Measuring cell ToMess, without fluid connections
Microstructured reactors have been successfully finding their way into the laboratories and production facilities of large-scale and fine chemistry as well as medicine for several years. Worldwide research activities impressively show that a multitude of reaction-related advantages in chemical processes can be achieved by using the micro-process engineering components.
In contrast, the selection of suitable sensors for monitoring the state variables is rather limited. In many cases, modified sensors from process technology are used. A disadvantage is the high internal volume compared to micro-process engineering components, which can be reduced by using different sensors for the four important state variables
- mass or volume flow, and
Measurement of the state variables is necessary for both process control and plant operational reliability. Based on the special requirements for sensors in micro-process engineering, a sensor module was developed that simultaneously records these variables and can be attached directly to the micro-process engineering
component via suitable interfaces.
Sensor principle and target parameters
The basis is a silicon flow cell with silicon-based sensors. The materials in contact with the media are largely chemically resistant and can be further passivated via layer systems, e.g. silicon nitride. The top image shows the complete flow cell with the channel plate that holds the four sensors and directs the fluid to the sensors. The pressure is evaluated piezoresistively via implanted measuring resistors in the silicon bending plate.
The channel plate is designed in such a way that the dwell time distribution is not significantly affected. A temperature diode is used for temperature measurement; this is integrated in the pressure sensor and also serves for temperature compensation of the measuring bridge on the software side. The volume flow is derived from the differential pressure at a narrow point. The final element is an interdigital structure for impedance measurement or impedance spectroscopy. Concentration measurement via impedance is only one possibility, this is only suitable if the impedance depends on the concentration, e.g. for ionic liquids. Via a suitable interface (USB, compatible with Labview ®, optional wireless data transmission), the direct integration of the sensor module into the micro process technology and the in situ measurement is possible. A Labview® program is available for evaluation of the sensor.
|State variable||Measuring principle||Measuring range (target value)|
|Process pressure||Process pressure||0-16 bar|
|Volume flow||Differential pressure and|
|Concentration||Electrical impedance spectroscopy||1 kHz-100 kHz|
|Temperature||Temperature diode||-30 to +130 °C|