Page 240 - MEMS Mechanical Sensors

9.3 Pressure Difference Flow Sensors 229

Sensirion AG, Switzerland [62], produces a flow sensor including the CMOS
measurement electronics on the same chip. The flow meters are based on Sensiri-
ons’s CMOSens technology, and use the calorimetric flow measurement principle.
They combine the thermal sensor element with the amplification and A/D converter
circuit on one single CMOS chip (Figure 9.20). This renders them very resistant to
electromagnetic disturbances. The measurement data is fully calibrated and tem-
perature compensated by means of an internal microcontroller. Chemical resistance
and biocompatibility are achieved by measuring heat transfer through the tubing
material of a capillary made of PEEK or fused silica (Figure 9.21). Therefore, the
media is not in direct contact with the sensor chip. Flow sensors in CMOSens tech-
nology have been sold since 1999. The sales for gas flow sensors are significantly
higher than for liquid flow sensors, indicating that the market for liquid flow sens-
ing is not yet significant. The sensors can be bought as plug-and-play units for labo-
ratory use, or as an OEM solution.
Micromachined gas flow sensors are also available from Leister Process Tech-
nologies, Switzerland [77], and SLS Micro Technology GmbH, Germany [78]. A
liquid flow sensor is available from GeSiM mbH, Germany [79]. A wind sensor has
been commercialized by Mierij Meteo [80]. Data for the various commercial flow
sensors are listed in Table 9.3.

9.3 Pressure Difference Flow Sensors

This flow sensing principle relies on the measurement of the differential pressure p
in a flowing fluid. Pressure sensors can be used to measure flow by sampling the

Sensor
Figure 9.20 Photograph of a CMOSens chip. On the right is the flow sensing element, and on
the left the CMOS electronics. (Courtesy Sensirion AG.)

Digital signals
from chip
CMOS
sensor chip

Sensor
Measures through capillary
tubing material

Figure 9.21 Measurement setup of Sensirion’s sensor chip for liquid flow. (After: [62].)