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Determine absorption spectrum of body

Discussion in 'Electronic Design' started by Bruce Epstein, Apr 22, 2011.

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  1. I would like to experimentally determine the absorption spectrum of a
    human body by applying low current white noise, via skin electrodes,
    at one anatomical location and monitoring at another.

    Through subtractive mixing, one might then obtain a unique energy
    signature for that individual. IOW the effect of non-linearities and
    resonance.

    Is this worth trying?

    If so, what are the relevant technical considerations?

    Bruce Epstein
     
  2. mike

    mike Guest

    What are you gonna do with the answer?
    If you have a specific objective, you have a chance of
    determining what effects to consider.
    If it's just "hey look what I can do", the technical
    issues are irrelevant.

    I think you're gonna find that the connectivity issues
    overshadow everything else.
     
  3. Rich Grise

    Rich Grise Guest

    No. It's been done and done; it's called "galvanic skin resistance," and
    it's been used in polygraph "lie detectors" for decades.

    They've even tried using RF to measure the "body mass index," i.e., what
    proportion of fat vs. muscle comprise your body, and IIRC it was quite a
    flop.

    Save your money.

    Cheers!
    Rich
     

  4. Yes, I am aware of these. Neither are the concept I described in my
    OP.

    Bruce
     

  5. So far, everyone has missed the point. What I want to do is not a
    passive system like GSR and skin conductance. It has nothing to do
    with monitoring psychological states.

    Please re-read the OP.

    Bruce
     
  6. Winston

    Winston Guest

    They're just pulling your chain, Bruce.

    This guy thinks that some time-varying electrical gradients
    appled to people can be harmful:
    <http://lofi.forum.physorg.com/Resonant-Response-Of-The-Human-Nervous-System_15393.html>

    This guy thinks that sensing the resulting magnetic field is the
    way to go:
    <http://www.sciencedirect.com/scienc...c90b01e105f35d84bdecf13e20861d20&searchtype=a>


    --Winston
     
  7. Winston

    Winston Guest

    Yes, very funny. Thank you John.

    --Winston
     
  8. Just my 0.02€ worth, but I would think that anything outside AF (e.g.
    20Hz-20kHz) would be an exercise in futility. However, having used
    myself as a conductor for audio signals during my experimentation in the
    past, I have this recollection that the human body will have different
    attenuation at different frequencies, as well as different phase shifts
    (my hunch is mostly capacitive (i.e. I leads U)).

    Whether or not that attenuation and phase diff (versus f) is fixed and
    only dependent on electrode positions, or if certain characteristics of
    the human body (e.g. Salinity, over all H2O content, etc) can cause it
    to change is an open question.

    As always, usual safety protocol applies, battery powered equipment
    only, low signal voltages and pay attention to electrode placement.

    /Teo.
     
  9. I'd say that we are purely resistive. No phase shifts. No inductance.
    No capacitance. Sheesh.
     
  10. Brian

    Brian Guest

    Has been defined in the Human Body Model for various product
    safety and EMC standards, and is ranges from 100pF series and
    1500ohm parallel for ESD, to 1.5nF and 1k for medical equipment
    touch to ground. For leakage current, the measurement is
    required to be made using an instrument with a 1MHz bw, so the
    phase will probably vary.

    Based on my anecdotal data (data from males, aged 28 to 59),
    measurements for HBM leakage and touch current indicate contact
    and body mass capacitance to be between 7pF and 85pF.
     
  11. Shaun

    Shaun Guest

    Archimedes Lever,

    You a fucking idiot. The Human body is modeled by resistors and capacitors.
    The measured signal from a white noise source applied to the human body will
    change with frequecy.



    Once again YOU'RE WRONG!

    Shaun
     
  12. Shaun

    Shaun Guest

    If you want to avoid just measuring skin resistance the most important
    thing to do is to use a four-terminal measurement technique so that
    you are not measuring through the same electrodes that drive the test
    signal into the body. Even then it will be difficult to avoid the
    results being dominated by electrode effects. At low frequencies a
    standard approach is to use "reversible" silver electrodes coated with
    silver chloride and ensure that they make good skin contact by
    abrading the skin and then applying "electrode gel" which contains
    salt solution. This reduces non-linearities at the electrodes which
    will undoubtedly dominate over any nonlinearities which might be
    present within the body. As for resonances, there will obviously be
    some related to the lengths of the limbs and the connecting wires.

    You have not indicated what frequency range you are interrested in -
    once the frequency exceeds a few MHz, conventionally attached
    electrodes will tend to give meaningless results and alternative
    methods such as coaxial electrodes would be needed. At this point a
    vector network analyser might be the test instrument of choice.

    However, I would be very surprised if after all your efforts you
    discover anything that could not be modelled by a bag of salt water.

    John


    Another factor that you'll have to take into account is that we act as an
    antenna. Have you ever touched the input of an audio amp, you get a 60hz
    and harmonic buzz from the speakers.

    Shaun
     
  13. Do you make your measurements within an anechoic chamber? (not that
    that would be the only particular you've missed)

    I'd say that you are not only wrong, but more likely do not even know
    how to find out any real numbers properly.
     

  14. Like I said, idiot, purely resistive.

    You lose!
     
  15. Jamie

    Jamie Guest

    Hmm, Human body, white noise, I see a comment there.

    Jamie
     
  16. Grant

    Grant Guest

    I don't suppose you could fix your news agent to quote properly?

    As far as the topic goes, OP didn't state any meaningful goal I could see.

    Silliest thing to come out in the last decade is that engineers finally
    solved the high schoolers' wet dream of seeing through clothes ;)

    There's a standard body hookup to read the heart signals, where one leg
    is used as the reference (I read that in a datasheet for body pickup
    amplifiers), OP needs to survey existing techniques and then decide what
    they're really looking for.

    Grant.
     
  17. Shaun

    Shaun Guest


    You've missed everything including an education dimbulb. I'm a Biomedical
    Technologist and have work in the field for 11 years, I know what I am
    talking about, you're just a fucking idiot who is always wrong.


    The electrodes are a resistive connection and are also capacitive. 10Kohms
    resistance and about 4 picofarads of capacitance. I can't find capacitance
    values for the Human body right now, but resistance is somewhere between 500
    ohms to 1000 ohms. Now if you use large paddles of a defibrillator, the
    resistance to the inside of the body drops down to about 50 ohms. If you
    have a break in the skin where the electrode is, the resistance will also
    drop. As for capacitance, it goes layer by layer throught the body. Each
    blood vessel is going to present a resistance and a capacitance to the fluid
    inside the blood vessel, same thing with muscle, organs and so forth. Each
    layer will have capacitance from outside to inside and resistance through
    it.

    Shaun
     
  18. Rich Grise

    Rich Grise Guest

    Is that really being an "antenna," or is it more like being one plate of a
    capacitor?

    Thanks,
    Rich
     

  19. To be more specific, let's say I feed a pre-recorded audio white noise
    track to a step-up transformer, output applied at a safe level via two
    skin electrodes.

    From two other electrodes I record the output. This is processed in
    software back to the same amplitude as the orginal noise signal.

    One is then inverted and mixed with the other to provide a difference
    signal.

    My interest is to see how this changes under varying conditions,
    elctrode placement, etc.

    Any further comments regarding the practicalities of this would be
    appreciated.

    Bruce
     
  20. Shaun

    Shaun Guest

    You're probably right. I guess it would be capacitive pickup. Antenna is
    the wrong term.

    Shaun
     
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