There are various types of flow meters commonly used for measuring small flows, such as differential pressure type, float type, volumetric type, thermal type, etc. Some of these flow meters are suitable for both medium-large and small flow measurements, while others are specifically designed for measuring small flows.
Differential Pressure Flow Meters: Differential pressure flow meters composed of standard orifice devices and differential pressure sensors have three limitations in small flow measurement.
The first limitation is the lower limit of the Reynolds number. The lower limit of the Reynolds number varies with different types of orifice devices. In general, a Reynolds number ≥10^4 is the usable boundary. The corresponding average velocity and flow rate to this lower limit of the Reynolds number represent the lower limit of small flow measurement. When the Reynolds number is too small, the discharge coefficient will significantly change with the variation of the Reynolds number, leading to increased uncertainty.
The second limitation is the size of the pipe diameter. Standard orifice devices are applicable to pipe diameters of 50mm and above. When the pipe diameter is too small, the diameter of the orifice device also becomes correspondingly smaller, making it difficult to process according to the similar shape specified in the standards.
The third limitation is the excessively small differential pressure. This differential pressure is directly proportional to the square of the flow velocity. When the flow velocity decreases to a certain value, the differential pressure becomes very small, making it impossible to distinguish.