Rotameter

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

_{is a simple industrial flow}

_{is a simple industrial flow meter that measures}

_{meter that measures}

volumetric flow of liquids and gases in a closed

tube.



_{they have linear scales, a relatively large}

measuremen

measurement range, low

t range, low pressure drop, and are

pressure drop, and are

simple to install and maintain.



_{are a subset of meters called}

_{are a subset of meters called variable area flow}

_{variable area flow}

meters

meters that measure the flow rate by allowing the

that measure the flow rate by allowing the

fluid to travel through a tapered tube where the

cross sectional area of the tube gradually

cross sectional area of the tube gradually becomes

becomes

greater as the fluid travels through the tube.

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Three basic components:



_{a uniformly tapered flow tube}



_{a float}



_{measurement scale}

A

A control valve may

control valve may be added

be added

if flow control is also desired. In

operation, the rotameter is positioned

vertically in the fluid system with the

smallest diameter end of the tapered

flow tube at the

flow tube at the bottom. This is the

bottom. This is the

fluid inlet

fluid inlet . The float, typically

. The float, typically

spherical, is located inside the flow

tube, and is engineered so that its

diameter is nearly identical to the flow

tube’s inlet diameter.

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The flow rate inside the

rotameter is measured using

a float that is lifted by the

fluid flow based on the

buoyancy and velocity of

the fluid opposing gravity

pulling the float down.

pulling the float down. For

For

gasses the float responds to

the velocity alone,

buoyancy is negligible.

The

The float

float is

is always

always

denser than the substance it

is resting in and does not

actually float on the

substance’s surface, but

rests somewhere between

the substance’s surface and

the bottom of

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As a liquid or gas passes through

As a liquid or gas passes through the tube,the tube, the flow causes the

the flow causes the float to rise. As the floatfloat to rise. As the float rises, more and more fluid flows by the float rises, more and more fluid flows by the float because the tapered

because the tapered tube’s tube’s diameter isdiameter is increasing. ltimately, a point is reached increasing. ltimately, a point is reached where the flow area is large enough to allow where the flow area is large enough to allow the entire volume of the fluid to flow past the the entire volume of the fluid to flow past the float. This flow area is called the

float. This flow area is called the annularannular passage

passage. The float is now . The float is now stationary at thatstationary at that level within the tube as its weight is being level within the tube as its weight is being supported by the fluid forces which caused it supported by the fluid forces which caused it to rise. This position corresponds to a point to rise. This position corresponds to a point on the tube’s measurement scale and

on the tube’s measurement scale and provides an indication of

provides an indication of the fluid’the fluid’s flows flow rate. The operator reads the flow

rate. The operator reads the flow from afrom a graduated scale on the

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!hanges in the flow

!hanges in the flow rate cause rotameter"s float to changerate cause rotameter"s float to change position inside the tub

position inside the tube. e. #ince the float posit#ince the float position is based on gravity ion is based on gravity it isit is important that all rotameters be mounted vertically and oriented with the important that all rotameters be mounted vertically and oriented with the widest end of the ta

widest end of the taper at the top. per at the top. It is also imIt is also important to remportant to remember that ifember that if there is no flow the float will sin$

there is no flow the float will sin$ to the bottom of the rotameter due toto the bottom of the rotameter due to its own weight.

its own weight.

%otameters can be calibrated for other fluids

%otameters can be calibrated for other fluids by understandingby understanding the basic operating pri

the basic operating principles. nciples. %otameter a%otameter accuracy is deteccuracy is determined by thermined by the accuracy of the pressure, temperature, and flow

accuracy of the pressure, temperature, and flow control during the initialcontrol during the initial calibration.

calibration. Any chaAny change in the density ange in the density and weight of the float nd weight of the float will havewill have impacts on the

impacts on the rotameter"s rotameter"s flow reading. flow reading. AdditionalAdditionally any changes thatly any changes that would affect the fluid such as pressure or temperature will also have an would affect the fluid such as pressure or temperature will also have an affect on

affect on the rotameter"s the rotameter"s accuracy. accuracy. &iven this, &iven this, rotameters should berotameters should be calibrated yearly to correct for any

calibrated yearly to correct for any changes in the system that may havechanges in the system that may have occurred.

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A

ADDVVAANNTTAAGGEES S DDIISSAADDVVAANNTTAAGGEESS 



_{'o e(ternal power}

required



_{)ou can see the process}



_{%otameters are cost}

effective



_{#imple to install and}

maintain



_{*ow pressure drop}



_{%epeatability}



_{+ffer wide flow}

measurement ranges or

rangeability



_{%otameter"s scale is linear}



_{ust always be installed}

vertically with the fluid

flowing up through it



_{&raduated scale will only}

be valid for the specific

fluid and conditions where

it was calibrated.



_{-ifficult to be adapted for}

machine reading



_{ust be made of glass or}

other transparent material

in order for the user to see

the float in the tube

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