Two sensor Microflown

Flow sensors commonly are used in, for example, micro liquid handling systems or for measuring gas flows. One example is a gas flow sensor used as a microphone: the Microflown [3]. This sensor is designed to measure particle velocity, very small flows in a frequency range up to twenty kilo Hertz. While the signal to noise ratio is a very important parameter of the microflown an innovative measuring principle is optimised with respect to the signal to noise ratio. To measure small air flows, mass flow sensors (MFS) are commonly used. This type of sensor consists of a heater and two temperature sensors equally spaced around this heater. The flow alters the temperature distribution around the heater. This is measured by the sensors and the temperature difference quantifies the flow. Another principle to measure a flow is an anemometer. This sensor consists of one heated wire which acts like a sensor at the same time. The heat loss, which is dependent on the air flow, is measured. A disadvantage of the anemometer is the low sensitivity for small DC flows [1]. The direction of the flow can not be obtained while for both positive and negative flows the heat loss is the same. The new proposed sensor, the Two Sensor Microflown (TSM), consists of two heated elements which are used as both sensor and heater (like the anemometer) and uses the differential temperature as a representation of the flow (like the MFS). Using only hot sensors (and no heater) will lead to an easier to make sensor, an improved signal to noise ratio and a larger dynamic range, as will be shown in this article.