Variable area flowmeters measure flow by allowing the flow stream to change the opening within the flowmeter by moving an internal part. When the flow increases, the fluid generates more force and moves the internal part farther.
One variable area flowmeter measures flow in a vertical metering tube by balancing the downward weight of a float with the upward force of the flowing fluid. Spring-opposed float designs allow this type of flowmeter to be installed in horizontal pipes, because the functioning of the float is not dependent upon gravity. These flowmeters can be read locally because their glass or plastic metering tubes have markings that relate the height of the float (that can be seen) with the flow rate of the fluid. Flowmeters with remote signals are typically constructed with metal tubes, and include a transmitter that senses the height of the float to determine fluid flow.
Vane-style variable area flowmeters have a spring-opposed vane that moves in relation to the flow rate. Similarly, piston variable area flowmeters use a spring-opposed piston that moves in relation to the flow rate and are less sensitive to viscosity than vane-style variable area flowmeters. Both vane-style and piston variable area flowmeters can be connected to an indicator or transmitter.
Variable area flowmeters inferentially measure the flow of liquids and gases, such as water, air, industrial gases, and chemicals. Be careful because using variable area flowmeters on dirty fluids, opaque fluids that do not pass light, or fluids that coat the glass, can plug the flowmeter or block view of the float, both of which can render the flowmeter inoperable.
Variable area flowmeters can be applied to relatively clean liquids and gases that do not coat the measuring tube or float. For local indication, the fluid should be clear and clean enough so the float could be seen. Variable area flowmeters are available in sizes up to approximately 3 inches with materials of construction that include stainless steel.
Variable area flowmeters are commonly used to provide cost-effective local indication of small liquid or gas flows, such as for purging in the mining, mineral processing, pulp and paper, petroleum, chemical, petrochemical, and wastewater industries. Fluid applications include water, deionized water, compressed air, chemicals, paint, lubrication oil, and brine. They are also incorporated as internal parts of other instruments and sampling systems to verify and adjust system flow rates.
Do not apply variable area flowmeters to fluids that are opaque, dirty, or prone to coat the metering tube or float, because these may render the flowmeter inoperable. Be sure to install variable area flowmeters with floats in the vertical orientation because their operation is dependent upon gravity. Variable area flowmeters that require upward flow may not suitable in many applications where the fluid flows using only gravity.
A potential safety hazard can be created if a glass metering tube breaks, especially when dangerous fluids are present in the flowmeter. Be careful to install variable area flowmeters with glass metering tubes in locations where the glass cannot be damaged. Also, the float can get stuck when flow turns on suddenly or when high flow rates cause the float to reach its highest mechanical position.