ROTAMETERS Metal Tube Flow Meter


Rotameters are simple industrial flow meters that measure the flow rate of liquid or gas in a closed tube.  Rotameters are popular because they have linear scales, a relatively large measurement range, low pressure drop, and are simple to install and maintain.  Rotameters are a subset of meters called variable area flow meters that measure the flow rate by allowing the fluid to travel through a tapered tube where the cross sectional area of the tube gradually becomes greater as the fluid travels through the tube.  The flow rate inside the rotameter is measured using a float that is lifted by the fluid flow based on the buoyancy and velocity of the fluid opposing gravity pulling the float down.  For gasses the float responds to the velocity alone, buoyancy is negligible.

The float moves up and down inside the rotameter's tapered tube proportionally to the flow rate of the fluid.  It reaches a constant position once the fluid and gravitational forces have equalized.  Changes in the flow rate cause rotameter's float to change position inside the tube.  Since the float position is based on gravity it is important that all rotameters be mounted vertically and oriented with the widest end of the taper at the top.  It is also important to remember that if there is no flow the float will sink to the bottom of the rotameter due to its own weight.

The operator reads the flow from a graduated scale on the side of the rotameter, which has been calibrated to a specific fluid with a known specific gravity.  Specific gravity or the weight of the fluid has a great impact on the rotameter's accuracy and reliability.  All of Global Water's rotameters have been calibrated using water as the standard fluid with a specific gravity of 1.0.

Rotameters can be calibrated for other fluids by understanding the basic operating principles.  Rotameter accuracy is determined by the accuracy of the pressure, temperature, and flow control during the initial calibration.  Any change in the density and weight of the float will have impacts on the rotameter's flow reading.  Additionally any changes that would affect the fluid such as pressure or temperature will also have an affect on the rotameter's accuracy.  Given this, rotameters should be calibrated yearly to correct for any changes in the system that may have occurred.

There are several advantages to a rotameter over a more complicated flow meter including:

· Rotameters can be installed in areas with no power since they only require the properties of the fluid and gravity to measure flow, so you do not have to be concerned with ensuring that the instrument is explosion proof when installed in areas with flammable fluids or gases.

· Rotameters can be installed with standard pipe fittings to existing piping or through a panel.  You do not have to worry about straight runs of pipe as with a magnetic or turbine flow meter.

· Rotameters are simple devices that are mass manufactured out of inexpensive materials keeping investment costs low.

· A glance at a Rotameter acts as a sight glass telling the operator that a filter needs cleaning, that there is some other problem causing discoloration of the water, or that the fluid is actually flowing.  With a transparent rotameter they can instantly see if there is any build up on the float or tube walls.

· With a properly maintained rotameter the operator can expect sustained high repeatability.

· Rotameters offer wide flow measurement ranges or rangeability.  A typical ratio of 10:1 from maximum to minimum flow rate can be expected.  Operators will be able to measure minimum flow rates as low as 1/10 of the rotameter's maximum flow rate without impairing the repeatability.

· The rotameter's scale is linear because the measure of flow rate is based on area variation.  This means that the flow rate can be read with the same degree of accuracy throughout the full range.

· Pressure loss due to the rotameter is minimal and relatively constant because the area through the tapered tube increases with flow rate.  This results in reduced pumping costs.

There are also a few disadvantages to the use of rotameters that you should keep in mind:

· Because gravity plays a key roll in the flow measurement the rotameter must always be installed vertically with the fluid flowing up through it.

· The graduated scale on the side of the rotameter will only be valid for the specific fluid and conditions where it was calibrated.  The specific gravity of the fluid is primary property to consider, however the fluid's viscosity and any temperature changes may also be significant.  Rotameter floats are generally designed to be insensitive to viscosity, but the operator should verify that any rotameters installed in their system are calibrated to their specific setup prior to relying on the flow measurements provided.

· It is difficult for rotameters to be adapted for machine reading, although a magnetic float may be used in some instances.

· Rotameters are typically made of transparent material, however all operators should check the chemical compatibility of the meter with their fluid prior to full installation.

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