A variable area flow meter is a type of differential pressure flow meter. Variable area flow meters are simple and versatile devices that operate at a relatively constant pressure drop and provide flow measurement of liquids, gases, and steam. The variable area flow meter is popular for industrial flow indication because it has a linear scale, a relatively long measurement range, and low pressure drop and they are quite simple to install and maintain. Variable area flow meters, like all other types of differential pressure flow meters, function on the principles of the Bernoulli equation, which states that as the flow of a fluid increases, pressure loss occurs.
The position of the variable area flow meter’s in-line float, piston, or vane is changed as the increasing flow rate opens a larger flow area to pass the flowing fluid. The position of the float, piston, or vane provides a direct visual indication of flow rate. Design variations include the rotameter (a float in a tapered tube), orifice/rotameter combination (bypass rotameter), open-channel variable gate, tapered plug, and vane or piston designs.
Either the force of gravity or a spring is used to return the flow element to its resting position when the flow lessens. Gravity-operated meters (rotameters) must be installed in a vertical position, whereas spring operated ones can be mounted in any position. All variable area flow meters are available with local indicators. Most can also be provided with position sensors and transmitters (pneumatic, electronic, digital, or fiberoptic) for connecting to remote displays or controls.
The variable area flow meter’s operation is based on the variable area principle: fluid flow raises a float in a tapered tube, increasing the area for passage of the fluid. The greater the flow, the higher the float is raised. The height of the float is directly proportional to the flow rate. With liquids, the float is raised by a combination of the buoyancy of the liquid and the velocity head of the fluid. With gas flow, buoyancy is negligible, and the float responds to the velocity head alone. The float moves up or down in the tube in proportion to the fluid flow rate and the annular area between the float and the tube wall. The float reaches a stable position in the tube when the upward force exerted by the flowing fluid equals the downward gravitational force exerted by the weight of the float. A change in flow rate upsets this balance of forces. The float then moves up or down, changing the annular area until it again reaches a position where the forces are in equilibrium. To satisfy the force equation, the variable area flow meter float assumes a distinct position for every constant flowrate. However, it is important to note that because the float position is gravity dependent, variable area flow meters must be vertically oriented and mounted.