CSEM Scientific and Technical Report 2008

Novel Flow Sensor Concept and Implementation
M. Fretz, N. Schmid, S. Bitterli, L. Neumann, C. Bosshard, H. F. Knapp, D. Fengels
We present a novel concept for a low-rate flow sensor with a targeted measurement range of 1 µl/s. The device is based on a commercially
available differential pressure sensor die, which is packaged using a proprietary design making the sensor less prone to damage when pressure
peaks occur.
Liquid flow is measured in a variety of industries. Small liquid
flow is particularly of interest in drug delivery, liquid handling,
pump control / calibration, analytical instrumentation, lab-on-achip,
molecular diagnostics and flow cytometry.
A novel concept for very low-rate flow sensors with a
measurement range of 1 µl/s (water) was developed and
implemented. Due to the small physical dimensions
(50x8x8 mm) it could be integrated in pipetting systems. The
sensor method is based on differential pressure measurement
at two ends of a well-defined fluidic channel. The pressures
are directed to one sensitive differential pressure sensor. The
advantages of this concept are the following:
• The flow sensor is insensitive to high system pressures
(e.g. 10 bar), because the single pressure sensor is
exposed to the system pressure on both sides of its
sensitive membrane.
• Since the pressure sensor membrane is entirely exposed
to the system pressure, a very sensitive differential
pressure sensor can be utilized.
• Compared to thermal based flow sensors it is quicker and
data processing is simpler.
• This flow sensor concept can basically be provided with
any commercially available pressure sensor dies.
The manufacturing process employs flip chip bonding of the
sensor die onto a carrier piece with anisotropic conductive
adhesive [ 1 ] . This technology provides both electrical
connection for signal read-out and fluid-tight sensor
attachment. A picture of the flow sensor is shown in Figure 1.
The 50x7x4 mm large PCB exhibits sites for the electronic
components, fluidic channels and space for the pressure
sensor. An integrated microcontroller allows the calibration of
sensor signals at different temperatures, providing an analog
temperature compensated output signal from 0 V to 5 V.
First characterization tests with air were carried out with a
syringe pump. Because of the sensitivity of the sensor, the
minute oscillations in the flow generated by this syringe pump
could be detected (see Figure 2). The output signal oscillated
with an amplitude of ~200 mV. The long rise time was due to
the connection tube with a length of 50 cm and the resulting
large volume of air between pump and sensor. Due to the
oscillation the measurement was repeated with a constant
Figure 1: Flow sensor without case, fluidic connection and electronic
components
flow, generated by the following setup: A sealed bottle –
partially filled with water – was connected by a tube to the flow
sensor. A second tube connected from the sealed bottle to a
lower positioned bottle, causing a water flow, which, in turn,
provided a negative-pressure in the higher positioned bottle.
This negative-pressure sucked air through the flow sensor into
the bottle. And in order to achieve a fast build up of the flow, a
manual-switch was placed between sensor and sealed bottle.
After the negative-pressure was built up, the manual-switch
was opened quickly. Continuous and constant flow was
reached within ~10 ms (Figure 3). The output signal of 4 V
corresponds to an air flow of ~44 µl/s.
Voltage [v]
Figure 2: Output signal of flow sensor after switch-on of a syringe
pump running at ~50 µl/s (air). Long connection tubes limit the
response time.
Voltage[v]
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
5.0
4.0
3.0
2.0
1.0
0.0
0 2 4 6 8 10 12 14 16 18 20
Time [s]
0 20 40 60 80 100 120 140 160 180 200
Time [ms]
Figure 3: Response of flow sensor after continuous flow was
switched on.
When a water flow was driven through the sensor, the signal
was saturated above a flow rate of 1 µl/s. The sensitivity was
increased by a factor of 50, in agreement with the viscosity
ratio between water and air.
This work was supported by the MCCS Micro Center Central
Switzerland. CSEM thanks them for their support.
[1] M. Fretz, et al., “Flip Chip Bonding on Polymers for Low-Cost
Microfluidic Application”, in this report, page 111
107