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January 2010 Vol. XVI, No.1 • www.FlowControlNetwork.com

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MEMS Coriolis FlowMini Technology Makes Its Move on Industry

14 January 2010 Flow Control

by Doug Sparks, Ph.D.

Coriolis mass flowmeters and resonant densitometers arenow being manufactured on silicon microchips. WhileCoriolis meters have been used in high flowrate industrial

fluid monitoring applications for decades, the emergence ofmicro-miniature Coriolis devices represents a new area of devel-opment in the flow measurement category.

Making the Case for MEMS Fluid HandlingCoriolis flowmeters utilize a resonating tube, usually made ofstainless steel or other corrosion-resistant metals or alloys or, insome cases, glass. Stainless steel Coriolis mass flowmeterswith 12-inch diameter tubes that are several meters in lengthcan handle flowrates up to 40,000kg/hr. Meanwhile, microCoriolis mass flow sensors with submillimeter to millimeterdiameter tubes cover the opposite end of the spectrum (Figure1). Micro Coriolis mass flow sensors are based on MEMS(MicroElecroMechanical Systems) technology.

MEMS are the integration of mechanical technology elements,like sensors and actuators, with electrical elements on a com-mon silicon or glass substrate. The small size, improved effi-ciency, and low cost in high volume quantities have enabledMEMS to revolutionize a variety of functional applications.Widely used MEMS sensors include pressure sensors, inertialsensors like accelerometers and gyroscopes, ink jet printerheads and optical projector display arrays.

Hundreds of millions of MEMS sensors and actuators havebeen made in the last two decades in the automotive, medical,and consumer electronics markets. The reason for this wide

adoption is not just the small size of the sensor chips, but alsothe method of manufacturing the sensors. As shown in Figure2, hundreds of sensing chips are made on each wafer andbatches of wafers are processed through the semiconductorwafer fan in each lot. This enables millions of individual sensorsto be produced with relatively low labor content and, as a result,cost.

Evolution of Micro Coriolis TechnologyMEMS-based flow sensors based on differential-pressure andthermal or hot-wire technology have been used for many years.Coriolis mass flow is the latest flow sensing methodology toemploy MEMS technology. Coriolis mass flow has measure-ment advantages over thermal or differential-pressure flow sen-sors in that true mass flow outputs are obtained and a fluiddensity output can also be generated with the sensor. The fluiddensity measurement methodology is simple – the heavier ordenser the fluid, the lower the resonant frequency of the fluid-filled tube. This can be used to obtain a simple density or spe-cific gravity measurement or in a binary solution to monitorchemical concentration. The MEMS chip also has a thin-filmplatinum RTD temperature sensor integrated on it. This temper-ature-sensing capability is needed for accurate density measure-ments and offers mass flow, density, binary concentration andtemperature sensor outputs.

At the core of the Coriolis MEMS mass flow sensor is a res-onating silicon microtube (Figure 1). To begin the silicon micro-

Figure 1.Micro Coriolis mass flow sensors with submillimeter to mil-limeter diameter tubes are now being positioned for use in a variety ofindustrial applications.

Figure 2. In the manufacture of MEMS-based Coriolis flow sensors, hun-dreds of sensing chips are made on each wafer and batches of wafers areprocessed through the semiconductor wafer fan in each lot.

tube fabrication process, the inner-chan-nel is plasma-etched into a silicon wafer.Another silicon wafer is fusion-bondedonto this. This bonding step forms thetube channel. The outer shape of the tubeis next defined using photolithographyand plasma etching. This silicon tubeslice is then anodically bonded to a met-alized glass wafer. Plasma etching setsthe tube wall thickness, so high-pressure,thick-walled flow sensors can be fabricat-ed with this process. The glass wafer hasholes drilled into it that will be the fluidinlet and outlet to the resonating tube.The glass wafer is also etched prior tometal deposition and patterning such thata gap is formed between the silicon tubeand the metal capacitive electrodes pres-ent on the glass surface. The metal elec-trodes will electrostatically drive the sili-con tube into resonance and capacitivelysense the frequency and twist motion ofthe tube. The metal layer also forms thethin-film temperature sensor and bondpads. The flow sensor microstructurenow is in the form shown in Figure 1.

