AN884 MAXIM | Alldatasheet
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www.maxim - ic.com/an884 Page 1 of 6 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 ele ctronic 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 ima gine 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 t he 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 su bstrate (Figure 1).
www.maxim - ic.com/an884 Page 4 of 6 The output signal is adjustable between 0.5V and 4.5V (when operating with 5V supplies), and it accommodates sensor sensitivities of 10mV/V and higher. All coefficients required for compensation are held in an EEPROM internal to the MAX1458. Thus, the setup is easily and au tomatically calibrated via a simple 4 - wire serial interface. Because the relationship between measured differential pressure and liquid flow rate depends on the liquid's viscosity, which in turn changes with temperature, the MAX1458 provides a temperature output signal for use in external signal processing. This setup (Figure 4) allows the measurement of flow rates no higher than 5.5µl/sec. The analog pressure signals, the temperature sensor terminals, and all other connections are made accessible at a con nector for external post?processing.