What is a Rotameter : Working & Its Applications

The first rotameter including turning float was invented by Karl Kueppers in Aachen in the year 1908. After that, this invention was improved through new float shapes & the glass tube. Karl Kueppers designed the special model within the glass tube to recognize a symmetrical flow scale. The rotameter is also known as the variable area flow meter.

The Rotameter brand name was listed by the British company namely “GEC Elliot automation. Similarly in Germany, this brand name is registered by Rota Yokogawa GmbH & Co. KG. So at present, it is owned by Yokogawa Electric Corp. This article discusses an overview of a rotameter and its working with applications.

What is a Rotameter?

Rotameter is a reliable, simple, and low-cost flow measuring instrument. So this instrument is used to measure the flow rate of a liquid or a gas. This meter includes a tapered tube through a moving internal float. Sometimes, rotameters are also called gravity flow meters, mechanical flow meters or variable area flow meters. In gravity flow meters, the term gravity means, the rotameter must be installed vertically. The rotameter would look as shown below.


Rotameters’ working principle is to measure the flow rate of liquid because the liquid supplies throughout a tapered tube. Once the liquid supplies throughout the tube then its cross-sectional region will become higher.

Rotameter Construction

The rotameter construction can be done by using different components like transparent tube, scale, float, and transmitter.

Rotameter Construction
Rotameter Construction
  • In the construction, the shape of the transparent tube is conical which includes a scale and floats within it. This transparent tube is very helpful in observing the measurements directly.
  • A float is a small device within the tube of a rotameter with precise dimensions. The float is used to indicate the rate of flow of liquid within the tube, which can be designed with plastic, glass, or metal.
  • The scale on this meter displays the flow measurements by showing with float.
  • Transmitters are very helpful in recording the flow measurements precisely as compared to we observer on the scale directly.
  • It includes a float and a tapered tube where the float is arranged within the tapered tube. At both ends of the device, nets are arranged through a pipeline using flanged connections. Rotameters are connected vertically always in the pipelines and a scale is available on the tube to check the flow rate values directly.

Rotameter Working

In a rotameter, liquid flows from the base of the tapered tube, after that some of the liquid directly hits the bottom of the float & the remaining liquid will flow away from the float. So, the float in the rotameter experiences two forces in the reverse direction, the gravitational force is a downside, and the drag force is on the upside.

The flow of liquid pushes the float up from gravity. After some time, the flowing region achieves a position wherever the force induced on the floating body accurately matches the float’s weight. So the float will discover equilibrium once the area in the region of float produces sufficient drag which is equivalent to weight.


When both the gravity & float weight are stable, then the distance float moved upward is relative to the flow velocity of the liquid which is flowing throughout the tapered tube.


The following parameters need to consider while selecting rotameters.

  • Flow rates are like the volumetric flow rate of fluid and gas.
  • Operating pressure
  • Temperature of liquid
  • Diameter of pipe
  • Indicating type of scale
  • Requirements of valve

Types of Rotameter

These are available in three types which include the following.

Rotameter Types
Rotameter Types
  • Glass Tube Flowmeters
  • Armoured Purgemeter
  • Flanged Armoured Rotameter
  • Glass Tube Purge & Low Flow Meters
  • Glass Tube Rotameter

Glass Tube Flowmeter

The glass tube rotameter is the most common type of meter, so widely used in industries, pilot plants & labs to calculate flow rates of different liquids and gases. These devices monitor natural gas supply into furnaces & ovens, cooling liquids to guard equipment, sampling systems within laboratories & flow rate of water in & out of huge tanks.

The Glass tube rotameter example is 10A4500 which is accessible through flanged connections or NPT and it includes a protective shield & snap-in tube.

Armoured Purgemeter

Armoured purge meters can clean the liquid if the system condition is not good. This rotameter is used for high pressure, corrosive & low flow rate-based applications.

These are used in various sampling systems, gas analyzer systems, and where glass tubes are not suitable for safety reasons. The 10A3200 is an example of armoured purge meter, available with NPT threads & needle valve option.

Flanged Armoured Rotameter

The flanged armoured rotameter is used within automated systems wherever the applications need quite opaque fluids at forceful conditions. So it is applicable mostly in high-pressure-based applications.
These rotameters are very helpful in measuring the opaque, aggressive & non-conductive liquids usually in petrochemical, chemical, pharmaceutical-based industries within high-pressure-based applications.

An example of the flanged armored type is FAM54 including flanged connections. It can be used with transmitter, optional alarms, HART communications, totalized pulse output & configurable digital display.

These devices are also available as a PTFE lined meter, metal tube meter otherwise a heat traced meter used for fluids that respond to temperature.

Glass Tube Purge & Low Flow Meters

Glass tube purge rotameters are suitable for low flow rate applications like instrument enclosures & purging control lines. The burgomaster is outstanding for liquid sampling, specific gravity, measurements of its level & other less flow of gases & liquids applications.

The Glass tube purge meter example is 10A6130 which includes a protective shield, needle valve & ball check valve at the inlet otherwise outlet.

