Construction of Rotameter:
A rotameter consists of a
tapered tube, typically made of glass with a 'float' (made either of anodized
aluminum, ceramic, or plastic), inside that is pushed up by the drag force of
the flow and pulled down by gravity. The drag force for a given fluid and float
cross-section is a function of the square of flow speed. A higher volumetric
flow rate through a given area increases flow speed and drag force, so the
float is pushed upwards. However, as the inside of the rotameter is cone-shaped,
the area around the float through which the fluid flows increases, the flow
speed, and drag force decrease until there is mechanical equilibrium with the
float's weight.
Floats are made in many
different shapes, with spheres and ellipsoids being the most common. The float
may be diagonally grooved and partially colored so that it rotates axially as
the fluid passes. This shows if the float is stuck since it will only rotate if
it is free. Readings are usually taken at the top of the widest part of the
float; the center for an ellipsoid, or the top for a cylinder. The float must
not float in the fluid and it has to have a higher density than the fluid;
otherwise, it will float to the top even if there is no flow. The mechanical
nature of the device does not require any electrical power. If the tube is made
of metal, the float position is transferred to an external indicator via a
magnetic coupling. The measurement can be made remotely from the processor used
for automatic control.
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| Fig.1: Rotameter (I) Schematic (II) Real One Used In Practice |
Working of Rotameter:
In these devices, the
falling and rising action of a float in a tapered tube provide a measure of
flow rate. Rota meters are also known as gravity-type flowmeters because they
are based on the opposition between the downward force of gravity and the
upward force of the flowing fluid. When the flow is constant, the float stays
in one position that can be related to the volumetric flow rate. That position
is indicated on a graduated scale. To keep the full force of gravity in effect,
this dynamic balancing act requires a vertical measuring tube.
Applications of Rotameter:
- The rotameter is used in process industries to measure fluid flow rates.
- It is used for monitoring gas and water flow in plants or labs.
- It is used for monitoring filtration loading.
Advantages of Rotameter:
- It has good accuracy for low and medium flow rates.
- The pressure loss is nearly constant and small.
- It can be used for corrosive fluids.
- It requires no external power or fuel; it uses only the inherent properties of the fluid, along with gravity.
- A rotameter is a relatively simple device that can be mass-manufactured out of cheap materials, allowing for its widespread use, and thus cost is low.
- Since the area of the flow passage increases as the float moves up the tube, the scale is approximately linear.
- Clear glass can be used as this is highly resistant to thermal shock and chemical action.
Disadvantages of Rotameter:
- It is not suitable for opaque fluids as float may not be visible through them.
- Glass tubes may be subjected to breakage.
- As this device is based on the gravitational force it requires to be installed in a vertical position only.
- The calibration scale on the rotameter needs to be accurate for a given substance at a given temperature.
- A separate rota meter is needed for fluids with different densities and viscosities, or are supplied with multiple scales on the same rotameter.
- Readout uncertainty gets worse near the bottom of the scale.
- Oscillations of the float and parallax may increase the uncertainty of the measurement.
- A transducer may be required for electronically measuring the position of the float.
- Rotameters are not easily adapted for reading automatically.
- Rotameters are not generally manufactured in sizes greater than 6 inches (150 mm).