The variable area flow meter is an industrial flow meter used to measure the flowrate of liquids and gases. The variable area meter consists of a tube and float. The float response to flowrate changes is linear, and a 10-to-1 flow range or turndown is standard.
In the case of OMEGA™ laboratory variable area flow meters, far greater flexibility is possible through the use of correlation equations. The variable area flow meter is popular because it has a linear scale, a relatively long measurement range, and low pressure drop. It is simple to install and maintain.
Learn more about variable area meters
Principle of Operation
The 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 flowrate. With liquids, the float is raised by a combination of the buoyancy of the liquid and the velocity head of the fluid. With gases, 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 flowrate 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 flowrate 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 meters must be vertically oriented and mounted.
How to select a variable area flow meter ?
What is the minimum and maximum flow rate for the flow meter?
What is the minimum and maximum process temperature?
What is the size of the pipe?
Would you like a direct reading meter or is a look up table acceptable?
What accuracy do you need?
Do you require a valve to regulate the flow?
Will there be back pressure?
What is the maximum process pressure?
Purge Flow Regulators
If a needle valve is placed at the inlet or outlet of a variable area flow meter, and a d/p regulator controls the pressure difference across this combination, the result is a purge-flow regulator. Such instrumentation packages are used as self-contained purge flow meters. These are among the least expensive and most widely used flow meters. Their main application is to control small gas or liquid purge streams. They are used to protect instruments from contacting hot and corrosive fluids, to protect pressure taps from plugging, to protect the cleanliness of optical devices, and to protect electrical devices from igniting upon contact with combustibles.
Purge meters are quite useful in adding nitrogen gas to the vapor spaces of tanks and other equipment. Purging with nitrogen gas reduces the possibility of developing a flammable mixture because it displaces flammable gases. The purge-flow regulator is reliable, intrinsically safe, and inexpensive.
As shown in the figure, purge meters can operate in the constant flow mode, where P2 - P0 is held constant at about 60 to 80 in H2O differential. In bubbler and purge applications, the inlet pressure (P1) is held constant and the outlet pressure (P0) is variable.The figure describes a configuration where the outlet pressure (P0) is held constant and the inlet pressure (P1) is variable.
They can handle extremely small flow rates from 0.01 cc/min for liquids and from 0.5 cc/min for gases. The most common size is a glass tube variable area flow meter with -in (6 mm) connections, a range of 0.05-0.5 gpm (0.2-2.0 lpm) on water or 0.2-2.0 scfm (0.3-3.0 cmph) in air service. Typical accuracy is ±5% FS over a 10:1 range, and the most common pressure rating is 150 psig (1 MPa).