Variable Area FlowmetersClick on a title or page number below to jump to that section.B

Technical Reference Section B-3

Laboratory Rotameters B-11

General-Purpose Glass Tube Rotameters B-15

Multi-Tube Rotameters B-26

Heavy Duty and High Flowrate Rotameters B-28

Economical Piston-Type Flowmeters B-57

Electrical Output Flowmeters B-60

Flow Switches B-66

Industrial Vane and Piston Flowmeters B-75

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B-3

I see that rotameters and variable area meters aregenerally calibrated for water or air operating at 70°Fand 14.7 psia. What if I am using other materials at other operating conditions?The reading from rotameters and variable area meters is sensitive to the condition of the fluid passing through it; namely, the gas temperature, pressure and density, or liquid viscosity and density. For air the temperature and pressure have been selected as 21°C (70°F) and 14.7 psia because this is representative of the many gasapplications where the gas is at room temperature andthe rotameter outlet vents into the open air or into an enclosure which vents to the open air. Mathematicalcorrelations have been developed so that when arotameter or variable area meter is used for a differentfluid at different conditions, a multiplying factor can beapplied to the reading of the flowmeter so the actual flowrate of the new fluid can be obtained.

HOW TO SELECT YOUR ROTAMETER

MARIOTTEBOTTLE

LIQUIDUNDER TEST

Liquid Flow

FLEXIBLE HOSEVALVE

FLOWMETER

GRADUATEDCYLINDER

This article is intended to help the user to select the best rotameter/variable area meter for a particular application. For general information on these flowmeters, please refer to the “Variable Area Flowmeters” flow reference section.

How do the correlations work?For OMEGA’s general purpose rotameters, theequations shown on page B-7 are utilized. For liquids,the correlations do not take into account viscosity,which has a very significant effect on rotameters withball floats or plumb bob floats. Empirical data hasbeen derived for certain general purpose rotametersas to the effect of density and viscosity on theirperformance. For further details, consult OMEGA’sengineering department. For gases, the specific gravity of the gas can becalculated by dividing the molecular weight of the gasby 28.8 (28.8 is the molecular weight of air). In gascorrelations, it is important to realize that the

Figure 1

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correlations generate the flowrate at standard conditions,not actual conditions (for example: in standard cubic feetper minute, not actual cubic feet per minute). Standardcubic feet represent the volume that a gas would occupyif its temperature and pressure were 21°C (70°F) and14.7 psia (not the actual operating temperature and pressure).To convert actual cubic feet per minute, (ACFM or actualliters per minute, or any actual flowrate) to standardconditions (SCFM), the following relation is used:Qs = Qa x (Pa/Ps) x (530/(Ta + 460))

where:

Pa = actual operating pressure (in psia)Ps = 14.7 psiaTa = actual operating temperature (in °F)Qa = volumetric flowrate at actual operating conditions(e.g., ACFM)Qs = volumetric flowrate at standard conditions(e.g., SCFM)

SOAPYWATER

GAS IN

ROTAMETER

FLEXIBLEHOSE

CALIBRATED PIPETTO DELIVER

ASPIRATOR BOTTLE

Bubble Meter forLow Gas Flow

What about variable area meters?Variable area meters of a piston-and-tapered coneconstruction are only slightly affected by viscosities upto 20 centipoise. Their correlation equations are givenon page B-8.

Are correlation equations all I need to determine my flowrate?Correlation equations are approximations; for highestaccuracy, field calibration is recommended.

How is field calibration performed?For liquids, the process fluid is moved through therotameter until a stable reading is obtained. The outflowfrom the rotameter is then timed and collected in a bucket and timed. Knowing the volume of flow overa given time period indicates flowrate. This is thenrepeated for different rotameter readings. (See Fig. 1.)For gases, the same technique is used, except that thegas must be collected with a graduated cylinder underwater, (or with a bubble meter for very low flows.) Whenthe gas is collected under water, the pressure of the gasand the water vapor in the gas must be compensatedfor to (determine accurately) the volume of the gascollected. (See Fig. 2.)

Figure 2: Soapy water is squirted into the calibrated pipette. As bubbles form and travel up the pipette, they are timed with a stopwatch to determine cc/min flow rate.

