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Highly Sensitive Sensor for Flow Velocity and Flow Direction Measurement

Miniaturized sensors for flow velocity and flow direction measurement based on thin-film germanium thermistors (TCR = -1.8%/K) offering extremely high sensitivity were developed. The thermistors are placed on a silicon nitride diaphragm (1.3 mum thick) which is carried by a silicon frame. To resolve the direction of the flow, eight thermistors are arranged circularly on the diaphragm. Two orthogonal pairs of diametrically opposed thermistors (e.g., N-S and E-W) feature a directional sensitivity of 152 muV/deg at a flow velocity of 1 m/s. An increase of the sensitivity of about 50% can be gained by analyzing the difference signal of two 90deg rotated thermistors (e.g., N-E), which are in the downstream position. The measurable gas flow rate ranges from 0.025 m/s to about 3 m/s for the constant power mode. The sensor has a high sensitivity to flow direction of 30 mVs/m at low flow rates from 0.025 m/s to 0.2 m/s.