A rotameter is a flow meter device that measures the volumetric flow rate of a gas or fluid.
It consists of a tapered tube with a moving internal float.
A rotameter is a device that measures the volumetric flow rate of fluid in a closed tube.[1]
It belongs to a class of meters called variable-area flowmeters, which measure flow rate by
allowing the cross-sectional area the fluid travels through to vary, causing a measurable
effect
Rotameters are also known as variable area flow meters.
Variable area flow meters work by measuring the flow rate as the fluid travels through a
tapered tube where the cross sectional area of the tube gradually becomes greater as the
fluid travels through the tube.
The term rotameter comes from the early version of the floats, which had slots to help
stabilize and center them within the fluid flow.
This caused the floats to rotate. Current float designs are a variety of shapes (spherical for
example) and are constructed of stainless steel, glass, metal, and plastic.
Rotameters are a type of flow meter. As the name indicates, a flow meter measures the flow of
gases and liquids through machinery.
Such measurements are often essential to maintaining the proper function of many machines.
Having too much or too little flow can cause malfunctioning and even destruction of certain
machines.
That is why apparatuses like rotameters are so important.
A rotameter is composed of three main parts:
Flow tube
Float
Measuring scale
The flow is measured by the gas or liquid lifting the float within the tube.
The weight of the float is supported by the force of the fluid moving through the apparatus and
provides a reading on the measuring scale.
A rotameter consists of a tapered tube, typically made of glass with a 'float' (a shaped weight,
made either of anodized aluminum or a ceramic), inside that is pushed up by the drag force of
the flow and pulled down by gravity.
A higher volumetric flow rate through a given area increases flow speed and drag force, so
the float will be pushed upwards.
However, as the inside of the rotameter is cone shaped (widens), the area around the float
through which the medium flows increases, the flow speed and drag force decrease until
there is mechanical equilibrium with the float's weight.
Rotameters have a graduated scale on the the side to allow the operator to take
measurements.
It's critical that the device is calibrated to a specific fluid with a known specific gravity since
specific gravity can have great impact on the accuracy and reliability of the rotameter.
An example of calibration fluid is water with a specific gravity of 1.0.
Rotameters can be calibrated for a wide range of fluids by understanding basic operating
principles.
The accuracy of a rotameter is determined by the accuracy of the pressure, temperature,
and flow control during the initial calibration.
Changes to the density and weight of the float, as well as changes which affect the fluid such
as pressure or temperature, will impact the rotameter reading.
No external power required – Rotameters are mechanical devices which do not require
power to provide flow measurement. This allows rotameters to be installed in hazardous
areas and remote areas where it would be expensive to supply power.
You can see the process – Customers not only get a flow measurement reading but a look into
their process. Is the process dirty or cloudy looking which could mean filters need to be
changed? Is the process the correct color, are their bubbles in the liquid.
Rotameters are cost effective – Both rotameter price and function contribute to savings on
the job. Rotameters can be installed with other flow measurement technologies and be used
to complement each other at an economical price.
Simple to install and maintain – Rotameters are quickly installed by connecting the process
line to the inlet and the outlet of the rotameter. Make sure the meter is vertical and you are
now ready to measure flow.
Low pressure drop – Most small rotameters have only a few inches of water column pressure
drop. This means rotameters can be installed in many places in the process. Small pressure
drops mean smaller pumps!
Repeatability – Given the same process conditions a rotameter will accurately repeat the flow
measurement day after day.
A rotameter requires no external power or fuel, it uses only the inherent properties of the
fluid, along with gravity, to measure flow rate. [3]
A rotameter is also a relatively simple device that can be mass manufactured out of cheap
materials, allowing for its widespread use.
Since the area of the flow passage increases as the float moves up the tube, the scale is
approximately linear.[2]
Clear glass is used which is highly resistant to thermal shock and chemical action.
Due to its reliance on the ability of the fluid or gas to displace the float, graduations on a
given rotameter will only be accurate for a given substance at a given temperature.
Because operation of a rotameter depends on the force of gravity for operation, a rotameter
must be oriented vertically. Significant error can result if the orientation deviates significantly
from the vertical.
Due to the direct flow indication the resolution is relatively poor compared to other
measurement principles. Readout uncertainty gets worse near the bottom of the
scale. Oscillations of the float and parallax may further increase the uncertainty of the
measurement.
Since the float must be read through the flowing medium, some fluids may obscure the
reading. A transducer may be required for electronically measuring the position of the float.
Rotameters are not easily adapted for reading by machine; although magnetic floats that
drive a follower outside the tube are available
Rotameters are not generally manufactured in sizes greater than 6 inches/150 mm, but bypass
designs are sometimes used on very large pipes
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