Subject matter includes a method comprising: applying a first electrical signal to particular locations of a subject via at least one electrode pair applying a second electrical signal to the particular locations of the subject via the at least one electrode pair, wherein the second electrical signal is applied alternately and repeatedly with the first electrical signal determining a first electrical impedance based, at least in part, on the first electrical signal determining a second electrical impedance based, at least in part, on the second electrical signal and removing at least a portion of varying electrical characteristics of the at least one electrode pair by calculating a difference between the first electrical impedance and the second electrical impedance.
