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I would like to build me own electrical body fact calculator from a microcontroller (the Arduino), but I would like to know how does it work. I have a 5V analog reader, So I guess I sould calculate the current passing through my skin compared to a 0 body fat skin resistance?

Is that it?

Thank you

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    \$\begingroup\$ This is not really a electronics question. I find it difficult to believe that it is possible to get a meaningful measure of body fat from a simple electrical measurement that otherwise doesn't hurt you. Sounds like snake oil to me. Do your research and determine the science behind the measurement and what exactly is to be measured, and we may be able to help with a circuit to accomplish the measurement \$\endgroup\$ – Olin Lathrop May 13 '12 at 17:49
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    \$\begingroup\$ @OlinLathrop it's possible to realize it, and I've seen several commercial instruments. But it's more about knowing the relation than the complexity of the circuit itself. \$\endgroup\$ – clabacchio May 13 '12 at 17:54
  • \$\begingroup\$ How do they work? Not very well. Most of the sites that say they work are trying to sell you one. \$\endgroup\$ – Jeanne Pindar May 13 '12 at 18:45
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    \$\begingroup\$ @JeannePindar it's not true, they work. My mother works in nutrition (in a hospital), and it's one of the instruments of her job. The principle is that fat mass and muscle have different conductivity. \$\endgroup\$ – clabacchio May 13 '12 at 19:58
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    \$\begingroup\$ The term used for these measurements is "bioimpedance" or "bioelectrical impedance analysis" (google for it). Measurements include complex impedance at specific frequencies. Be careful about shocking your victims! \$\endgroup\$ – Juancho May 13 '12 at 20:41
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I have designed cardiac output meters, which is something related to what you are asking, although more complex. Cardiac output meters measure transthoracic electrical impedance, and --knowing that the blood is a good conductor of electricity-- they correlate conductivity with the amount of blood present in the volume between the sets of electrodes, at any given time. They provide as a result a waveform of the instantaneous flow of blood through your heart, in liters/minute. Body fat "meters" provide a rough estimate of the proportion of fat in your body. I'm not very familiar with the latter ones, but I know they also work by measuring electrical impedance.

To measure electrical impedance, you inject an AC current through some portion of the body of a patient. You use two electrodes to inject current, and two electrodes to measure voltage. The amplitude of the AC current is kept constant, and the amplitude of the sensed voltage is proportional to the instantaneous impedance. Typical frequencies used range from 20 kHz to 100 kHz. If the current has to go through your heart, a maximum of 400 \$\mu\$A\$_{rms}\$ is allowed, at 50 kHz. If only the magnitude of Z(t) is needed, best is to use a synchronous demodulator, to extract the amplitude information from the sensed voltage. If phase information is needed, you need more complex methods (like very fast ADCs and FFT).

There is no way that a circuit like this one can be done by an amateur, so forget about this. You need to sense a few microvolts (with a good instrumentation amplifier), and at the cables right next to the sensed voltage you have your excitation signal, spanning several volts. You need a very good schematic, and a better PCB design. I ended up with 4 or 5 ground areas, plus the same number of -5 V and +5 V areas. One of my toughest challenges, but the results were very good.

All this was for electrical bioimpedance in general, and for transthoracic impedance in particular. Knowing that fat is a relatively bad conductor of electricity, body fat estimators must correlate conductivity with amount of fat between the electrodes. I guess they use weight and volume data to help in that correlation, and data from MRI scans to have some absolute references.

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  • \$\begingroup\$ Wow, 4 or 5 ground and power layers? Why is that? Btw, I am guessing that you don't want to limit that current actively right? Or can you trust semiconductors? :) \$\endgroup\$ – abdullah kahraman May 14 '12 at 15:27
  • \$\begingroup\$ @abdullahkahraman I said areas, not layers. The PCB had four layers, but many more areas, connected as stars. \$\endgroup\$ – Telaclavo May 14 '12 at 16:32
  • \$\begingroup\$ An alternating current current? \$\endgroup\$ – Tom Paris May 14 '12 at 18:59
  • \$\begingroup\$ @Telaclavo ah, sorry. Would be interesting to see a design like that though.. \$\endgroup\$ – abdullah kahraman May 15 '12 at 2:52
  • \$\begingroup\$ @TomParis yes, why not? :) \$\endgroup\$ – clabacchio May 15 '12 at 14:32
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ANSWER: Capacitance is Voltage @ F using a current source sine wave is proportional to gap between electrodes or calipers which equates to BMI.

I believe "body mass index" (BMI) measurements are just pinching a gap of fatty skin between calipers or "capacitance measuring electrodes" grabbing onto the excess fat on the surface of the body with a wide smooth jaw. Each jaw is isolated so that the result is between the two conductors. Many readings are averaged to get an Index value.

To replicate this electronically would be to measure the distance between two metal conductors measuring the dielectric capacitance.

Resistance will be nulled out and has no significance as it is largely skin surface resistance from moist salty skin. By cancelling out voltage that is "in-phase" with the sinusoidal current source, the instrument can measure the reactive vector component. Since the body dielectric is capacitive the voltage will have a 90deg phase lead relative to the current source.

They are completely non-invasive and harmless signal levels.

Frequencies of useful range are 1~100KHz. I have a 4 digit autoranging LRC meter that displays capacitance at different frequencies, but I have never attempted to calibrate my BMI. I do have my fingers calibrated for capacitance though and resistance , whenever I have to tune my circuits.... ;)

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    \$\begingroup\$ Body Mass Index only takes weight and length into account. \$\endgroup\$ – stevenvh May 14 '12 at 11:06
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The body fat calculator works measuring the conductivity of the body, so it applies a signal (an AC voltage) and measures the (very small, for safety) current. So in principle it's simple, but what I think is hard is to calibrate it, because you need to have very good reference values.

This Wiki seems to suggest that these instruments measure resistance, as it's dependent on the water volume, greater in adipose tissues.

You also need a quite sensitive and accurate current meter, but it's less than an issue. What you need the most is another - commercial - instrument to take the reference values and maybe create a look-up table and define a function that relates current to fat percentage.

An alternative: according to Body Mass Percentage wiki, you may achieve better results engineering your bath tub to measure the water displacement when you get in. But do it before using bathfoam :)

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  • \$\begingroup\$ It's not a simple DC current measurement (this would be dominated by the skin's surface resistance, like in a "lie detector"). It seems to involve impedance at different frequencies. \$\endgroup\$ – Stefan Paul Noack May 14 '12 at 10:45
  • \$\begingroup\$ @noah1989 it's not or it seems not? \$\endgroup\$ – clabacchio May 14 '12 at 11:22
  • \$\begingroup\$ All resources (such as en.wikipedia.org/wiki/Bioelectrical_impedance_analysis) I found mention AC, some also talk about phase-angle measurements. but reading your answer again, it mentions neither DC nor AC, maybe you could add that AC is required. \$\endgroup\$ – Stefan Paul Noack May 14 '12 at 11:50

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