Environmental Engineering Reference
In-Depth Information
Fig. 6.9
Representative results of DO MEA sensor characterization. (
a
) Cyclic voltammogram
(CV) curve for 0 mg/L and 8.7 mg/L DO in saline solution at a scan rate of 10 mV/s. (
b
) Calibration
curves in 0.85% NaCl saline and mineral salt solutions. (
c
) Representative current response for an
MEA sensor at three DO concentrations. (
d
) Representative stirring effect results that compare
response of recessed and non-recessed sensor tips at 500 rpm agitation. Adapted from [57, 58]
lower sensitivities of ~13 pA/mg/L [26]. This increased sensitivity is attributed to
the differences in recess dimensions and electrode surface areas.
The stirring effect is an important factor for the sensitivity, the reliability and
the stability of this recess-type DO sensor. The stirring effect test compared DO
measurements of open (Fig. 6.4b) and recessed (Fig. 6.4c) tips at 500 rpm agitation.
For the exposed open tip the current signal changed drastically when the test solution
was stirred, and rebounded back to the initial current level when the stirrer was
turned off (Fig. 6.9d). The stirring effect (Re) can be calculated in this case as [64]:
(
I
−
I
1
)
Re
=
×
100%
(6.2)
I
where
I
1
is the current measured from the unstirred sample, and
I
is the current
measured from the stirred sample. The test result reported that the non-recessed
MEA (i.e., prior to etching steps) exhibited a strong variation in signal due to stirring
at 500 rpm, with Re
~
23%. The recessed cathodes exhibited no change in signal,
with Re ~ 0%. This stirring effect result agreed with the previous reports [68-70].
Thus, the recessed structure with the inner cathode improves sensitivity and stability
of the DO microelectrode.
