Linearity of electrochemical cells and electrical conductivity measurement

I've measured the output signal of a few commercially available conductivity meters and was surprised to see that the signal was typically a square wave of amplitude in the range of 1.5 to 6 Vpp (peak-to-peak) and a frequency ranging from 4 to 20 kHz. One of the meters even had a dc (direct current) offset of about 200 mV.


My understanding is that for an electrochemical cell to behave as a linear system, and to avoid creating harmonic distortions, we typically use a sine-wave of around 10 mVrms (root-mean-square), but not much higher.


So, I'm a bit surprised with the characteristics of these meters.
Does anyone have an explanation for these meter characteristics? Mainly:


* Why they use amplitudes higher than 10 mVrms?
* Why the square wave instead of a sine wave?
* Why this variability in the used frequency and how can one possibly determine the correct value to use?
* What could explain the use of a dc offset although we know it could change the properties of our cell!


Any help is appreciated.


