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Lake Monitors - Variable Area Flow Meters
15 Sep 2011

Variable Area Flow Meters

Lake Monitors offers a rugged line of flow rate gauges, alarms/switches, transmitters and hydraulic system analyzers. The basis for our product, the sharp-edged, variable-area, measurement method, provides accurate and repeatable flow rate measurements for both liquids and gases.

Flow Meter

Flow Meter Operating Theory

Enclosed within a high pressure casing (A), a high strength magnet (F) in tandem with the sharp-edged annular orifice disk (E) is pressed towards the zero flow rate position by a linear rate compression spring (G). A tapered metering pin(D) is positioned concentrically within the annular orifice disk and provides a variable-area opening that increases by the square of linear displacement of the orifice disk. Fluid flow creates a pressure differential across the orifice disk, pressing the magnet/orifice disk duo against the compression spring. Flow rate is hellorolex.io read by aligning the magnetically coupled follower (C) with the graduated scale located within the environmentally sealed window (B). The variable-area orifice design provides pressure differential and orifice displacements that are linearly proportional to fluid flow rate.

  • Lake Monitors' sharp-edged orifice provides a more reliable and accurate reading in applications where fluid viscosity varies.
  • The unique design of the monitor allows installation in any piping orientation.
  • The high-strength magnetic coupling between internal and external components eliminates mechanical seals and linkages that can fail.
  • With more than 20 different port options and three materials of construction, Lake Monitors has the correct product to match your system requirements.
  • Most Lake variable area meters are backed by a five year parts and labor warranty against all defects in materials and workmanship.


Variable-area flow rate monitors, often referred to as rotameters, measure flow rate of a liquid or gas by relating linear displacement of an internal "float" or sharp-edged orifice plate (Lake Monitor) to a corresponding flow rate.

Two common variable-area designs are shown in Illustration 1A and 1B. As flow rate increases, the orifice area that the flow moves through also increases - thus, the term variable-area. Variable-area monitors either allow flow through a peripheral orifice formed between a tapered wall and a float as in the traditional rotameter (Ill. 1A), or an annular orifice and an internal tapered metering pin (Ill 1B). The increase in orifice area is proportional to the square of flow rate as depicted in Graph 1.

The non-linear increase in orifice area compensates for, what would be, a non-linear increase in the pressure differential characteristic in fixed orifice monitors. The resultant attributes of the variable-area monitor are a linear relationship between flow rate, pressure differential and piston displacement. Flow rate is read on variable-area meters by aligning the position of the piston/float to a calibrated scale adjacent to the piston/float.


variable-area designs



variable-area monitor

Compression Springs vs. Gravity:

It is also evident from Illustration 1A and 1B that two methods are used to urge the piston/float to the zero flow rate position: the rotameter example utilizes gravity and the Lake Monitor uses a compression spring. The advantage to using gravity as a return force is the repeatability of gravity, and the relatively low pressure differential that is developed by its force as it pushes the float back towards the zero flow rate position. The disadvantage of using gravity is that monitors are limited to vertical, inlet-side-down mounting only.

The compression spring technique provides a pressure differential that is relatively high, but allows the meters to be mounted horizontally or inverted. As a result, the compression-spring return meters (Lake Monitors) are typically only used when system pressures exceed 5-10 PSIG (pounds per square inch gauge - pressure).

traditional rotameter vs Lake Monitor

Annular vs. Peripheral Orifices

The other significant difference between the traditional rotameter and the Lake's Flow Meter is the design of the fluid path in which the measured fluid is forced to take. Rotameters form a peripheral orifice between a tapered wall and a float, while Lake's provide an annular orifice between a tapered metering pin and a sharp-edged orifice plate.

The sharp-edged orifice design has the advantage of having a much smaller parallel orifice area than the peripheral design and thus a smaller area for shear forces to act upon. The shaded area in Illustration 2represents the parallel orifice area in which the fluid flows. The following equation mathematically models the effects of viscosity and orifice area:

Shear force in the orifice/float = nAv/L 
(Physics for Scientists and Engineers; McGraw-Hill 1979)


n = the viscosity of the fluid
A = the parallel orifice area
v = the velocity of the fluid
L = the distance between the components in which fluid flows

(Note that the fluid viscosity (n) and parallel orifice area (A) are directly proportional to shear forces exerted on the orifice. This force is not related to flow rate and causes inaccurate readings.)

This minimal orifice area characteristic of the Lake Monitor's flow meter is very important when changes in fluid viscosity can be encountered, as the force exerted on the piston/float assembly due to the viscosity of the fluid is directly proportional to the parallel area of the orifice.

Additional Operating Pressures

To satisfy system and media requirements, variable-area flow meters are available in numerous materials of construction including: plastics, aluminum, brass, stainless, glass and cast iron. Operating pressure ratings range from a maximum of 6000+ PSIG for steel monitors, to less than 50 PSIG in many plastic and glass monitor designs. The typical accuracy of these monitors ranges from ±0.5% to ±10% of full-scale, with repeatability within ±1% of full scale. Lake Monitors has the ability to sense and electronically output the position of the piston inside of the monitor. The purpose of doing this is to provide flow rate switch points and flow rate proportional analog output signals.

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