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Determine absorption spectrum of body

B

Bruce Epstein

Jan 1, 1970
0
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
 
M

mike

Jan 1, 1970
0
Bruce said:
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

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.
 
R

Rich Grise

Jan 1, 1970
0
Bruce said:
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?
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
 
B

Bruce Epstein

Jan 1, 1970
0
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.


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

Bruce
 
W

Winston

Jan 1, 1970
0
Bruce said:
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.

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
 
T

Teodor Väänänen

Jan 1, 1970
0
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.

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.
 
A

Archimedes' Lever

Jan 1, 1970
0
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.

I'd say that we are purely resistive. No phase shifts. No inductance.
No capacitance. Sheesh.
 
B

Brian

Jan 1, 1970
0
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.
 
S

Shaun

Jan 1, 1970
0
Archimedes' Lever said:
I'd say that we are purely resistive. No phase shifts. No inductance.
No capacitance. Sheesh.

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
 
S

Shaun

Jan 1, 1970
0
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

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
 
A

Archimedes' Lever

Jan 1, 1970
0
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

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.
 
A

Archimedes' Lever

Jan 1, 1970
0
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.


Like I said, idiot, purely resistive.

You lose!
 
J

Jamie

Jan 1, 1970
0
Shaun said:
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!

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

Jamie
 
G

Grant

Jan 1, 1970
0
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.

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.
 
S

Shaun

Jan 1, 1970
0
Archimedes' Lever said:
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.


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
 
R

Rich Grise

Jan 1, 1970
0
Shaun said:
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.
Is that really being an "antenna," or is it more like being one plate of a
capacitor?

Thanks,
Rich
 
B

Bruce Epstein

Jan 1, 1970
0
I didn't understand it the first time, so I doubt re-reading would
help much.


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
 
S

Shaun

Jan 1, 1970
0
Rich Grise said:
Is that really being an "antenna," or is it more like being one plate of a
capacitor?

Thanks,
Rich

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

Shaun
 
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