A MEMS-based Coriolis Mass Flow Sensor for Industrial Applications

A microfluidic Coriolis mass flow sensor is discussed. The micromachined flow sensors are made using silicon tubes bonded onto a metalized glass substrate. True mass flow rates with better than +/- 0.5% accuracy were measured between 1 g/hr to 500 g/hr. The sensor also provides a temperature and density output. The sensor output was resistant to pressure, temperature, vibration fluid density and viscosity. Unlike conventional steel Coriolis mass flow meters, MEMS-based sensors are immune to external vibration.

Applications for these low flow rate devices includes, chemical mixing, additives, biotechnology, chromatography, pharmaceutical development and other areas where extremely small volumes of liquids are mixed, studied or metered and where shock and vibration are encountered.

The majority of MEMS-based flow sensors employ volumetric flow measurement methods such as thermal hot wire sensors [1,2]. The advantages of a Coriolis mass flow sensor [3-9] over other methods include the ability to measure true mass flow regardless of the fluid going through the resonating tube. Coriolis mass flow technology also provides a fluid density output which can be used for fluid identification, concentration measurement and quality monitoring. Conventional Coriolis mass flow sensors [8,9] have been commercially available for over 30 years.

These flow meters generally employ large diameter stainless steel tubes. Unlike steel
tube meters which are fabricated one at a time, MEMS-based sensors [3-7] employ wafer fabrication enables hundreds of micromachined silicon Coriolis mass flow tubes and even assembled subsystems to be produced with one wafer stack. This batch fabrication method reduces the manufacturing costs enabling a wider use of Coriolis mass flow technology.

The basic function of an ideal resonating Coriolis mass flow sensor can be expressed by the following equations. The mass flow rate q is given by: q = Ksθ/(4ωLr) (1) Where, Ks angular spring constant of the flow tube, θ is the twisting angle of the tube, ω is the resonance frequency, L is the length of the tube and r is the radius of the U-bend of the tube. Therefore, the mass flow rate is directly proportional to the twisting angle and inversely proportional to the resonance frequency. The density of a liquid ρ is given by the expression:
ρ = 1/V [(Ks/4π2f2) –mt](2)
where V is the internal volume of the resonant tube, mt is tube mass, Ks is the spring constant of the tube and f is the resonance frequency of the tube. As can be seen by the expression above, the density is inversely proportional to the square of the resonance frequency.

Any process requiring the metering or mixing of small amounts of liquids or gases such as semiconductor doping, leak detection, cleaning chemicals, additives, pharmaceutical formulation, fragrance and flavor additions can benefit from this technology. Precise
mixing requires the measurement of true mass flow, not an estimate based on a volumetric measurement. The performance of a MEMS-based Coriolis mass flow
sensor, designed for industrial applications will be covered in this paper.

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