This circuit was originally designed to measure the volume of fluid inside a medical syringe.   As designed, it produces a zero to 5 volt output, corresponding to a capacitance change of about 10 picofarads.  With a digital voltmeter, at its output, it can resolve a capacitance change of 0.002 picofarads or 2 femtofarads.

Low Value Capacitance Meter – [[[LINK]]]