According to the measurement principle, it can be divided into the following major categories:

1. Mechanics principle: Instruments belonging to such principles have differential pressure and rotor type using Bernoulli's theorem; impulse type and movable tube type using momentum theorem; direct mass type using Newton's second law; using fluid momentum The target type of the principle; the turbine type using the angular momentum theorem; the vortex type using the principle of fluid oscillation, the vortex type; the pitot tube type using the total static pressure difference, and the volumetric type, the enthalpy, the trough type and the like.

2, electrical principles: instruments used in such principles are electromagnetic, differential capacitive, inductive, strain resistance and so on.

3. Acoustic principle: Ultrasonic type is used to measure the flow using the acoustic principle. Acoustic (shock wave) and the like.

4, the principle of thermal: the use of thermal principles to measure the flow of heat, direct calorimetry, indirect calorimetry and so on.

5, optical principle: laser, photoelectric, etc. are instruments of this type.

6, the principle of atomic physics: nuclear magnetic resonance, nuclear radiation, etc. are instruments of this type.

7, other principles: the principle of marking (trace principle, NMR principle), related principles.