1. Mechanics Principle: Instruments which belong to this kind of principle are differential pressure type and rotor type using Bernoulli theorem; impulse type and movable tube type using momentum theorem; direct mass type using Newton's second law; target type using fluid momentum principle; turbine type using angular momentum theorem; vortex type and vortex street type using fluid oscillation principle; pitot tube type using total static pressure difference. As well as the volume type and weir, trough type and so on.
2. Electrical Principle: Instruments used in this kind of principle include electromagnetic type, differential capacitance type, inductance type, strain resistance type, etc.
3. Acoustic Principle: Flow measurement based on acoustic principle includes ultrasonic, acoustic (shock wave) and so on.
4. Thermal Principle: There are heat type, direct type and indirect type to measure flow rate by thermal principle.
5. Optical principle: Laser type, photoelectric type and so on are instruments belonging to this kind of principle.
6. Atomic Physics Principles: Nuclear Magnetic Resonance (NMR) and Nuclear Radiation (NR) are instruments of this kind.
7. Other Principles: Labeling Principle (Tracer Principle, Nuclear Magnetic Resonance Principle), Related Principle, etc.