Measurement of Pipe Flow Rate with a Rotameter Flow Meter

written by: Harlan Bengtson • edited by: Swagatam • updated: 3/19/2010

Values of pipe flow rate are needed for many purposes. Pipe flow measurement can be done with various types of flow meter, including the rotameter, which is covered in this article. The principle of operation and advantages and disadvantages of the rotameter for pipe flow measurement are discussed.

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Introduction

Many fluids are transported in pipes and their flow rate needs to be monitored for many applications. There are quite a variety of types of flow meter for pipe flow measurement. One of them, the rotameter, is being considered in this article. The term pipe flow is being used here, and indeed circular pipes are the most common method for conveying fluids, but most types of pipe flow meter, including the rotameter, can be adapted for use with pressure flow of fluids in conduits of cross-sectional shape other than circular. This could include rectangular shaped conduits or the annular shape for flow in a double pipe heat exchanger.

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General Description of the Rotameter

A rotameter, also sometimes called a 'variable area' flow meter, is simply a tapered plastic or glass tube with a float whose position in the tube is determined by the flow rate of fluid through the rotameter. The diagram at the left illustrates this with the tapering of the tube accentuated. Pictures of a couple of rotameters are shown  at the right.

As a fluid flows through the rotameter at a greater flow rate, a larger cross-sectional area of flow is needed, so the 'float' rises until it reaches an equilibrium position where the upward force on it by the fluid is the same as the downward force of gravity. Note that the density of the 'float' must be greater than the density of the fluid or it would simply float to the top of the fluid at any flow rate. When used for pipe flow measurement with a particular fluid, the position of the float in the tube can be calibrated to read flow rate directly for that fluid.

Because of the way that gravity helps to determine the float position, a rotameter must be installed vertically, as shown in the diagrams. Also it should be mentioned that a rotameter works well for visually noting flow rate readings, but it doesn't lend itself well to machine reading and continuous recording of flow rate.

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• A rotameter doesn't require power, so it is safer to use with flammable fluids than a flow meter that does use electrical power.
• A rotameter is flexible in the type of applications it is suited for. It can be used for clean and relatively dirty liquids and gases.
• The cost of a rotameter is low in comparison with most other types of pipe flow meter.

• Typical accuracy for a rotameter is 1% to 10% of full scale. Several other types of flow meter have typical accuracy better than this.
• As noted above the rotameter doesn't work well for machine reading or continuous recording of flow rate.

Neutral Rotameter Characteristics:

• Pressure drop for flow through a rotameter is medium. Some other types of pipe flow meter have lower pressure drop and some cause a higher pressure drop than a rotameter.
• The effect of fluid viscosity on the operation of a rotameter for pipe flow measurement is medium. The effect of fluid viscosity is less for some types of pipe flow meter and it is greater for some.
• The typical turndown ratio for a rotameter is 10:1, that is: a flow rate as low as one tenth of the full scale reading can be measured accurately. This mid-range for flow meter turndown ratio. Some types of flow meter have a higher typical turndown ratio and some have a lower value for this parameter.
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Image Credit

Rotameter picture: http://www.magicsoil.com/MSREV2/oxygen_realities_in_compost.htm

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