The variable-area flowmeter (Figure 1) is among the oldest technologies out there and arguably probably the most well -known. It's constructed of a tapered tube (usually plastic or glass) and a metal or glass float. The
volumetric flowrate by means of the tapered tube is proportional to the displacement of the float.
Fluid shifting by means of the tube type bottom to top causes a pressure drop throughout the float, which produces an upward force that causes the float to move up the tube. As this occurs, the cross-sectional space between the tube partitions and the float (the annulus) will increase (therefore the term variable-area). As a result of the variable-area flow meter relies on gravity, it have to be installed vertically (with the flow tube perpendicular to the ground). Some variable-area meters overcome this slight inconvenience by spring loading the float within the tube (Figure 2). Such a design can simplify installation and add operator flexibility, particularly when the meter have to be put in a good physical area and a vertical installation will not be possible.
Two varieties of variable-area flow meters are usually available: direct-reading and correlated. The direct-reading meter allow the user to read the liquid or gas flowrate in engineering units (i.e., gal/min and L/min) printed directly on the tube, by aligning the top of the float with the tick mark on the flowtube. The benefit of a direct-reading flowmeter is that the flowrate is actually read straight off the flowtube. Correlated meters, however, have a unitless scale (sometimes tick marks from 0 to 65, or 0 to 150 ), and come with a separate data sheet that correlates the scale reading on the flowtube to the flowrate in a particular engineering unit. The correlation sheets often give 25 or so data factors alongside the dimensions of the flowtube, allowing the consumer to decide the precise flowrate in gal/min, L/min, or whatever engineering unit is needed.
The advantage of the correlated meter is that the identical flowmeter can be used for numerous gases and liquids (whose stream is represented by completely different units) by choosing the suitable correlation sheets, the place further direct-reading meters would be required for different fluid applications. Equally, if strain or temperature parameters change for a given application, the person would simply use a special correlation sheet to reflect these new parameters. By comparability, for a direct-reading meter, a change in
operating parameters will compromise the meter's accuracy, forcing it to be returned to the factory for recalibration. Usually, the typical accuracy of a variable-area flowmeter is ±2-4% of fullscale flow.
Advantages: The main benefit of the variable-area flowmeter is its relative low cost and ease of installation. Due to its simplicity of design, the variable-area meter is virtually maintenance-free and, hence, tends to have a long working life.
Another advantage is its flexibility in dealing with a wide range of chemicals. As we speak, all-Teflon meters
can be found to resist corrosive injury by aggressive chemicals. The advantage of a Teflon flowmeter with a built-in valve is that you would be able to not only monitor the fluid flowrate, but you can control it, as well, by opening and shutting the valve. If the application requires an all-Teflon meter, chances are the fluid is fairly corrosive, and plenty of users would like the option of controlling the flowrate by merely turning a valve that is constructed into the flowmeter itself.
Disadvantages: One potential drawback of a variable-area flowmeter occurs when the fluid temperature and pressure deviate from the calibration temperature and pressure. As a result of temperature and strain variations will cause a gas to increase and contract, thereby changing density and viscosity, the calibration of a specific variable-area flowmeter will now not be valid as these circumstances fluctuate. Manufacturers sometimes calibrate their gas flowmeters to a standard temperature and pressure (often 70°F with the flowmeter outlet open to the environment, i.e., with no back pressure). During operation, the flowmeter accuracy can rapidly degrade as soon as the temperatures and pressures start fluctuating from the standard calibration temperature and pressure. Meters used for water have a tendency to indicate much less variability, since water viscosity and density changes little with normal temperature and pressure fluctuations. While there's a strategy to correlate the circulation from precise operating situations back to the calibration conditions, the conventional formulas used are very simplified, and don't bear in mind the impact of viscosity, which can cause large errors.