Variable Area Flow Meters

Variable area flow meters are offered primarily as reliable, low-cost in-line visual flow rate indicators with adequate versatility to monitor a wide range of industrial liquids and gases.

These simple in-line flow meters utilize the time-tested variable area technology, in which the velocity of the flowing fluid forces the (free moving) float, or piston to shift position, increasing or decreasing the size of the opening (variable area) to allow the fluid to pass.

There are three types of flow meters (Rotameters, Orifice/Tapered Plug Meters and Piston-type), using the variable area metering principle. They offer the industrial user a variety of advantages including:

• Ease of Installation
• No Special Piping (Upstream or Downstream)
• Simple to Maintain
• No Electrical Connections
• Low Pressure Drop
• Operate in Any Position (Piston-type)
• Relatively Insensitive to Shock and Vibration
• Direct Reading
• Reliable Low Flow Rate Indication
• Good Repeatability
• Broad Rangeability (Turndown Ratios)
• Variety of Construction Materials
• Good Viscosity Stability

Rotameter Operating Principle

The Rotameter contains a float that is free to move vertically, within a tapered conical flow tube. As the flowing media moves upward through the annular area between the float and the expanding (tapered) metering tube, the float is lifted to a level of equilibrium at which the weight of the float is equal to the upward force of the fluid. The vertical rise (distance) of the float is proportional to the change in flow rate. The flow rate is indicated by visually checking the position of the float against a graduated flow scale, affixed to the outside of the tapered (transparent) tube.

Orifice and Tapered Plug Principle

This type of flow meter uses a fixed orifice within a vertical meter enclosure. A tapered float (narrow at the bottom) is free to move up and down through the fixed orifice. The flow rate is indicated by observing the position of the float, relative to the orifice.

Piston-type Flow Metering Principle

The Piston-type Variable Area Flow Meter uses a sharp-edged annular orifice, formed between the open-centered piston and a tapered metering cone. The piston is held in a "no flow" position at the base of the tapered cone by a calibrated retention spring. Flow through the meter creates a differential pressure across the piston orifice, moving the piston against the spring. The greater the volume, the further the piston moves.

Externally, the flow indicator ring is magnetically-coupled to the moving piston. A specific line on the indicator ring is visually "read" against a graduated flow scale mounted on the inside of the transparent dust guard.

Variable Area Flow Meters are calibrated for water at 1.0 specific gravity. They are calibrated for oil at 0.876 specific gravity. In selecting a meter for monitoring other liquids, it should be noted that the "indicated flow rate" will read higher for denser liquids and low for liquids of less density. For these applications, a properly recalibrated scale can be used, or a correction factor can be applied to the standard scale. A correction formula is available for liquids from 1.25 to 0.65 specific gravity. The correction formula is the square root of 1.0 / specific gravity for water meters, or the square root of 0.876 / specific gravity for oil meters.

Liquid Selection Chart

Field conversion of...
Flow meter scale to other special fluids

Special fluid flow / Correction factor = Scale reading

Special fluid flow = Scale reading x correction factor (Consult Instrumart for Special Scales)

Flow Meter Selection/Sizing - Air/Gases

The Variable Area Flow Meter is calibrated for air (1.0 specific gravity), in graduated standard cubic feet/minute (SCFM), at 100 PSIG inlet pressure, at 70 F. A standard cubic foot of air is defined as a cubic foot of air at 70 F at atmospheric pressure (14.7 PSI at sea level).

Each flow meter is also furnished with a "Correction Factor Chart" offering quick, convenient formulas to recalibrate the meter for changes in operating pressure, temperature and/or specific gravity.