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Six Questions to Ask about Swagelok Variable Area Flowmeters

by Katie Dennis, on Wed, Apr 15, 2015 @ 08:04 AM

Some people call them rotameters, but either way they are simple and effective


The M Series (left) and the G Series (right) are highly accurate measurement devices with individually calibrated scales based on flow tests. 

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Some readers of this blog may be old enough to remember gasolines pumps that had a small glass dome with balls inside .As the gasoline flowed through the pump, it would make the balls spin, giving the customer a visual confirmation that the pump was delivering the goods.

We have a sophisticated tool that works on a similar principal. Some people call them variable area flowmeters. Other people call them rotameters. Either way, it’s a simple device to visually check the flow of fluid through a system.

We start with a slightly flared, vertical tube. Inside is a buoyant ball. Fluid enters the bottom of the tube and flows out through the top. Along the way it pushes the ball upward. Gravity pushes the ball downward. Because the tube is tapered, the higher the ball rises, the more space there is between the ball and the tube wall. The wider gap lets more fluid flow around the ball. When the upward and downward forces are balanced, the ball stays in one place. A scale next to the glass tube indicates the rate of flow at that position.

Because the system fluid is doing all the work, the variable area flowmeter doesn't need any external power. And because it doesn't significantly restrict flow, it causes minimal pressure drop.


Our rotameters are made by Krohne, a German company known for their high quality. They make several models:

  • The G Series features the basic glass tube, good for lower-pressure applications. In general, the longer the tube, the more accurate the reading, because the scale is spread over a longer length.
  • The M1 Series uses a metal tube and a magnetized float that moves an external needle.
  • The M2 Series is similar, but comes with a digital readout option.
  • The M3 Series has bigger metal tubes, and can handle higher flow rates. 

Six questions

Here are the six questions to answer when you want to order a rotameter. Hand the answers to your Edmonton Valve & Fitting representative, and it will be easy to supply what you need.

  1. What is the fluid you will be measuring? Every variable area flowmeter is specifically calibrated to a particular fluid. Some are calibrated for air and water, but most are custom-calibrated.
  2. What is the fluid density?
  3. What is the fluid viscosity?
  4. What is the typical operating pressure? Changing pressure can make a fluid more dense, which means you've changed the viscosity. Process conditions must stay stable for a rotameter to work.
  5. What is the typical operating temperature? A change in temperature likewise can change viscosity. Again, stable process conditions are essential.
  6. What measurement range do you want? All of our variable area flowmeters have a range of 10 to 1. That is, the maximum measured pressure can't be any more than 10 times the minimum measured pressure.

Now that you know how a variable area flowmeter works and the six questions to answer, you are ready to call Edmonton Valve & Fitting and get the model you need. /ask

Additional resources

In a hurry or have a question? Please click here to get in touch - we respond fast! Or call 708.437.0640


Topics:Measurement Devices

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