All About Variable-Area Flowmeters and Rotameters

Variable-area flowmeters, or rotameters, operate on the principle that the variation in area of flow stream required to produce a constant pressure differential is proportional to the flow rate. The flowing fluid enters the bottom of the meter, passes upward through a metering tube, and around the float, exiting at the top. The flow rate is read by noting the position of the float against the calibrated scale etched on the glass.

Polysulfone Flowmeter for Water, 0.5 to 5 GPM

Polysulfone Flowmeter for Water, 0.5 to 5 GPM

To learn more about these flowmeters, see the questions and answers below.

What are the advantages of using a variable-area flowmeter?

  • Economical
  • Somewhat self-cleaning
  • No power required
  • Available in different materials for chemical compatibility

What are the limitations?

  • No output for data transmission
  • Sensitive to differing gas types and changes in temperature and pressure

What is the difference between correlated and direct reading rotameters?

A direct reading flowmeter indicates the flow rate on its scale in specific engineering units (e.g. ml/min or scfh). Direct reading scales are designed for a specific gas or liquid at a given temperature and pressure. While a direct reading flowmeter is more convenient than a correlated flowmeter, it is less accurate and is limited in its applications.

A correlated flowmeter is scaled along either a 65mm or a 150mm length, from which a reading is taken. The reading is then compared to a correlation table for a specific gas or liquid. This will give the actual flow in engineering units. One correlated flowmeter can be used with a variety of fluids or gases.

Can I use a rotameter in a vacuum application or with back pressure?

Yes, but if you have a valve, it must be placed at the outlet (top of the flowmeter). This is done by inverting the tube inside the frame, and then turning over the frame. At this position, the tube should read correctly from the original perspective and the valve should be at the outlet, or top of the flowmeter. This allows for proper control of the vacuum.

What are the differences between a 150-mm vs. a 65-mm flowmeter?

A 150-mm flowmeter has a 150 mm scale length and is graduated accordingly. It provides better resolution than the more economical 65-mm flowmeter.

Which float do I have?

Glass floats are black and sapphire floats are red. Carboloy and stainless steel floats both look metallic, but the Carboloy floats are magnetic.

View all variable-area flowmeters.

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