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• Broad Rangeability• Glass, Plastic and Metal Tube Designs

• Especially Suited for Low Flowrate Metering

• Piston and Vane Units Can BeMounted in Any Position

ROTAMETERSThe rotameter is an industrialflowmeter used to measure the flowrate of liquids and gases. Itsoperation is based on the variablearea principle: fluid flow raises afloat in a tapered tube, increasingthe area for passage of the fluid.The greater the flow, the higher thefloat is raised. The height of the floatis directly proportional to theflowrate. With liquids, the float israised by a combination of thebuoyancy of the liquid and thevelocity head of the fluid. Withgases, buoyancy is negligible, and the float responds to thevelocity head alone.The float moves up or down in thetube in proportion to the fluid

flowrate and the annular areabetween the float and the tube wall. The float reaches a stable positionin the tube when the upward forceexerted by the flowing fluid equalsthe downward gravitational forceexerted by the weight of the float. A change in flowrate upsets thisbalance of forces. The float thenmoves up or down, changing theannular area until it again reaches aposition where the forces are inequilibrium. To satisfy the forceequation, the rotameter floatassumes a distinct position for everyconstant flowrate. However, it isimportant to note that because thefloat position is gravity dependent,rotameters must be verticallyoriented and mounted.The rotameter is popular because it has a linear scale, a relatively long measurement range, and lowpressure drop. It is simple to install and maintain. It can bemanufactured in a variety ofconstruction materials and designed to cover a wide range of

B-5

INTRODUCTIONVariable area flowmeters are very simple yet versatile flowmeasurement devices for use on alltypes of liquids, gases and steam.They operate on the variable areaprinciple, whereby a flowing fluidchanges the position of a float,piston, or vane to open a larger area for the passage of the fluid.The position of the float, piston, orvane is used to give a direct visualindication of the flowrate.The variable area family offlowmeters includes the followingfeatures and capabilities:• Flowrate Meters and Controllers• Measurement of Liquids andGases

• Direct Visual Indication• Low Pressure Drop• 6 to 76 mm (1⁄4 to 3") Typical Size• Visual, Transmitting andAlarm Models

• Up to 2% of Reading Accuracy• Up to 1⁄4% of Reading Repeatability• Simple to Install and Maintain• No Up- or Downstream PipingLimitations

VARIABLE AREA FLOWMETERSFlow Reference Section

B-6B-6

but the operating pressure for alarge 51 mm (2") tube may be aslow as 100 psig. The practicaltemperature limit for glassrotameters is 204°C (400°F),although operation at such hightemperatures substantially reducesthe operating pressure of the meter.In general, there is a linearrelationship between the operatingtemperature and pressure.

Metal Tube RotametersFor higher pressures andtemperatures beyond the practicalrange of glass tubes, metal tubesare used. These are usuallymanufactured of stainless steel, with stainless steel floats. Theposition of the float is determined by magnetic or mechanical followersthat can be read from the outside ofthe stainless steel metering tube. As with glass tube rotameters, the tube-and-float combinationdetermines the flowrate, and thefittings and materials of constructionmust be chosen so as to satisfy the demands of the applications. These meters are used for serviceswhere high operating pressure ortemperature, water hammer, orother forces would damage glassmetering tubes. Like the generalpurpose type, armored rotameterscan be used for most fluids,including corrosive liquids andgases. They are particularly wellsuited for steam applications, whereglass tubes are unacceptable.

Plastic Tube RotametersPlastic tubes are also used in somerotameter designs due to their lowercost and high impact strength. Theyare typically constructed ofpolycarbonate, with either metal or plastic end fittings. With plasticend fittings, care must be taken ininstallation, not to distort thethreads. Rotameters with all plasticconstruction are available forapplications where metal wettedparts cannot be tolerated, such aswith deionized water or corrosives.

Sizing RotametersThe reading of a rotameter isdependent upon the nature of thefluid being metered. Rotameters aretypically supplied with either a directreading scale for air or water, orcalibration data for air and water. It is therefore necessary to performmathematical calculations whenusing rotameters with other fluids.

The meter’s ability to repeat orreproduce this flowrate is of primary importance. Rotameters are repeatable up to ±1⁄4% of theinstantaneous flowrate. This featureenables the operator to reset oradjust the flow with confidence.

