There are many kinds of common flow meters suitable for measuring small flow, such as differential pressure, float type, volumetric type, thermal type, etc. Some of them are suitable for measuring medium and large flow,and are also suitable for measuring small flow,while others are specially designed for measuring small flow.
- Float flow meter
The float flow meter is mainly composed of a float and a conical tube.The conical tube in the glass tube float flow meter is a glass tube; The conical tube in the metal tube float flow meter is made of metal with a fluid temperature of up to 180 ° C and a fluid pressure of 13 MPa.
The float flow meter is a medium and low precision meter. The basic error of the metal float flow meter is 1%~2.5%FS for the local indication type, 1%~4%FS for the remote transmission type; The small-diameter glass tube float flow meter is 2.5%~ 5% FS. Therefore, it is generally only applicable to flow monitoring and not for accounting measurement.
Glass tube float flow meter is only suitable for gas and liquid with high transparency, otherwise the height of float in conical tube is not easy to see. The metal tube floater flow meter has no such limitation.
The flow measurement range of the small-caliber float flow meter can be made very small,in which water is 0.3-3 L/h and air is 5-50 L/h.Miniature glass float flow meters, such as atmospheric samplers, on-line flowsheet analyzers and laboratory analyzers, are widely used in environmental protection, and their measurement ranges can be smaller.
In the measurement of liquid level, flow rate and density in the industrial process, if the measured medium has a high viscosity or corrosivity, it is usually isolated by means of air or liquid blowing. The flow of air and liquid blowing is often measured by float flow meter. In the case that the pressure of the gas (liquid) source fluctuates greatly or the pressure of the medium under test fluctuates greatly, in order to make the flow of the air and the liquid to be measured stably and accurately, some products match the float flow meter with the regulator (constant current device) into a set of constant differential pressure flow regulator
2. Differential pressure flow meter
The differential pressure flow meter consisting of a standard throttle device and a differential pressure type is limited in three aspects in small flow measurement.
The first limitation is the size of the pipe diameter. The standard throttle device is suitable for pipe diameters of 50mm and above. When the pipe diameter is too small, the diameter of the throttle device becomes correspondingly small, and it is difficult to perform similar processing according to the shape specified in the standard.
The second limitation is that the differential pressure is too small. This differential pressure is proportional to the square of the flow rate. When the flow rate is low to a certain value, the differential pressure becomes so small that it cannot be resolved.
The third limitation is the limit of the Reynolds number lower limit.The type of throttling device is different, and the lower limit of the Reynolds number is also different. In general, a Reynolds number ≥ 104 is a limit that can be used. The average flow rate and flow velocity corresponding to the lower limit of the Reynolds number is the lower measurement limit of the small flow rate.The Reynolds number is too small, and the outflow coefficient changes significantly with the change of Reynolds number, so that the uncertainty increases.