A Flow Measurement Primer - Variable Area Flowmeters    

Variable Area Flowmeters, sometimes referred to as Rotameters, employ a float in a vertically mounted, tapered tube.  Process Fluids/Gasses enter the tube from the bottom and exit at the top and in doing so, lift the float with the height of the float being directly proportional to the flow rate. The greater the flow, the higher the float is raised. 

Construction/Design Principle:
VA Flowmeters are constructed of a uniformly tapered flow tube (Glass, Metal or Plastic), a float (most commonly made from metal or glass) and a measurement scale (either etched directly on the metering tube or on a detached plate to the side of the tube. 

With liquids, the float is raised by combination of buoyancy of the liquid and velocity head of the fluid. Buoyancy is negligible with gases and the float responds to the velocity head alone. The float reaches a stable position in the tube when the upward force exerted by the flowing liquid or gas equals the downward gravitational force exerted on the float.

Because the float responds to gravity, the meter must be mounted in a vertical position with the smallest diameter end of the flow tube at the bottom.  The float, designed such that it's diameter is nearly identical to the tubes diameter at the inlet (bottom), is placed within the tapered tube.  As the process fluid (gas or liquid) begins to flow, the float is raised from it's resting position at the inlet, allowing the fluid to pass between it and the tapered wall of the tube. 

As it rises, an increasing amount of fluid is allowed to pass between the tube's wall and float because the increasing diameter of the tapered tube.  When an equilibrium is reached, the entire volume of flow can physically pass between the wall and the now resting float. By noting the position of the float relative to the reading on the scale, the operator is given an indication of the process flowrate.  Generally, the reading is taken at the center of the float, but may vary by manufacturer.  

Process Data required for sizing: 

NOTE: Generally, published sizing tables for VA Meters are based on Air @ 70°F and 14.7 PSIA or Water.  Adjustments must be made for process fluids with differing characteristics or process conditions.  To make the conversion from Air or Water @ STP (Standard Temperature and Pressure) the sizing engineer will need to know the following:

·         Gas or Liquid – Name and Properties  
1.     Specific Gravity  
2.     Viscosity  

·         Process Temperature

·         Back Pressure across the meter (required for gas sizing).  
1.     With Integral Valve on Inlet   – Downstream Pressure  
2.     With Integral Valve on Outlet – Upstream Pressure

Advantages:   

Inexpensive

Ease of operation
Virtually Maintenance Free

Long Life
No power required 
(For basic - read only meters)		 Variety of available materials allows for use in applications with a wide range of chemical compatibility 

Disadvantages:  

Because variations in temperature and pressure can cause a gas to expand contract, process conditions must remain the same for the meter to true to it's calibrated range.
Local Indication of Rate Only Note: 
Remote Totalization is available by utilizing a meter equipped with a microprocessor transmitter.  In this scenario, the signal originates when a magnet  (embedded in the float) passes a magnetic sensor mounted on the transmitter. This allows for the both a local indication of rate of flow on the meter scale and remote display of all flow data (rate and total). 

Typical Applications: 
Variable Area Flowmeters are well suited for a wide variety of industrial applications

  • Measuring water and gas flow in plants or labs
  • Monitoring chemical lines
  • Purging instrument air lines
  • Monitoring filtration loading
  • Monitoring flow in material-blending applications
  • Monitoring hydraulic oils

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Copyright © 2002 by Cross Instrumentation 
Specifications subject to change without notice - 10/01/02
 NOTE: The information provided above  has been compiled and published as a convenience to our customers. All technical designs, advice, and recommendations, including but not limited to, that appearing on this document are rendered free of charge. As a result, Cross Instrumentation disclaims any responsibility or liability to the buyer or user of this information for any result obtained or damages incurred by reason of the use or application of the design or other information contained herein.