There are many different types, turbine, rotometers, magnetic, and positive displacement types measure volumetric flow whereas thermal and coriolis types are mass flow meters. You need to think about the application. Things like cost, user interface, accuracy, compatability with the material...
Measuring GOhms is not easy! I'm an engineer at Keithley Instruments, where we make instruments that measure up to TOhms, among other things. To do this be prepared for the heavy use of Teflon, guard traces, plastic gloves, MOSFET input opamps. The OPA602 is a JFET input with up to 10pA of bias...
