Page 224 - MEMS Mechanical Sensors

CHAPTER 9
Flow Sensors

Christian G. J. Schabmueller

Where fluids flow, the question of quantity arises. A fluid flow can be either a gas
flow or a liquid flow. Measurands can be either the amount of mass moved (weight
per second), the distance moved (meters per second), or the volume moved (volume
per second). A variety of conventional flow sensors exist, but they are often of little
use in the micro domain. Limited sensitivity, large size, high dead volume, and diffi-
culties in interfacing with microfluidic devices restrict their use. Microfabrication,
however, offers the benefits of high spatial resolution, fast time response, integrated
signal processing, and potentially low costs. Micromachined flow sensors are able
to measure a broad range of fluid flows from liters per minute down to a few drop-
lets an hour. They have matured from the research stage to commercial applications
and are now real competitors for conventional sensors and not limited to microflu-
idic applications, as the examples below will show.
The first micromachined flow sensors were presented by van Putten et al. [1]
and van Riet et al. [2] about 30 years ago. They used the thermal domain as the
measurement principle. Since then the performance of flow sensors has been
improved and several other flow measuring principles were transferred from the
macro into the micro world.
The intention of this chapter is to give an overview of the various flow-
measuring principles. References to papers published on numerical analysis or ana-
lytical models are given at the appropriate places in the text. The necessary parame-
ters of fluids and other materials (e.g. the dynamic viscosity, density, specific heat
capacity, thermal conductivity) can be found in [3].
The chapter starts with an introduction to microfluidics, which is relevant for
flow sensors. The microfluidic phenomena, the formulas from the fluid mechanics
or other relevant aspects are only mentioned briefly, without full explanation, as a
detailed description of that matter would exceed the scope of this chapter. Rather,
the reader is made aware of these matters and is directed to references where
detailed information is available. In the same section, various applications for micro
flow sensors are given. Thereafter follows the description of the flow-sensing
principles using MEMS fabrication. The section dealing with flow sensors operat-
ing in the thermal domain is the most elaborate, as it is one of the most important
areas.

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