Glass Tube RotametersThe basic rotameter is the glasstube indicating-type. The tube isprecision formed of borosilicateglass, and the float is preciselymachined from metal, glass orplastic. The metal float is usuallymade of stainless steel to providecorrosion resistance. The float has a sharp metering edge where thereading is observed by means of ascale mounted alongside the tube.End fittings and connections ofvarious materials and styles areavailable. The important elementsare the tube and float, often calledthe tube-and-float combination,because it is this portion of therotameter which provides themeasurement. In fact, similar glass tube and stainless steel float combinations are generallyavailable, regardless of the type ofcase or end fittings the applicationcan demand, so as best to meetcustomer requirements. The scaleof the rotameter can be calibratedfor direct reading of air or water, or it may have a scale to read apercent of range or an arbitraryscale to be used with conversionequations or charts. Safety-shieldedglass tube rotameters are in general use throughout industry for measuring both liquids andgases. They provide flow capacitiesto about 60 GPM, and aremanufactured with end fittings of metal or plastic to meet thechemical characteristics of the fluid being metered.The only fluids for which thesemeters are not suited are thosewhich attack glass metering tubes,such as water over 90°C (194°F),with its high pH which softens glass;wet steam, which has the sameeffect; caustic soda, which dissolvesglass; and hydrofluoric acid, whichetches glass.The primary limitations of generalpurpose rotameters are thepressure and temperature limits ofthe glass metering tube. Small,6 mm (1⁄4") tubes are suitable forworking pressures up to 500 psig,

pressures and temperatures. Therotameter can easily be sized orconverted from one kind of serviceto another. In general, it owes itswide use to its versatility ofconstruction and applications.Because of its functionaladvantages the rotameter is an exceptionally practical flowmeasurement device. The pressuredrop across the float is low andremains essentially constant as theflowrate changes. Float response to flowrate changes is linear, and a 10-to-1 flow range or turndown isstandard. In the case of OMEGA®

laboratory rotameters, far greaterrangeability is possible through theuse of correlation equations.Rotameters can be installed directlyadjacent to pipe fittings withoutadverse effects on their meteringaccuracy, and the meters areinherently self-cleaning. Flowingbetween the tube wall and the float,the fluid provides a scouring actionwhich discourages the build-up offoreign matter.Variable area flowmeters are used primarily to set flowrates. The operator observes the meter,and adjusts the valve to bring theprocess flow to the proper flowrate.

FLOW

FLOAT

100

90

80

70

60

50

40

30

20

10

R

(Scale)

EQUILIBRIUM

TaperedMeteringTube

GRAVITY

Figure 1: Variable area flowmeter, also called a rotameter, has a float that moves up or down in a tapered tube. The distance it moves is proportional to the liquid flowrate and the annular area between the float and thetube well.

B-7

FLOW CALIBRATION SERVICES

� Air Velocity Calibrations� Mass Flow Meter and

Controller Calibrations� Turbine Meter and Rotameter

Calibrations� Paddlewheel Flow Sensors� Highly Accurate NIST

Comparison Standards� In-House 15' x 25'

Recirculating Wind Tunnel� Fast Turnaround Times

NIST

For Calibration Services Call:

To Order (Specify Calibration Level)Cal Level Price DescriptionCAL-1 $ 0 Statement of Conformance.

(Must be requested at time of purchase).CAL-2* 0 Statement of Traceability, no data points.

(Must be requested at time of purchase).CAL-3-FLOW 225 Calibration traceable to NIST with data pointsCAL-4-FLOW 225 Same as CAL-3 above plus the addition of

+25/PT customer selected data points*Not all products are available with NIST traceabilityNote: Because of the large selection of flow sensors, meters and instrumentation OMEGA offers, thepricing and calibration data points shown above are only an example and do not apply to every flowcalibration. Contact OMEGA’s Customer Service prior to returning any instrument for calibration service.Ordering Example: CAL-3-FLOW, designate this calibration level when NIST traceability is required on theflow sensor or instrumentation when you are returning previously purchased equipment for recalibration,$225. Note: To request NIST traceable calibration on new flow products add suffix “-NIST” to the modelnumber at time of purchase.

OMEGA’s laboratory grade large temperature andhumidity controlled recirculating wind tunnel.

OMEGA’s benchtop wind tunnel series model WT44 shown being utilized to calibrate the FMA900series hot-wire anemometer

OMEGA maintains a full line of standardsfor calibrating paddlewheel sensors, massflow meters, hot wire anemometers andlarge vane style handheld units. Picturedabove is our large wind tunnel, which isfully equipped with chillers, pumps andcondensers to keep the recirculating air ata constant temperature and flow ratewhile achieving flow rates from 25 to9000 AFPM. This large wind tunnel isused for anemometer and vane typesensor calibration.

