Variable area Flow Meters

Advantages: 

1. Major advantages of the variable-area flowmeter include it’s relatively low cost and it’s ease of installation. It’s simplicity of design lends to low maintenance operation, and hence, tends to have a long operating life.
2. Another advantage is it’s flexibility to be used in different applications having a wide range of chemicals. With the PTFE-lined metal tube construction,  variable area meters are able to resist corrosive damage from aggressive chemicals.
3. The variable area meter does not require power and will provide accurate flow rate readings much the same as a glycerin-filled pressure gauge reads pressure readings without any electrical power. This allows the meter to be deployed where it is not practical or possible to run power cable to the device.

Disadvantages:

1. One potential disadvantage of a variable-area flowmeter occurs in applications where the media temperature and pressure deviate from the calibration temperature and pressure. Temperature pressure variations will cause a changes in density for both gasses and liquids while changes in pressure will cause a gas to expand and contract. For liquids, viscosity will also change with temperature. Since the variable area measurement principle is volumetric in nature and is also density and viscosity-sensitive, these changes in pressure and temperature will lead to calibration shift. 
2. During operation, the flowmeter accuracy can quickly degrade once the temperatures and pressures start fluctuating from the standard calibration temperature and pressure.  VA 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.
3. VA flowmeter readings must be corrected when the calibration conditions and operating conditions for gas flows vary. Some manufacturers have a correction that can be used.  However, this correction factor is based on a constant pressure and temperature, just as is for all volumetric flowmeters such as vortex, turbine, PD, DP and others.  For liquids, the effect of viscosity changes is another potential disadvantage because 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.