The third generation measures the impedance of the tooth at two different frequencies (a high one as 8 kHz and a low one as 400 Hz). In the coronal portion of the canal, the impedance difference between the frequencies is constant. As the file is advanced through apical constriction, the difference in the impedance value will increase and reach a maximum value at the apical area.
In comparative with second generation, such units may be equipped with powerful microprocessors and are able to process the mathematical quotient or other calculations required to give accurate readings.
As the magnitude of impedance depends on the measurement frequency the use of two frequencies give the opportunity of observing the difference between the two results as the needle advances in the root canal. We can also say that this is a “comparative impedance” method because it measures the impedance difference which can be converted into length information.