With the many flowmeters available today, choosing the most appropriate one for a given application can be difficult. This article discusses six popular flowmeter technologies, in terms of the major advantages and disadvantages of each type, describes some unique designs, and gives several application examples.

Dozens of flowmeter technologies are available. This article covers six flowmeter designs - variable-area, mass, Coriolis, differential-pressure, turbine, and oval-gear. Table 1 compares the various technologies.

Variable-area flowmeters
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 from 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 crosssectional 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 within 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: directreading and correlated. The directreading 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 ususally 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.

With the many flowmeters available today, choosing the most appropriate one for a given application can be difficult. This article discusses six popular flowmeter technologies, in terms of the major advantages and disadvantages of each type, describes some unique designs, and gives several application examples.

Dozens of flowmeter technologies are available. This article covers six flowmeter designs - variable-area, mass, Coriolis, differential-pressure, turbine, and oval-gear. Table 1 compares the various technologies.

Variable-area flowmeters