Venturi tube flowmeter

Venturi tube flowmeters are devices used to measure fluid speeds in pipes. Figure 5.11 illustrates the general construction of a Venturi tube. These are very simple devices that consist of a middle section with a small diameter connected on both ends to larger diameter sections via smooth transitions in order to prevent turbulence. A U-tube containing a fluid of known density connects the large and small diameter tubes. The U-tube is a manometer, and we can use it to measure differences in pressure. 

We ll call this last equation the Venturi tube flowmeter equation, a highly descriptive, if not generally recognized, name. This is the equation that can be used to determine vv the speed of a fluid in a pipe. 

In order to apply the Venturi tube flowmeter equation, we will need to know the cross-sectional areas A1 and A2 of the two different parts of the Venturi tube. Let s be careful and work in SI units. 

Substituting all of the necessary information into the Venturi tube flowmeter equation we get ... 

Flow Rate. The values for volumetric or mass flow rate measurement are often determined by measuring pressure difference across an orifice, nozzle, or venturi tube. Other flow measurement techniques include positive displacement meters, turbine flowmeters, and airflow-measuring hoods. 

Head-type flowmeters include orifice plates, venturi tubes, weirs, flumes, and many others. They change the velocity or direction of the flow, creating a measurable differential pressure, or "pressure head," in the fluid. Head metering is one of the most ancient of flow detection techniques. There is evidence that the Egyptians used weirs for measurement of irrigation water flows in the days of the Pharaohs and that the Romans used orifices to meter water to households in Caesar s time. In the 18th century, Bernoulli established the basic relationship between the pressure head and velocity head, and Venturi published on the flow tube bearing his name. 

Meters that measure differential pressures over the flowmeter and such pressure changes that can be interpreted as flowrates. Such flowmeters with a large number of designs include orifices, venturi tubes, pitot tubes, elbow taps, etc. Fluids that result in changes of the cross-sectional area due to erosion, corrosion, or deposition of solids obviously change the calibrations. These meters tend to be relatively cheap but are often not very accurate. 

The Venturi Tube as a Liquefied-Gas Flow Measuring Device (5) 282 A Volumetric Flowmeter for Liquid Oxygen (5) 299... 

The flow of fluids is most commonly measured using head flowmeters. The operation of these flowmeters is based on the Bernoulli equation. A constriction in the flow path is used to increase the flow velocity. This is accompanied by a decrease in pressure head and since the resultant pressure drop is a function of the flow rate of fluid, the latter can be evaluated. The flowmeters for closed conduits can be used for both gases and liquids. The flowmeters for open conduits can only be used for liquids. Head flowmeters include orifice and venturi meters, flow nozzles, Pitot tubes and weirs. They consist of a primary element which causes the pressure or head loss and a secondary element which measures it. The primary element does not contain any moving parts. The most common secondary elements for closed conduit flowmeters are U-tube manometers and differential pressure transducers. 

Instruments which measure the rate of flow (velocity) of liquids and gases are called flowmeters they may be broadly defined as being mechanical or electronic in operation. Examples of mechanical flowmeters are orifice plate and float meters (Fig. 5.8), venturi meters, and pitot tube meters, all of which depend on a constriction being introduced into the flow stream in order to produce a difference in pressure across the constriction. The rate of flow can then be obtained from the difference in pressure. 

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