Design overview: The variable-area flowmeter (Figure 1) is one of the oldest technologies available and arguably the most well-known. It is constructed of a tapered tube (usually plastic or glass) and a metal or glass float. The volumetric flowrate through the tapered tube is proportional to the displacement of the float.

Fluid moving through the tube form bottom to top causes a pressure drop across the float, which produces an upward force that causes the float to move up the tube. As this happens, the cross-sectional area between the tube walls and the float (the annulus) increases (hence the term variable-area).

Because the variable-area flowmeter relies on gravity, it must be installed vertically (with the flowtube perpendicular to the floor). Some variable-area meters overcome this slight inconvenience by spring loading the float withing the tube (Figure 2). Such a design can simplify installation and add operator flexibility, especially when the meter must be installed in a tight physical space and a vertical installation is not possible.

Two types of variable-area flowmeters are generally available: direct-reading and correlated. The direct-reading meter allows 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 advantage of a direct-reading flowmeter is that the flowrate is literally read directly off the flowtube. Correlated meters, on the other hand, have a unitless scale (typically 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 usually give 25 or so data points along the scale of the flowtube, allowing the user to determine the actual flowrate in gal/min, L/min, or whatever engineering unit is needed.

The advantage of the correlated meter is that the same flowmeter can be used for various gases and liquids (whose flow is represented by different units) by selecting the appropriate correlation sheets, where additional direct-reading meters would be required for different fluid applications. Similarly, if pressure or temperature parameters change for a given application, the user would simply use a different correlation sheet to reflect these new parameters. By comparison, 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. In general, the average accuracy of a variable-area flowmeter is ±2-4% of fullscale flow.

Advantages: The major advantage of the variable-area flowmeter is its relative low cost and ease of installation. Because of its simplicity of design, the variable-area meter is virtually maintenance-free and, hence, tends to have a long operating life.

Another advantage is its flexibility in handling a wide range of chemicals. Today, all-Teflon meters are available to resist corrosive damage by aggressive chemicals. The advantage of a Teflon flowmeter with a built-in valve is that you can not only monitor the fluid flowrate, but you can control it, as well, by opening and closing the valve. If the application requires an all-Teflon meter, chances are the fluid is pretty corrosive, and many users would like the option of controlling the flowrate by simply turning a valve that is built into the flowmeter itself.

Disadvantages: One potential disadvantage of a variable-area flowmeter occurs when the fluid temperature and pressure deviate from the calibration temperature and pressure. Because temperature and pressure variations will cause a gas to expand and contract, thereby changing density and viscosity, the calibration of a particular variable-area flowmeter will no longer be valid as these conditions fluctuate. Manufacturers typically calibrate their gas flowmeters to a standard temperature and pressure (usually 70°F with the flowmeter outlet open to the atmosphere, i.e., with no backpressure).

During operation, the flowmeter accuracy can quickly degrade once the temperatures and pressures start fluctuating from the standard calibration temperature and pressure. Meters used for water tend to show less variability, since water viscosity and density changes very little with normal temperature and pressure fluctuations. While there is a way to correlate the flow from actual operating conditions back to the calibration conditions, the conventional formulas used are very simplified, and don't take into account the effect of viscosity, which can cause large errors

Table 2
The Effect of Pressure Deviations on a Variable-Area Flowmeter

As Table 2 shows, the effect of pressure deviations can be quite significant. This table was created using data from a variable-area flowmeter that was calibrated for air at 70°F and with the outlet of the flowmeter vented to the open atmosphere (i.e. , 0 psi of outlet pressure).

The flowmeter was calibrated to read a maximum of 2.23 L/min at this temperature and pressure. When the outlet pressure increases as all other parameters remain constant, the flowrate drops off. This pressure change affects the viscosity and density of the gas and will cause the actual flowrate to deviate from the theoretical, calibrated flowrate. This relationship is extremely important to be aware of, and underscores the difficulty in measuring gas flow. Also note that even though gas flowrate changes with a change in gas temperature (with all other parameters remaining constant), this effect is much less significant with air than with other gases.

Table 2 shows this same variation with a meter calibrated for water at 9 psi venting pressure and a temperature of 70°F. Here, one can assume water to be incompressible. As shown, there is no direct effect on water flow with a change in back-pressure. The temp-erature change is not that significant either. But, for various fluids, a change in temperature could change the viscosity enough to degrade the accuracy below acceptable limits.

The bottom line is that the user must be aware of any variation between calibration conditions and operating conditions for gas flows, and must correct the reading according to the manufacturer's recommendations. Some users have the manufacturer calibrate the meter to existing conditions, but this presumes that operating conditions will remain the same—which they rarely do.

The effect of viscosity changes is another potential disadvantage of the variable-area meter when measuring liquids. When a viscous liquid makes its way through a variable-area flowmeter, drag layers of fluid will build up on the float. this will cause a slower-moving viscous liquid to yield the same buoyant force as a faster-moving fluid of lower viscosity. The larger the viscosity, the higher the error. The general rule of thumb is as follows—unless the meter has been specifically calibrated for a higher-viscosity liquid, only water-like liquids should be run through a variable-area flowmeter.

Sometimes, for liquids that are slightly thicker than water, a manufacturer-supplied correction factor can be used without the need to recalibrate the whole meter. As always, check with the manufacturer if you plan on deviating from its calibration fluid and calibration conditions. For a more-detailed discussion of the proper correction equations to apply to variable-area flowmeters in both water and gas service when they deviate from standard conditions, consult Refs. 9 and 10.

Applications:
Variable-area flowmeters are well suited for a wide variety of liquid and gas applications, including the following:

• Measuring water and gas flow in plants or labs
• Monitoring chemical lines
• Purging instrument air lines (i.e., lines that use a valved meter)
• Monitoring filtration loading
• Monitoring flow in material-blending applications (i.e., lines that use a valved meter)
• Monitoring hydraulic oils (although this may require special calibration)
• Monitor makeup water for food & beverage plants