We use around about 1V peak to peak The third factor is frequency - if you change the frequency, you change the resistance reading. We stick with 1kHz. And finally, if you change the waveform of the AC excitation, you change the resistance in unpleasant ways. These get distorted within the block, as you can see if you try and drive a block with a square-wave form something like a 555 oscillator mentioned by one correspondent. One final hazard - block interaction.
Data loggers impose quite different conditions on reading gypsum blocks than do hand-held readers.
The result is weird readings as blocks at different levels of moisture content interact. Data looks spiky. The gypsum blocks were connected with one side in common. Same problem - interaction. So you need to galvanically isolate gypsum blocks one from the other if you want to connect them to a data logger.