A Pulsed Thermal-Flow (PTF) Sensor Measures Velocity of Flow and Thermal Diffusivity

The objective of our development project was twofold: first, to realize a micro-sensor of low cost and ultra-low energy consumption that, secondly, accurately and rapidly measures gas flow velocities within a wide temperature range. The sensor of size 4.2  × 3.2 × 0.6 mm3 is of micro-beam type. It consists of one Joule heating element and several resistance thermometers at distinct distances from the heater. The heater is operated in a single short pulse mode of a fixed duration that can be chosen between 10 and 100  µs. Due to this ultra-short time of excitation, there is no natural convection during a run. For typical values of pulse width (30 µs) and heater power (6 mW), the energy per measurement is smaller than 0.2 µJ. Typically, the excess temperature of the heater is limited to 5 K. This relative ly small overtemperature considerably reduces radiative heat transfers otherwise interfering with the process of measuring. The velocity of flow is determined by the time-of-flight technique of the imposed heat pulse as the tracer in a few milliseconds. The measurement uncertainty is preliminary ass essed to be 1.5% within a range from 0 to 7 m/s. The working temperature may vary between − 100 and 300 °C. The sensor not only measures the velocity of flow but also the thermal diffusivity of the gas under test. The report provides a complete analytical model of the functional principle of the sensor, a numerical FEM analysis of the 2-dim. isotherms, the...
Source: Sensing and Imaging - Category: Biomedical Engineering Source Type: research