Figure 2 shows two products madeusing these chips, a Coriolis mass flowsensor (right) and the much smaller bina-ry concentration/density sensor. To pro-duce a flow sensor for industrial applica-tions requires a stable output and accu-rate performance over a wide tempera-ture and pressure range. The ability tomeasure different fluids with varying fluiddensities and viscosities is beneficial. Inthe industrial, automotive and aerospacemarkets, resistance to vibration andshock is key to enabling widespreadapplication

Performance PointsThe most common measurement usedto judge the point of reading accuracy ofan industrial Coriolis mass flowmeter is aflowrate trumpet curve. In a trumpetcurve, the relative measurement error isplotted as a function of flowrate. The out-lying trumpet curve boundaries are thezero flowrate stability (0.223 g/hr) plus

the 0.5 percent for a +/- 0.5 percent accu-rate meter. To take the mass flowratedata, a micro-scale is employed alongwith a timer card to independently meas-ure the mass of liquid flowing throughthe sensor. Figure 3 shows the flowratetrumpet curve for the micro Coriolis sen-sor is within the +/- 0.5% accuracy limitsfor water at room temperature.

Industrial sensors must operate over areasonable temperature range. For labo-ratory, micro-reactors and pilot lineinstruments, where the micro Coriolisflowmeter is finding its first applications,a temperature range of 15 C to 55 C is ofinterest. The sensor temperature is meas-ured with the on-chip platinum thin-filmtemperature sensor. This sensor wassuccessfully operated at up to 85 C with-out failure for prolonged periods of time.Operating temperatures can go as high as125 C in the engine compartment and ashigh as 150 C when mounted on an inter-nal combustion engine or testing petro-chemicals. The upper limit on functionali-ty was found to be the electronics, notthe MEMS resonator. With upgradedelectronics, these MEMS sensors have

been used at temperatures as high as160 C.

One big advantage to Coriolis massflowmeters is that in addition to flowrate,the density of the fluid can be monitored.In a laboratory, density measurementsare generally made using static fluid sam-ples loaded with a syringe with no pres-sure. A number of fluids have been testedwith the MEMS-based Coriolis mass flowsensor density error of five different liq-uids: IPA, methanol, N4 (viscosity stan-dard), water and 30% dextrose in water.The density error was less than 0.0002g/cc for these five fluids. Different vis-cosities did not affect the density orflowrate accuracy. Viscosities as high as750 cps have been tested with this sen-sor with no significant mass flowrate ordensity error.

Effects of Viscosity, HighFlowrate & Chemical ReactivityHigh viscosities do limit what fluids canbe used with microtube sensors.Eventually the pressures needed to pusha viscous fluid through a small orificebecome impractical. For thick petrochem-

Figure 3. Flowrate measurement error versus flowrate through the sensor for awater-based application.

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16 January 2010 Flow Control

icals, users have resorted to heating thesensor to 50-70 C to enable testing ofthicker fluids. Also, high flowrates canbe a limit for micro Coriolis mass flowmeasurements, although for density orbinary concentration, a bypass has beenused to accommodate high flowrates.Since the microtubes are made of sili-con, they cannot accommodate as widea range of chemicals as stainless steel.These three parameters – high viscosity,high flowrates and chemical reactivity –are the primary limits for this microflu-idic technology.

Pressure effects on the sensor outputhave also been examined. Applying 100PSI (689 KPa) to the fluid line resultedin a density error above atmosphericpressure value of just 0.0003 g/cc forwater. Burst pressure is a parameter ofinterest for industrial applications. Theburst pressure of the microtubes used toproduce this Coriolis mass flow sensorwas found to be in the 1,100 PSI to1,300 PSI range.

Since all Coriolis mass flowmeters arevibratory devices, vibration sensitivityhas been an underlying problem withthis technology. This is a critical problemfor industrial, automotive, and aerospaceapplications where shock and vibrationare commonplace. Conventional metaltube Coriolis mass flowmeters resonateat 100 Hz to 1,500 Hz, leaving them sus-ceptible to the spectrum of commonexternal mechanical vibration and shockfrequencies, which are under 2,000 Hz.

