygen in the heliox. Based on measuring the thermal conductivity meter. The material is a scientific notebooks Naval Academy in Poland. An analysis of their usefulness in protecting decompression chambers are discussed measures of thermal conductivity cell and paramagnetic properties of oxygen. Both types of meters measure the binary mixture well, and are suitable for quasi-binary mixtures (oxygen and nitrogen have very similar properties). Scientific Papers ZN-89 p 135-146.
    Meters to measure ultrasonic helium were developed by Victor Bolek and Technical University of Wroclaw. Even were used at CERN for the determination of helium on the premises with the installation of helium. Czechs use this option aided measurements. in latest CCR.
    http://www.rebreatherworld.com/showthread.php?t=42586
    There is already a little of this type of ideas in the development of the CCR.

    rc greet
    Last edited by anarchista; 16th May 2012 at 19:55. Reason: addition to an entry

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    RBW Member Bottom Dollar Diver is an unknown quantity at this point Bottom Dollar Diver's Avatar
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    Re: Acoustic nitrox analyser

    Hi,
    I am thinking about working on this issue also. You used the term "acoustic" but you then refered to thermal properties. As far as I understand, this is two different methods of doing the same thing. The acoustic method involves measuring sound velocity in the mix while the thermal method involves measuring the heat transfer properties. Both work well with helium because of the big difference in properties between He and air. Not so easy between O2 and air or N and O2.

    I like the acoustic method since it may be less affected by the very wide range of temperatures that diving involves. Less problem with calibration.

    Can you explain what you are doing exactly. It sounds very interesting.

    Mark

  7. #7
    RBW Member anarchista is an unknown quantity at this point anarchista's Avatar
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    Re: Acoustic nitrox analyser

    Quote Originally Posted by Bottom Dollar Diver  View Original Post
    Both work well with helium because of the big difference in properties between He and air. Not so easy between O2 and air or N and O2.

    I like the acoustic method since it may be less affected by the very wide range of temperatures that diving involves. Less problem with calibration.
    It is easy to measure binary mixtures, helium / oxygen and nitrogen / oxygen.
    The problem occurs with mixtures of TMX.
    Additionally, it all depends on the pressure, which is also required measurement of this parameter to calculate ppO2.

    Therefore, the Czechs program the breathing mixture composition, so that extra information can bring ppO2, but less precise. (The effects of decompressing and speed of off-gassing of the various components TMX)

    rc greet

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    RBW Member pegcjs will become famous soon enough pegcjs will become famous soon enough pegcjs will become famous soon enough pegcjs's Avatar
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    Drager Ray

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    Re: Acoustic nitrox analyser

    Hi folks,
    several people have asked for some clarifcation of this so I'll try to summarise. Best if you are interested just PM me for the paper.
    Basically the speed of sound in a gas outside of the van de walls limit (ie well above the liquid - gas phase transition, is exactly equal to:

    v=sqrt(RTg/M)

    where v is the sound velocity, R is the moplar gas constant (8.314 J/K/mol), g is the specific heat ratio (almost exactly 1.4 for O2 and 1.404 for N2) and T is the thermodynamic (absolute) temperature in K. M is the mean molar mass of the gas under consideration. for pure N2 this is 28.013g/mol and pure O2 its 31.9988 g/mol.

    These data mean that for pure O2 at 300K we have 330.3m/s whilst pure N2 is 353.1m/s. - ie the whole range for 0-100% o2 is a net change of ~6%. Basically this method is weighing the average gas molecule.

    the formula shows a significant issue - this requires very accurate knowledge of the thermodynamic temperature to get a reliable measurement of M.

    To enable the measurement I've built a little resonator tube whose frequency is determined by the speed of sound and its length. It's 14.7mm long and 27mm in diameter I'm capping this tube with two piezoelectric speakers of the sort used in mobile phones etc. halfway down the tube I've got two holes to allow the gas under test to enter and leave the tube.

    I'll try to put some pictures up on this when I can find the time.


    The longitudinal resonance of this device for normal air comes to about 11.5kHz with a 10% range over the whole range. This is a nice freqeuecny in terms of electornics but annoying as you can hear it - a shorter tube would be better in this case but otherwise should work just the same.

    I read out the sensor by scanning the drive frequency of one of the piezos whilst reading out the amplitude and phase of the signal arriving at the other. Using typical lab instruments I can get a frequency resolution of better than 1/10 Hz.

    I'll put some pics up later.

  9. #9
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    rEvo II Mini Hybrid

    Re: Acoustic nitrox analyser

    What about variable humidity?

  10. #10
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    Re: Acoustic nitrox analyser

    It sounds interesting, but please bear in mind that a for a Nitrox-dive in the south the loop consists of:

    P_Tot = P_Watervapor + P_N2 +P_O2 + P_Ar + (P_He)

    The Argon arrives when you use poor oxygen supply made from PSA towers (Zeolite).
    So you meassurements have to determined theese 4 variables.
    And makes that 5 when you add helium to the mix...

    I woul dlove to have two different methods of PPO-meassurements. Right now we are all making do, with normal cells, but having two different methods woudl be interesting.

    Hanssing.

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