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One standard method of the flow material has any conductance is to use the Hall effect in what is called the electromagnetic flow meter. You run the fluid through a rectangular cross section area and put an AC magnetic field across one pair of opposite faces and electrodes on the other two faces. You measure the voltage on them with a high impedance volt meter.
Another method that only works for high speed flow is ultrasonic doppler. You TX from one sensor along the flow direction and RX the reflected signal and compare the frequencies. This can be made contactless by putting the sensors at the end of a rectangular box shape area and run the fluid in one side and out the other.