ISO 9001CERTIFIED

CORPORATE QUALITY

STAMFORD, CT

B-7B-9

D

A. INLET PORTB. FLAPPER VANEC. OUTLET PORTD. SPRINGE. ENLARGED AREA TO ALLOW HIGHER FLOWS WITH NO INCREASE IN PRESSURE DROPF. INDICATING POINTERG. CAMH. SWITCH

C

H

12 8 4D

2016 0

F

G

E

B

A

OP

1 3 5

1 3 5

Figure 5: Flowmeters Operating Principle ofFL-O Series

Figure 6: Flowmeters Operating Principle of FL-X Series

VARIABLE AREA FLOWMETERSFlow Reference Section

Figure 4: Piston-type Variable Area Flowmeter

General Purpose RotametersFL-1000, FL-1100, FL-1200, FL-1300, FL-1500, FL-1600, FL-1700, FL-1900 SeriesFor liquids:GPM water equivalent = GPM metered liquid flow x

where: SGL= specific gravity of metered liquid atoperating conditions and SGF= specificgravity of rotameter float.This equation converts the reading of arotameter calibrated for water to areading appropriate for another fluid ofdifferent density. Most rotameters arevery sensitive to liquid viscosity, and thisequation does not take viscosity intoaccount. Therefore, it must be consideredto provide only a rough approximations.

Specific gravity of Glass = 2.53Specific gravity of 316 SS = 8.04For Gases:SCFM air flow from rotameter reading =SCFM true gas flow rate x

where:SG = specific gravity of metered gas (air = 1.0) at STP T0=temperature at operating conditions,Rankine (F + 460) P0 = pressure at operating conditions inpsia (psig + 14.7).This formula converts the metered flow,making allowance for the operatingtemperature and pressure, to anequivalent flow of air in SCFM at 21°C (70°F) and 14.7 psia. Capacitytables are in SCFM at standardtemperature and pressure of 14.7 psiaand 21°C (70°F).

End Cap

RetainingRing

Body FlowIndicator

DustGuard

ScaleAssembly

SealBumper

SpringMagnet/PistonAssembly

MeteringConeSpider

Plate

EndFitting

Flow

(SGF-1) x SGL(SGF - SGL)

(SG) (T0) (14.7)

(1.0) (530) (P0)

B-10

When used with purge rotameters(meters with +10% full scaleaccuracy) these equations aresatisfactory, although not precise.When used with ±2% full scaleaccuracy meters, the correlationholds as long as the viscosity of thefluid does not exceed 6 centistokes.Above this viscosity, field calibrationis required.

General PurposeRotametersFL-1400 and FL-1800 SeriesGeneral purpose glass tuberotameters with ball floats cannot becorrelated with precision, due totheir strong sensitivity to variationsin viscosity. OMEGA® FL-1400 andFL-1800 Series rotameters aresupplied with correlation charts fornumerous gases at varioustemperatures and pressures. Forhighest precision, field calibration isrecommended.Piston and Vane Type VariableArea FlowmetersPiston-type flowmeters use anannular orifice formed by a pistonand a tapered cone. The piston isheld in place at the base of the cone(in the “no flow position”) by acalibrated spring. Flow through

by the following correction factorsto account for fluid density:

�.84/spec. grav. (for oil meters) or

�.1.0/spec. grav. (for water meters)

These units are only slightlyaffected by viscosity, and nocorrection is normally required.Correlation of FL-2700, 6700,7700, 8700These units are calibrated toindicate standard cubic feet perminute (SCFM) of air at 100 psigand 21°C (70°F). When used atother conditions, the followingcorrections apply:SCFM (true flowrate) = SCFM(indicated)/ (f1 x f2 x f3) where:

f1 = � 114.7/(14.7 + operating psig)

f2 = � (460 + operating °F)/530

f3 = � molecular weight of gas

29

the meter results in a differentialpressure across the piston orificethat moves the piston against thespring. Piston movement and orificearea are proportional to flowrate. Incase of the vaned-type units, thefluid flow forces the vane to rotateagainst a spring, increasing theorifice area for flow. The position ofthe piston or vane is then read on ascale to give the flowrate. Since theforce of a spring opposes the flow(in comparison to rotameters, whichuse gravity), these units may bemounted in any position. Scales arebased on specific gravities of 0.84for oil meters, and 1.0 for watermeters. Their simplicity of designand the ease with which they can be equipped to transmit electricalsignals has made them aneconomical alternative to rotametersfor flowrate indication and control.Correlation of FL-2000, 6000,7000, 8000Although normally calibrated for oilor water, these units can be usedfor other fluids as well. The readingon the flowmeter must be multiplied