Rotameter Working Principle

A Rotameter is a form of  Variable area flow meter  which has a simplistic operation one of the oldest and mature principles in flow measurement with its simple design: a float rises inside a conically shaped glass tube as the flow increases and its position on a scale can be read off as the flow rate. Since this measuring method is purely mechanic, it as simple as it is reliable.

As fluid flows upward through the tube, a pressure differential develops across the plummet. This pressure differential, acting on the effective area of the plummet body, develops an upward force (F = PA). If this force exceeds the weight of the plummet, the plummet moves up. As the plummet moves farther up in the tapered tube, the area between the plummet and the tube walls (through which the fluid must travel) grows larger. This increased flowing area allows the fluid to make it past the plummet without having to accelerate as much, thereby developing less pressure drop across the plummet’s body. At some point, the flowing area reaches a point where the pressure-induced force on the plummet body exactly matches the weight of the plummet. This is the point in the tube where the plummet stops moving, indicating flow rate by it position relative to a scale mounted (or etched) on the outside of the tube.

Parts Of Rotameter

The rotameter is a variable-area-type flowmeter. It consists of a tapered metering tube and a float that is free to move up and down within the tube. The metering tube is mounted vertically with the small end at the bottom. The fluid to be measured enters at the bottom of the tube, passes upward around the float, and flows out at the top. show in figure is a representation of a rotameter.

Sizing of Rotameter

To size a rotameter, it is customary to convert the actual flow to standard flow. For liquid flows, it is necessary to calculate the GPM (l/min or l/hr) water equivalent. For gases, it is necessary to determine the SCFM (l/min or l/hr) air equivalent. Capacity tables are based on these standard flows of GPM or cm 3 /min of water and SCFM or cm 3 /min of air at standard conditions.

Rotameter Characteristics

A very wide range of liquids can be handled by the rota meter. A wide choice of tube, float, end fitting, and packing or O-ring materials are available for the particular service being considered. Even liquid metals such as mercury and liquid lead can be metered. Because these metals are denser than the stainless-steel float, they are metered by an inverted rotameter. In this case, the flow is from top to bottom. When the meter is full of the liquid metal but there is no flow, the stainless-steel float is buoyed up by the heavier liquid and rests at the inlet, which is at the top. When there is flow, the flow forces the float down against the net buoyant force, and the float takes a position related to the flow rate.

The rotameter is an inexpensive flowmeter for gas flow measurement. The pressure drop across the meter is essentially constant over the full 10:1 operating range. Pressure drop is low, generally less than 1 PSI (6.89 kPa). Special designs are available for even lower pressure drops.

The position of the float in the metering tube varies in a linear relationship with flow rate. This is true over ranges up to 10:1. Percent of a maximum, universal millimeter, and directreading scales are used. Direct reading scales are convenient; however, they are valid only for the fluid for which they were designed and only at a unique set of pressure and temperature conditions. Universal millimeter scales are used in conjunction with separate calibration charts relating the elevations of the float to flow rates. The advantage of universal scales the capability of a single flowmeter to be used for different fluids at various pressure and temperature conditions. Rota-meters can directly measure flows as high as 4000 GPM (9201/h). Even higher flow rates can be economically handled using

the bypass-type rotameter. The capacity of the rotameter can be changed by changing the float. Various float configurations are available for higher capacities and generally permit a 2:1change in capacity (Figure ). By using the same housing but changing both the metering tube and the float, a gross change in capacity is possible. These changes can be required by both a change in flow rate and a change in fluid density.

Rotameter Types :

Various types of rota meter is there
  • Tapered tube Rotameter
  • Tapered plug Rotameter
  • Piston perforated Rotameter
  • Flexing Vane Rotameter

Rotameter floats Types :

Variation in the shape of rotameter floats. The float on the right is provided with slots, which caused the early floats to rotate (for stabilizing and centering purposes); hence the name “rotameter.”

Standard Design Pressure :

350 PSIG (2.4 MPa) average maximum for glass metering tubes, depending on sizeUp to 720 PSIG (5 MPa) for metal tubes, special designs to 6000 PSIG (41 MPa).

Standard Design Temperature :- 

Up to 400 ° F (204 ° C) for glass tubes and up to 1000 ° F (538 ° C)

Fluids :

Liquids, gases, and vapors

Flow Range :

0.01 cm 3 /min to 4000 GPM (920 m 3 /h) of liquid, 0.3 cm 3 /min to 1300 SCFM(2210 m 3 /h) of gas

Inaccuracy :

Laboratory rotameters can be accurate to ± 0.5% of actual flow; most industrial rotameters will perform within ± 1 to ± 2% of full scale over a 10:1 range, dual float rotameters can detect flow over a 20:1 range, and purge or bypass meters ± 5to ± 10% of full range.

Materials of Construction  :

Tube: Borosilicate glass, stainless steel, Hastelloy  , Monel  , Alloy 20  , PFA, Acrylic, Polysulfone, and polycarbonate. Float: Conventional type— brass, stain-less steel, Hastelloy  , Monel  , Alloy 20  , nickel, titanium, or tantalum, and special plastic floats. Ball type glass, stainless steel, tungsten carbide, sapphire, or tantalum. End Fittings: aluminum, brass, stainless steel, PTFE, or alloys for corrosive fluids. Packing: The generally available elastomers are used and O-rings of commercially available materials; Teflon  is also available.

