H.E. de Bree

Bi-directional fast flow sensor with a large dynamic range

In this article an extended mass-flow sensor is presented. Apart from the magnitude of the flow, as an add-on to the traditional anemometer, this sensor also measures the direction of the flow. This is of interest for the flow sensor market in general, and more in particular for the safety monitoring of low over-pressure systems like surgeries and cleanrooms where the risk of reverse flow needs to be avoided at all times. It detects the smallest amount of gas-flow, down to approximately 100 µm s-1 airflow up to high flow levels of 1 m s-1. The response time is of the order of milliseconds.

Experiments with a new acoustic particle velocity sensor in an impedance tube

The development of a new acoustic sensor makes it possible to measure acoustic particle velocity instead of sound pressure. The new sensor, called the Microflown, can be used in the frequency range of 0 to approximately 20 kHz. As one of the first applications, the new sensors are applied in an impedance tube to determine the impedance of an aluminium sample at the end of the tube. The sample has an orifice which accounts for the sound absorption. Comparing the results with theory and measurements with microphones leads to an excellent agreement. The characteristics of the Microflowns, like the simplicity and the small dimensions, make it a very attractive alternative to the microphones. Furthermore, the sensitivity to the direction of the acoustic waves and the matching of the phase and sensitivity of two sensors can be used in other research fields in acoustics as well.

Towards thermal flowsensing with pL/s resolution

This paper focuses on the development of highly sensitive calorimetric flow sensors. Both the hydrodynamics of the flow channel, as well as the heat transfer are analyzed in detail. With the expressions for the hydraulic resistance of the flow channel and the hydraulic system requirements for the flow sensor, it is possible to optimize the design of the flow channel. It is theoretically shown why for small v the calorimetric flow sensor output is linear in v. Also it follows from theory that for a symmetrical configuration, in this linear regime the heater temperature is independent of v for constant heating power. This suggests that for low Reynolds Number Kings Law has t obe modified. Different configurations and methods are analyzed: absolute, differential, two beam, three beam, CPA, CTA and TBA. The latter is a real thermal balance measurement and allows the use of non-linear sensing elements. From our experience with acoustical measurements it is possible to estimate practical attainable sensitivity. In combination with proper flow channel design, and a fabrication technology for narrow channels, it is shown that pL/s sensitivity is in reach.

An electronical characterisation method for an acoustic and thermal flow sensor

The Microflown is an acoustic sensor measuring particle velocity instead of sound pressure, which is usually measured by conventional microphones [2, 3, 4]. Originally a thermal DC-flow sensor[1], it is optimized for sound measurements. For most applications the Microflown should be calibrated [2–7], which is usually performed acoustically, using a standing-wave-tube [8,9]. Here an electronical method for characterisation of its relevant thermal parameters, which is more convenient, is presented.