Why would my signal from a piezoelectric device changes while using a voltmeter and electrometer?
nano voltmeter specs >10 GΩ

eleectrometer specs > 200 TΩ

If result changes 100x then DUT Z is > 100x10G = 1 TΩ

If result is dynamic and changes 10x then consider fixture capacitance loading.  One needs a Charge Amplifier for this or special 0 pF fixture right at instrument.


Charge is proportional to pressure
Rate of charge is equal to current Ic=dQ/dt = C dV/dt
thus dynamic pressure from acceleration (ac) is proportional to Iac into load R. DC Leakage resistance must be > TΩ to measure static pressure which makes this difficult to do.
this is the fundamental property that occurs in accelerometers useful for sensing g and integrating to v but not useful for static pressure. For this they use load cells.
piezo effect also makes ceramic caps microphonic.