Rota meter Price :

(6-mm) glass tube purge meter starts at $75. A -in. stainless-steel meter is about $300. Transmitting rotameters in -in. (13-mm) size and with 2.5% error start at about $1200 whereas, with 0.5% of rate accuracy, their costs are over $2500.

Advantages of Rotameter :

  • 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.

Disadvantages to the rotameters :

  • Because gravity plays a key role 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.

Rotameter Technical Specification 

Rotameter Inspection and Test Plan

This content provides you with a sample Rotameter inspection and test plan. This is a draft Rotameter ITP and must be modified based on your purchase order requirements.

TPI InspectionClient
Rotameter Mill test reportsHRR
Rotameter Non-destructive examination, when specifiedHRR
Rotameter Pressure testHWR
Rotameter Performance testHWR
Rotameter Insulation resistance testHWR
Rotameter Final visual / Dimension InspectionHHR
Rotameter Documentation review prior to release(Final activity of Technical Inspection)HHR
Pre-shipment InspectionHHR

H: Hold Point implies that relevant production activities shall not proceed until the continuation of work is permitted by Purchaser. (Notification req’d)

W: Witness point implies that Purchaser intends to witness the designated inspection feature. If Purchaser decides not to witness the relevant feature, production can proceed without permission of Purchaser. (Notification req’d)

S: Witness, but spot check basis. Initial operation will be witness point and subsequent operation will be witnessed at discretion of Purchaser considering the results of previous inspection. (Notification not req’d : Randam Inspection)

R: Review inspection & test records (Notification not req’d)

General Note for Rotameter ITP :

1) Rotameter witness/hold point by Code Inspector, when applied by Code /Local Regulation, is to comply with its requirements and to specify in Vendor’s ITP.

2) Type of Inspection by Rotameter Purchaser may be changed according to equipment criticality and vendor QC evaluation at job stage.


3) When requested in the requisition, detailed Rotameter shall be submitted by Vendor.


4) Monitoring/Observation of Rotameter Vendor shop daily routine works (i.e. Storage of materials ,Adherence to approved procedures, Testing tool calibration check, workmanship, cleanliness and etc.) shall be done by Purchaser during inspection visit.


5) 100% Rotameter inspection which is covered by the combination of witness / spot check / record review inspection & tests listed above and vendor’s original QC activity shall be confirmed by Purchaser’s inspector prior to shipment.


6) Unless otherwise specified in Rotameter REQ., the sampling Q’ty of ” S (Spot Witness Check) ” shall be as follows:

3 to 20→3Pcs(all if total 2 and less) , 21 to 50→5Pcs, 51 to 100→10Pcs,101 to 200→20Pcs, 201 to 300→30Pcs, 301 to 500→50Pcs

Minimum 25 pcs or more of the commodity shall be arranged for H/W/S inspection and test at one time inspector’s visit as much as practical.


7) Final Rotameter Visual Inspection/Documentation Review/Pre-Shipment Inspection may be combined within one time visit.


How to Install a Rotameter

A rotameter, also known as a variable area flowmeter, is a device that measures the flow rate of liquid or gas in a closed tube. It measures flow rate by allowing the cross-sectional area the fluid travels through, to vary, causing a measurable effect. They are a cost-effective flow measuring device that provides excellent repeatability, requires no external power, can be made from a wide variety of materials, and may be designed for high pressure and high temperature applications.

  1. Inspect meter for damage that may have occurred during shipping. Report any damage to the container to the freight carrier immediately.
  2. Make sure your pressure, temperature, fluid and other requirements are compatible with the meter and components (including o-rings).
  3. Select a suitable location for installation to prevent excess stress on the meter which may result from:
    • Misaligned pipe.
    • The weight of related plumbing.
    • “Water Hammer” which is most likely to occur when flow is suddenly stopped as with quick closing solenoid operated valves. (If necessary, a surge chamber should be installed. This will also be useful in pressure start-up situations.)
    • Thermal expansion of liquid in a stagnated or valve isolated system.
    • Instantaneous pressurization which will stress the meter and could result in tube failure. note: In closed thermal transfer or cooling systems, install the meter in the cool side of the line to minimize meter expansion and contraction and possible fluid leaks at the threaded connections.
  4. Handle the meter carefully during installation.
    • Use an appropriate amount of teflon tape on external pipe threads before making connections. Do not use paste or stick type thread sealing products.
    • Over tightening of plastic connections may result in fitting damage.
  5. Install the meter vertically with the inlet port at the bottom.
  6. Meters with stainless steel fittings will support several feet of pipe as long as significant vibration or stress resulting from misaligned pipe are not factors.
  7. Meters with plastic fittings must be installed so that fittings are not made to support any part of the associated plumbing. In addition, meter frame should be fastened to bulkhead, panel or column.
  8. Meters used in gas service should have suitable valves plumbed in at the inlet and outlet of the meter. These valves should be no more than 1-1/2 pipe diameters from the meter ports. The valve at the outlet should be used to create back pressure as required to prevent float bounce. It should be set initially and then left alone. The inlet valve should be used for throttling purposes. Depending on the installation, valves may not be essential, but they are most useful in many installations. Remember: To get a correct reading of flow in gas service, it is necessary to know the pressure right at the outlet of the meter (before the valve).
  9. Pressure and temperature maximums must never be exceeded.

Rotameter Manufacturers :-

Rotameter Manufacturers / VendorsCountryWebsitePhone No. 6419931
KROHNE MESSTECHNIK GMBH & Co. 2033010 491602263 1162822822
SOLARTRON 1243833333

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