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=Details=
Body fat scales work by comparing the electrical resistance of a direct current to alternating current. The direct current does not penetrate the cell walls whereas the alternating current does. This creates a difference that allows an estimate of extracellular water. Typically the alternating current is around 50 kHz, with dual frequency scales adding in 6.25 kHz for improved accuracy. Single frequency scales generally treat the body is a single cylinder with uniform resistance. Unfortunately because the arms and legs are thinner than the trunk they contributed disproportionate amount of resistance. Typically arms and legs are about 5 to 20% of body weight, but form about half of the overall resistance, while the trunk contains 50% of the body mass and contribute only 5 to 10% of the overall resistance. Dual frequency scales on the other hand segment the body up into five cylinders, two for the arms to for the legs and one for the trunk, giving much higher accuracy.
=WiFi Scales=The [http://www.withings.com/en/bodyscale Withings body fat scale] is noteworthy because it is Wi-Fi enabled. This allows it to upload the data directly to the Internet where it can be viewed on any web browser. This makes it easy to visualize and track the changes in your weight and body fat. Unfortunately the scales only measure between the feet, and so their body fat accuracy is limited.=References==
<references>
<ref name="BIA1">Body composition changes assessed by bioelectrical impedance measurements http://www.ncbi.nlm.nih.gov/pubmed/2923071</ref>
<ref name="BIA2">Changes in fat-free mass during weight loss measured by bioelectrical impedance and by densitometry http://www.ncbi.nlm.nih.gov/pubmed/2912008</ref>
</references>
