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*** Tanita Body Fat Scale ***

The Tanita Body Fat Monitor/Scale performs a body fat
measurement using a bio-impedance technique. For
alternating current circuits, impedance is the mathematical
concept of how much electrical resistance is encountered by
a signal in a circuit, taking into account both the magnitude
of the signal and the phase of the signal. Mathematically,
impedance is a complex number, where the real part and the
imaginary part together yield both the phase and amplitude.

The "bio-impedance" measurement occurs when the user stands
barefoot on two metal pads on the scale. A 9-volt battery
powers a very low power 50 KHZ radio frequency (RF) transmitter,
which sends the RF signal through the human body. The user
feels nothing, and is unaware that he is part of an electrical
circuit! The instrument must be programmed, because 3
parameters: (1) the gender, (2) athletic status
(normal or athlete), and (3) body height, affect the computation
of the final answer, which comes from a computer chip
delivering the final readout on the digital display.

How hydrated the person is (water in the stomach, and water
in the tissues) affects the answer slightly by 1 or 2 percent.
In the mornings, an average person is dehydrated.
So the morning readings of body fat will tend to be
slightly different from the evening readings. My
body fat readings are very consistent, no matter whether
taken in the morning or the evening. They vary between
5 percent body fat to 7 percent body fat.

http://www.tanita.com/BodyFatOverview.shtml