Glass Tube Rotameter

A glass tube rotameter is mainly used to shut down the heavy equipment automatically once the bearing lubricant supply becomes very low, electrical equipment shuts down once the flow of cool water drops under a fixed limit. An example of a Glass tube rotameter is 10A2235 which is available with a hollow bronze body, unique piping arrangements & NPT threads.

Difference b/w Rotameter vs Flow Meter

A rotameter is a flow meter used to measure the volumetric flow of fluids & gases. There is no difference b/w a rotameter & a flow meter because these two terms are utilized interchangeably.

Rotameter Equations and Derivations

The Rotameter can be arranged vertically where the base of the tube is thin and gets broad when the top is reached. The flow begins from the base and moves up to the place where the float’s weight balances the strength used by the liquid flow. If the fluid stays lower as compared to the speed of sound, the incompressible Bernoulli’s theorem equation is applied simply as equilibrium on the rotameter system.

1/2 V2/g + z+p/pg = C

In the above equation,

‘g’ is the gravitational acceleration

‘V’ is the fluid’s velocity

‘z’ height on top of an arbitrary origin

‘C’ is constant

First, we need to set the above equation to zero as all powers are balanced on the rotameter once the float is fixed.

ρg[1/2 V2/g + z+p/pg] = 0

1/2 V2/g + z+p/pg = 0

After that, compare for points at top (b) & bottom (a) of the liquid float.

1/2 ρV22/g – 1/2 ρV12z+ ρgz2- ρgz1 = p1-p2

The above equation can be simplified to

Δρ = ρghf + 1/2 ρVb2 + [1-(va/vb)2]

In the above equation, ‘f’ denotes the properties of the float where the ‘hf’ is the float’s height. The volumetric liquid flow rate is similar at the top & the bottom of the liquid float, so

Q =  VaAa = VbAb

In the above equation, ‘Q’ is the volumetric supply, ‘V’ is the volumetric flow & ‘A’ is the area.

From the above equation, Vb = Va [Aa/Ab] = Q/Ab

Substitute this ‘Vb’ value in Bernoulli’s equation.

Δp = ρghf + 1/2ρ [Q/Ab]2[1-(Ab/Aa)2]

From the above equation, for solving ‘Q’

[Q/Ab]2 = 2 [Δp – ρghf]/ρ[1-(Ab/Aa)2]

Q = [√2 [Δp – ρghf]/ρ[1-(Ab/Aa)2]] Ab

The change within pressure can be found to be typically from the weight of the float.

Δp = F/A

Δp = Vf (ρf-ρ)g/Af

In the above equation, ‘Vf’is the amount of the float, ‘ρf’ is the float’s density & ‘fA’ is the region of the float.

Ideal inviscid liquids would allow the equation of flow which is shown above, however, the energy can be changed into heat. Generally, lowers the actual velocity of the liquid. The viscosity of the fluid can be accounted for using the discharge coefficient (C).

Advantages and Disadvantages

The advantages of a rotameter include the following.

  • Inexpensive
  • The pressure drop is low.
  • Good rangeability.
  • It is appropriate for small flow rates.
  • It is simply used with switches, alarms, or any transmitting devices.
  • By using these meters, corrosive liquids flow rate can be measured.

The disadvantages of rotameter include the following.

  • It must be connected in vertical
  • Handling the glass tube is difficult.
  • It is not apt for pulsating services.
  • It is restricted to little pipe services.
  • It is restricted to low temperatures.
  • Its accuracy is ± 1/2 to 10%.
  • It uses in-line mounting.

What is a rotameter used for?/ what is rotameter Applications ?

The applications of rotameter include the following.

  • Rotameters are used in different industries.
  • These are used where there is a quality air supply is required.
  • This is used where low pressure is necessary.
  • Used in laboratories.
  • Process and oil industries.
  • Used in the medical field for measurement of oxygen flow rate.


The characteristics of the rotameter include the following.

  • Linearity
  • Accuracy
  • Pressure Drop
  • Differential
  • Corrosive
  • Capacity
  • Minimum Piping Requirement

What are the main parts of the rotameter?

The main parts of a rotameter include metering tubes and floats.

What is rotameter used to measure?

Rotameter is used to measure the flow rate of liquids & gases within a closed tube.

How does a rotameter measure flow?

The method used to measure the liquid flow can be achieved through a moving float within a tapered tube.

Why rotameter is installed vertically?

This device must be installed vertically because, in the fluid stream, the float must be in the center.

What is annulus in rotameter?

In a rotameter, the annulus is the area for the flow of fluid which is formed between the wall of the tube & float.

Which is the device for measuring flow velocity?

A Pitot-tube is used to measure fluid flow velocity

What is the discharge coefficient of the rotameter?

The coefficient of the rotameter mainly depends on the location of taps. The discharge coefficient (cd) changes significantly by changing in area ratio & the Reynolds number. So the standard discharge coefficient value is 0.60, but it changes obviously at low Reynolds number values.

Thus, this is all about an overview of a rotameter and its working with applications. It is a flow meter, used to measure the volumetric flow rate of a fluid or a gas. Sometimes, these are also called variable area flowmeters, mechanical flow meters & gravity flowmeters. Here is a question for you, what are the drawbacks of a rotameter?

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