Details

Autor(en) / Beteiligte

• Hammerschmidt, U.
• Sosna, C.
• Benkert, A.
• Meier, A.
• Völklein, F.

Titel

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

Ist Teil von

• Sensing and imaging, 2019-12, Vol.20 (1), p.1-22, Article 41

Ort / Verlag

New York: Springer US

Erscheinungsjahr

2019

Link zum Volltext

Quelle

SpringerLink

Beschreibungen/Notizen

• A prototype pulsed thermal-flow (PTF) sensor was designed, fabricated in micro-electro-mechanical system (MEMS) technology and tested on various gases. 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 mm 3 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 relatively 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 assessed 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 heat flow pattern of the active surface of the sensor and an empirical model to fit the temperature response of the thermometers. The latter one effectively supports the evaluation of the acquired data. Exemplary experimental results are presented for nitrogen, methane and the two gas binary mixtures 80CH 4 –20N 2 and 80CH 4 –20H 2 . The results deviate from their analytically predicted values by at most  ± 1%. To the best of our knowledge, the PTF-sensor operates at the lowest energy consumption known so far. This characteristic makes it the perfect flow sensor for battery-operated long-term flow velocity recording systems.