To examine the difference between theMEMS sensor in this study and a con-ventional steel tube and MEMS-basedCoriolis mass flow sensor, both wereplaced on a vibratory test stand andcycled from 10 Hz to 1,000 Hz, startingat 0.5 g and going to 2 g accelerationwhile monitoring the zero flowrate out-put of a water-filled tube. The conven-tional low-flowrate, steel Coriolis meterhad both large flow and density outputspikes at its resonance frequency at verylow accelerations, 0.5 g. The silicon tubeused in the MEMS sensor in this studyhad resonant frequencies ranging from20 KHz to 30 KHz, well above what istypically experienced in industrial, auto-

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MEMS-based flow sensors based on differ-ential-pressure and thermal or hot-wiretechnology have been used for many years.Coriolis mass flow is the latest flow sensingmethodology to employ MEMS technology.

motive, or aerospace applications.The zero flowrate output of the MEMS

tube was within a +/- 1 g/hr band at allexternal vibrational frequencies at 2 g.The density output was not affected byvibration on the MEMS sensor. This isan advantage for the MEMS-basedCoriolis mass flowmeter over conven-tional technology and can broaden thefield of use to include applications withsignificant vibration. As such, theseMEMS-based Coriolis mass flowmeterswould seem to be a good fit for vehi-cles, mounted on moving platforms androbotic pipette systems undergoingconstant start and stop motion.

Emerging ApplicationsSome of the emerging applications forthis technology are chemical concentra-tion, including alcohol to water concen-trations in fuel cells and ethanol togasoline concentrations in ethanol-blended fuels in automobiles. SmallMEMS-based sensors are a good fit forportable applications like fuel cell-pow-ered laptop computers, where an opti-mized water-to-methanol concentrationis needed to reduce membranecrossover and optimize the efficiency ofthe fuel cell.

The smaller sensor in Figure 4 iswidely used in direct methanol fuelcells. For ethanol-blended fuels, thesensor lets the engine control moduleknow the gasoline-to-ethanol ratio tomaintain optimum operating conditions.Other applications for Coriolis massflow and density sensors exist in thepharmaceutical, microreactors, biomed-ical, nuclear, perfume, petroleum and

beverage industries, as well as in distill-eries, hematology and urology. Thedevice can also be used to measureproof, ºBrix, ºPlato and API gravity. Thebeverage industry uses the meter fordetermining the sucrose, alcohol andextract percentages. Since distilleriesare taxed based on alcohol content,measurement accuracy is of greatimportance. MEMS-based Coriolis massflow sensors have been applied to the

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Figure 4. ACoriolis flow sen-sor typically usedfor direct methanolfuel cells in blend-ed ethanol fuelapplications.

18 January 2010 Flow Control

medical field in the area of drug infu-sion monitoring and pumping. TheCoriolis effect, which causes the tubesto twist under flowing conditions, issensed capacitively on the microchip.An FDA-approved drug flow monitoruses a chip to measure drug flowratesand the total volume of infused drugsinto a patient in the 5 mL/hr to 200mL/hr flowrate range. The IV line fromgravity-fed IV bags are connected inseries with the small flow sensor toprovide an extra layer of protection tothe patient and reduce the incidence ofdrug-infusion errors.

Doug Sparks, Ph.D. is the executivevice president and director of FlowProducts at ISSYS, where he overseasthe development of microfluidic prod-ucts such as density meters, Coriolismass flow sensors and drug infusionsystems. He is also a cofounder andpresident of NanoGetters, a whollyowned subsidiary of ISSYS specializ-ing in vacuum packaging. Prior to join-ing ISSYS in 2001, Dr. Sparks workedat Delphi Automotive System’s DelcoElectronics Division in the area ofMEMS and integrated circuits. He haspublished numerous technical papers,has more than 30 patents and earned aPh.D. in Material Science &Engineering from Purdue University.Dr. Sparks can be reached atdsparks@mems-issys.com or 734-547-9896 ext 119.

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ISSYS, E+H Partner on MEMS Coriolis for Industrial ApplicationsEarlier this year Integrated Sensing Systems, Inc. (ISSYS, www.mems-issys.com) and Endress+Hauser Flowtec AG (www.flowtec.endress.com)in Switzerland announced that they have entered into a strategic partner-ship. The objective of this partnership is to collaboratively develop andcommercialize advanced sensing fluidic products based on ISSYS’MEMS (Micro-Electro-Mechanical-Systems) technology. Targeted mar-kets include both the traditional industrial process industries as well asemerging process and OEM applications that demand novel, high per-formance measurement capabilities.

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