AN884 Datasheet, PDF (1/6 Pages) Maxim Integrated Products – Miniature Flow Sensor Has Electronic Temperature Compensation

SENSOR SIGNAL CONDITIONERS
Application Note 884: Dec 13, 2001
Miniature Flow Sensor Has Electronic Temperature Compensation
Fluid analysis is essential in a wide range of current applications. Biology, medical
analysis, genetic engineering, and many other fields rely on fast, precise, and
reproducible chemical and biological analyses. Tools that automate the dosing and
analysis of fluid samples are essential for affordable performance.
Highly specialized electronic sensors have been designed to automate sample analysis, but the
dosing of liquids – still a prominent issue – must be done physically. This operation is usually
accomplished with specialized micro-syringes driven by stepper motors. It's easy to imagine the
difficulty and expense associated with such setups.
In a novel approach to this problem, DASA IMT and Seyonic SA (both of Neuchâtel,
Switzerland) developed flow-through micro-fluidic dosing as part of a toolkit for experiments
performed in space. As described in the following, a key element of that design was a micro-
flow sensor device. Requirements were:
• Small size
• Chemical inertia
• Temperature stability
• Long-term stability
• Simple, easy, and fully automated recalibration
• Linear voltage-versus-pressure output
One way to measure micro-fluidic flow is by measuring the pressure difference across a
restriction integrated into a micro-flow channel. Pressure measurements are performed with
dual piezo-resistive pressure sensors, one placed before the restriction and the other behind.
To insure that the sensor is not affected or altered by chemically aggressive fluids, the fluid
under pressure is applied to the back of the sensor diaphragm (whose monocrystal-line silicon
is relatively insensitive to the chemicals) rather than the front. This unusual configuration
protects sensitive microelectronic circuits on top of the sensor by shielding them from the
liquid. To prevent error from mechanical-tension forces, the sensor is mounted on a thick
ceramic substrate (Figure 1).
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