The application of flow meters in the measurement of very small flow rates

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There are various commonly used flow meters suitable for measuring small flow rates, such as differential pressure type, float type, positive displacement type, and thermal type. Some of these flow meters are suitable for measuring both medium to large flow rates as well as small flow rates, while others are designed specifically for measuring small flow rates. 1. Float flow meter: A float flow meter is mainly composed of a float and a conical tube. The cone tube in the glass tube float flow meter is made of glass ； The cone tube of the metal tube float flow meter is made of metal, and it can handle fluid temperatures up to 180°C and fluid pressures up to 13 MPa. Float flowmeters are instruments with medium to low precision. The basic error of metal float flowmeters is 1%–2.5% FS for those with on-site indication, and 1%–4% FS for those with remote transmission ； The float flow meter with small-diameter glass tube has a precision of 2.5%–5% FS. Therefore, it is generally only suitable for traffic monitoring, not for accounting and metering. Glass tube float flowmeters are only suitable for gases and liquids with high transparency; otherwise, it is difficult to see the height of the float in the conical tube. Metal tube float flowmeters, however, have no such limitation. The flow measurement range of small-diameter float flow meters can be made very narrow; for water, it is 0.3–3 L/h, and for air, it is 5–50 L/h. Micro glass float flowmeters, which are widely used in environmental protection for things such as air samplers, on-line process analyzers, and laboratory analyzers, can have an even narrower measurement range. In the measurement of liquid level, flow rate, and density in industrial processes, when the medium being measured has high viscosity or is corrosive. Isolation is commonly achieved by blowing air or liquid, and the flow rate of the blown air or liquid is usually measured using a float flow meter. In situations where the pressure of the gas (liquid) source fluctuates significantly or where the pressure of the medium being measured varies greatly, in order to ensure a stable flow rate of the gas or liquid and enable accurate measurement, some products combine a float flow meter with a regulator (flow stabilizer) to form a constant-differential-pressure flow control device. 2. Differential pressure flow meters: These flow meters, which consist of a standard throttling device and a differential pressure element, are subject to three types of limitations when measuring low flow rates. The first limitation is the size of the pipe diameter. Standard throttling devices are suitable for pipe diameters of 50 mm and above. When the pipe diameter is too small, the diameter of the throttling device becomes correspondingly smaller, making it difficult to carry out similar processing in accordance with the shapes specified in the standards. The second limitation is that the differential pressure is too low. This differential pressure is proportional to the square of the flow velocity; when the flow velocity drops to a certain level, the differential pressure becomes so small that it cannot be detected. The third limit is the constraint on the lower Reynolds number. Different types of throttling devices have different lower limits for the Reynolds number; generally speaking, a Reynolds number of ≥104 is the acceptable threshold. The average flow velocity and flow rate corresponding to this lower Reynolds number represent the lower limit for measuring low-flow rates. When the Reynolds number is too low, the discharge coefficient changes significantly with the Reynolds number, thereby increasing the uncertainty.