Flow Measurement

Rotameter – Characteristics, Components, advantages and disadvantages

In the Rotameter, a weighted float or a plumb bob contained in a vertical conical tube is raised to the equilibrium position between the downward force of the plumb bob and the upward force of the fluid in addition to the floating effect of the fluid flowing past of the float through the annular orifice.


Linearity: The flow rate (volume) through a variable area meter is essentially proportional to the area and, as a result, most of these meters have increments of equal scale. A typical indication rotameter scale is not linear at about 5%.

Differential:An important characteristic of the variable area meter is that the loss of pressure through the float is a constant. The general differential across the meter will increase at higher flow rates due to friction losses through the fittings.

Accuracy: The most common accuracy is ± 2% of full scale reading. This increases considerably with the individual calibration and the length of the scale. The repeatability is excellent

Capacity: Variable-area flow meters are the most commonly used means to measure low flow rates. The large-scale capacities range from 0.5 cm3 / min of water and 30 cm3 standard / min of air in the smallest units to more than 1200 liters / min of water and 1700 m3 / h of air in meters of 8 cm height.

Minimum Piping Requirement: An area meter can usually be installed without taking into account the connections or lengths of straight pipe procedures or following the meter.

Corrosive or difficult to handle liquid: These can often be successfully managed in an area meter. They include materials such as oil, tar, coolants, sulfuric acid, black liquor, beverages, aqua regia and molten sulfur.

Pressure Drop: By placing very light floats on larger gauges, the flow rates can be managed with a combination of very low pressure loss (often 2.5 cm water column or less) and a 10: 1 flow rate

Components of rotameter:

Metering Tubes:

In modern practice, they are formed in a mandrel and annealed to avoid internal stresses, so that strong and uniform tubes are obtained. This method also allows the formation of tubes with greater reproducibility and interchangeability and the formation of special shapes, such as non-conical tubes with curved elements designed to extend the graduations at the lower end of the range.


The floats can be made from various materials to obtain resistance to corrosion or modification of capacity. The classifications are generally in terms of meter capacity, using a stainless steel float.

It has been found that the shape of the float determines to a large extent the way in which a rotameter will be affected by changes in the viscosity of the fluid measured. It has been found that floats having sharp edges are relatively insensitive to changes in viscosity in a range of considerable viscosity.

Advantages of rotameter

  • Low cost.
  • Low pressure drop.
  • Rangeability is better.
  • It is suitable for small flow rates.
  • It is easily equipped with alarms and switches or any transmitting devices.
  • It also measures the flow rate of corrosive fluids.
  • There is an availability of viscosity-immune floats.
  • It can be used in some light slurry services.

Disadvantages of rotameter:

  • It is difficult to handle the glass tube type.
  • It must be mounted vertically.
  • It is not suitable for pulsating services.
  • Generally it is limited to small pipe services.
  • It is limited to low temperatures.
  • Accuracy is = ± 1/2 to 10%.
  • It requires in-line mounting.


Instrumentation Engineer

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