Some fun facts about float flowmeters!

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Hi, this is the Industrial Control Assistant. Do you know what a float flow meter is? A float flow meter is a volume flow meter that measures flow by using a float which rises and falls within a vertical conical tube as the flow rate changes, thereby altering the area between them; it is also known as a rotor flow meter. The principle behind the float flowmeter was conceived in the 1860s, and commercial versions appeared at the beginning of the 20th century. In terms of the proportion of devices in use, a survey conducted in the UK in 1985 on 72 companies and their 17,000 flow meters showed that float flow meters accounted for 19.2% of them. In 1996, the production volume of float flowmeters in our country was estimated to be between 150,000 and 170,000 units, of which about 95% were glass tube float flowmeters. The working principle of a float flow meter is as follows: When the fluid to be measured flows from bottom to top through the annular gap 3 formed by the conical tube 1 and the float 2, a pressure difference is created between the upper and lower ends of the float, which generates a force that pushes the float upward. When this upward force exceeds the weight of the float submerged in the fluid, the float rises; as a result, the area of the annular gap increases, and the flow velocity of the fluid in that gap decreases immediately. The pressure difference between the upper and lower ends of the float thus decreases, and the upward force acting on the float also decreases. Eventually, when the upward force equals the weight of the float submerged in the fluid, the float stabilizes at a certain height. There is a corresponding relationship between the height of the float in the conical tube and the flow rate passing through it. Features of float flowmeters: Float flowmeters are used for small pipe diameters and low flow rates. The common diameter of instruments is below 40-50 mm, with the smallest diameter ranging from 1.5-4 mm. Suitable for measuring low flow rates and small volumes of fluid; taking liquids as an example, for glass tube float flow meters with a diameter of 10 mm or less, the nominal pipe diameter at full scale corresponds to flow velocities in the range of 0.2–0.6 m/s, or even below 0.1 m/s ； Metal tube float flow meters and glass tube float flow meters with a diameter greater than 15 mm have slightly higher values, with flow velocities ranging from 0.5 to 1.5 m/s. Float flow meters can be used at lower Reynolds numbers; by selecting a float with a shape that is not sensitive to viscosity, the Reynolds number in the flow gap needs only to be greater than 40 or 500 for the flow coefficient to remain constant, meaning that changes in fluid viscosity do not affect the flow coefficient. This value is far below the requirement of 104–105 for the minimum Reynolds number of throttling differential pressure instruments such as standard orifice plates. Most float flowmeters do not require a straight pipe section upstream, or they have low requirements regarding such a straight pipe section. Float flow meters have a wide range of flow rates, typically 10:1, with a minimum of 5:1 and a maximum of 25:1. The output of the flow detection element is nearly linear. The pressure loss is low. Glass tube float flowmeters have a simple structure and are inexpensive. As long as it is convenient for those who control the flow on-site, the downside is the risk of the glass tube breaking, especially when a float without a guiding structure is used for gases. Metal tube float flowmeters have no risk of cone tube rupture. Compared to glass tube float flowmeters, it has a wider operating temperature and pressure range. Most structured float flowmeters can only be installed in pipes with vertical flow from bottom to top. The application of float flowmeters is limited to medium and small pipe diameters; conventional full-flow float flowmeters cannot be used for large diameters. Glass tube float flowmeters have a maximum diameter of 100 mm, while metal tube float flowmeters have a maximum diameter of 150 mm. For larger diameters, split-flow meters must be used. When using a fluid different from the factory-calibrated fluid, a correction to the flow reading is required. Liquid float flow meters are usually calibrated with water, while those for gases are calibrated with air. If the density and viscosity of the actual fluid used differ from these values, the flow rate will deviate from the original scale readings, and a correction factor must be applied. Well, that’s all for today’s content. If you have any further questions, feel free to leave a message. New updates will be posted regularly, so please stay tuned.