FAQ
  •  How does a rotameter work?
  •  Where do I take the reading?
  •  What is the difference between correlated and direct reading rotameters?
  •  What if I use a gas or liquid other than water or air?
  •  Can I use a rotameter in a vacuum application or with back pressure?
  •  Can I use a rotameter in a vacuum application or with back pressure?
  •  Can I use one flowmeter to measure different flow rates?
  •  Must a rotameter be mounted vertically?
  •  What are the advantages of using a variable area flowmeter?
How does a rotameter work?
Rotameters, or variable area flowmeters, 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.
Where do I take the reading?
With the CM flowmeters, the reading is taken at the top of the cone-shaped float, and at the center of ball-shaped floats. It is recommended that the float be at eye level to minimize reading errors.
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 it is more convenient than a correlated flowmeter, a direct reading flowmeter is less accurate and 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.
What if I use a gas or liquid other than water or air?
What if I use distilled water? If you have a correlated flowmeter, details of the tube number and type of float can be provided to get a correlation chart for the gas in question. For distilled water, use the correlation chart for water.
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.
Can I use one flowmeter to measure different flow rates?
Yes. If a correlated flow tube is used, different flow rates can be attained by using different floats, i.e. carboloy, stainless steel, glass, or sapphire.
Must a rotameter be mounted vertically?
Generally, rotameters must be mounted vertically, because the float must center itself in the fluid stream. At high flow rates, the float assumes a position towards the tip of the metering tube and at low flow rates positions itself lower in the tube. Some of our rotameters have spring loaded floats and therefore may be mounted in any orientation.
What are the advantages of using a variable area flowmeter?
Inexpensive somewhat self-cleaning no power required available in different materials for chemical compatibility Low and constant pressure losses Suitable for very low flow rates Rangeability 10:1 Capable of measuring fluids of varying density and viscosity (compensation given by float design).

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