Determine absorption spectrum of body

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.

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

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

Bruce

Well, you *could* start a religion...

John

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

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

What does "unique energy signature" mean?

John

They're just pulling your chain, Bruce.

This guy thinks that some time-varying electrical gradients appled to people can be harmful:

This guy thinks that sensing the resulting magnetic field is the way to go:

--Winston

Hey, that kills a lot of people every year.

John

Yes, very funny. Thank you John.

--Winston

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

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.

--
Teodor Väänänen            | Don't meddle in the affairs of wizards,
teostupiditydor@algonet.se | for you are good and crunchy with
Remove stupidity to reply  | ketchup.

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

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.

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

Like I said, idiot, purely resistive.

You lose!

On Sat, 23 Apr 2011 12:27:06 -0500, "Shaun" wrote: ...

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.

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

Thanks, Rich

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

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

Shaun