Overview

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 difficulties 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 droplets an hour. They have matured from the research stage to commercial applications and are now real competitors for conventional sensors and not limited to microfluidic 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.