Page 252 - MEMS Mechanical Sensors

9.6 Flow Sensor Based on the Faraday Principle 241

Flow channel
SU8 Polymer cover

Silicon
Upstream Downstream
cell (RE,WE,CE) cell (RE,WE,CE)
Figure 9.33 Schematic cross-section of an electrochemical time of flight flow sensor (RE:
reference electrode, WE: working electrode, CE: counter electrode). The electrodes are 1 mm wide
and 100 nm thick, and the two cells have a distance of 1.5 cm from each other. The height of the
channel is 100 µm. (After: [101].)

of an electrical pulse (2V, 100 ms), water is dissociated at the working electrode and
a pulse of oxygen is released [see (9.1)] and transported downstream by the fluid
flow. There, the electrochemical cell functions as an amperometric oxygen sensor.
At a potential of −600 mV, the dissolved oxygen in the solution is reduced [see
(9.1)]. The current of the oxygen sensor is determined by the oxygen concentration
in the solution. Data for the electrochemical type flow sensor is given in Table 9.10.
+
2HO ⇔ 4H + O + 4e − (9.1)
2
2
The diffusion coefficient of oxygen, D, varies with temperature. Normally, D
changes by 2% per degree Celsius in aqueous solutions. Convection, however, is
–5
2
usually very fast compared to diffusion (D is ∼10 cm /s for oxygen at 25°C in
aqueous solutions) [101]. Therefore, it is possible to omit the diffusion in the flow
direction as long as the flow rate is not extremely slow, which implies that the meas-
urement of the flow sensor is temperature independent.
The flow sensor can also be set up for impedance measurement. Then the plati-
num working electrode and platinum counter electrode are connected to an imped-
ance meter. The produced oxygen forms microbubbles in the solution, which
increases the impedance of the solution. This setup was used for sensing the flow in
tap water. The applied potential to the oxygen producer was 4V.

9.6 Flow Sensor Based on the Faraday Principle

This technique uses the physical principle that an electric potential is developed
when a fluid of low conductivity passes perpendicularly through a magnetic filed
[102]. By using an ac magnetic field (here 65 kHz), it is possible to extract the elec-
tric potential across capacitors formed through plates on both channel sides and an
isolating layer. This eliminates the need for electrodes in contact with the fluid. The

Table 9.10 Data for Electrochemical Flow Sensor
Author; Year Flow Range Sensitivity Response Time Fluid Chip Size
Wu et al. [101]; 2002 1–15 µl/min — — Water —