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מדי זרימה מסוג מצוף (רוטמטר)

What are Variable Area Flow Meters?

How they work, where they excel, advantages, and limitations.

How they Work:

Known also as rotameters, or float-type meters, variable area technology has been around for more than 100 years. It is by far the most commonly quoted, sold, and installed flow meter in the world. They can be used to measure the flow rate of either gas or liquid in a tube. They are comprised of a few simple parts, a tube, a float, the scale, fittings, and any outputs or switches or protective external elements.

The theory of operation is straight-forward. There is a float that is able to move freely up and down within the flow tube. The bouyancy force acting on the float, the flow resistance, and the float weight are all in equilibrium. Guide rods are sometimes used to stabilize the float. Guides are common for gas meters or large flow liquid meters.

The float bouyancy is dependent on the density and mass of the float as well as its materials of construction and volume. The float must have a higher density than the media or it will move to the top even when there is no flow, resulting in an error of measurement. Essentially, the float must not float on top of the media.

Advantages:

Linear scale
Typically cost less than other types
Simple, reliable design
Easy to install
Minimal to no straight run requirements
Easy start-up and minimal maintenance
Variety of materials possible
No external power needed
Good accuracy for low to medium viscosity media
Repeatable measurement
Can be used with corrosive liquids if compatible build material is selected
Low pressure drop that is usually constant
Visual flow indication with transparent tubes
Multiple instruments can be installed on a single line without significant pressure loss

Considerations:

Important to pick a meter that will operate in 60% to 80% of the meter’s range most of the time
Armored versions allow for use in extreme conditions

Limitations:

Usually limited to a 10:1 turndown
Cannot tolerate coating or crystalizing
Most do not handle large solids or debris
Has defined operating conditions
Changes in SG, pressure, and viscosity cause significant reading errors
Must be calibrated for density, viscosity, and compressed gases
Orientation must be vertical installation, with flow from bottom to top in most cases
Non-transparent liquids obscure reading the float
Not good for pulsating flow
Without protection, glass/plastic tube models can break