let u03b1 denote the complex value obtained when the switch 90 is open and u03b2 the complex value obtained when the switch 90 is closed u03b1 is obtained by calculating u03b1 u03b1 1 j u03b1 2 where u03b1 1 is the voltage measured at microcontroller input 70 when the phase of the multiplexer 48 with respect to the guard sine wave is u03c6 u03c6 need not be known but it must be constant and u03b1 2 is the voltage measured at microcontroller input 70 when the phase of the multiplexer 48 with respect to the guard sine wave is u03c6 90 u00b0 for the measurements of both u03b1 1 and u03b1 2 the switch 90 is open u03b2 is obtained by calculating u03b2 u03b2 1 j u03b2 2 where u03b2 1 is the voltage measured at microcontroller input 70 when the phase of the multiplexer 48 with respect to the guard sine wave is u03c6 same value as for the determination of u03b1 and u03b2 2 is the voltage measured at microcontroller input 70 when the phase of the multiplexer 48 with respect to the guard sine wave is u03c6 90 u00b0 for the measurements of both u03b2 1 and u03b2 2 the switch 90 is closed the difference u03b2 u2212 u03b1 between these two measured complex values corresponds to the complex impedance of the reference impedance 88 because the reference impedance 88 is in parallel with the unknown impedance 16 which will be denoted z x in the following since the complex impedance z ref of reference impedance 88 is known the ratio between the difference u03b2 u2212 u03b1.