36
Commercial field instruments have ranges to 120% of AEC and initial accuracy
of about 2% of saturation or close to 8 μmole.kg
−1
. Robust stability (0.5%) makes
this instrument capable of deployment for over about 4 months, but the manufacturer stresses that a clean membrane is required, a difficult feat to achieve in highly
productive coastal environments or near-surface waters subject to oil pollution.
Response time varies 8–20 s, making it unsuitable for detecting rapid changes such
as fast vertical profiling of natural waters but useful for medium-term mooring
applications where burst sampling of a few minutes per hour is common for saving
power and reducing data density.
Oxygen Optode
Electro-optical fluorescence-based sensors (known as optodes) that bypass many of
the difficulties of the amperometric approach have demonstrated improved performance and are well on their way to being the primary method for DO measurement.
Fluorescent materials are organic compounds that absorb visible or ultraviolet radiation and subsequently lose the resulting excited state through reemission at longer
wavelengths. Initially developed for medical applications (as was the amperometric
electrode), the optode makes use of the effect of oxygen on the fluorescence emitted
by light-absorbing organic metal complexes providing a novel means of measuring
DO without consuming it. Porphyrin complexes of platinum and other transition
metals emit red light following excitation (Mills 1997). This emission is modulated
by the presence of oxygen which impedes or quenches fluorescence. Thus fluorescence is greatest in the absence of DO and least at high DO concentrations. In practice such a complex embedded in a sensing foil target exposed to the aqueous
medium is irradiated with blue light provided by a LED device and the resulting red
emission is detected by photodiode. Modulation of the excitation signal and measurement of the phase of the emitted radiation allows a measure of fluorescence
decay time, further increasing precision. An additional red LED is used as a stability
reference source. The sensing foil is provided with a black optical isolation coating
to block stray ambient light from reaching the photodiode. Initial response times for
current generation field instruments now stand below 30 s with accuracy below 8
μmole.kg
−1
.
Under the auspices of Alliance for Coastal Technologies, field tests were performed on three commercial optode instruments and one pulsed polarographic sensor using Winkler titrations for reference under widely varying environmental
conditions at seven sites in US waters varying from coral reefs to the Great Lakes to
inshore waters of Chesapeake Bay (see ACT evaluation reports). Stability during
deployments of 4 weeks, sampling at 5 min intervals, was excellent for all instruments in some low productivity environments but most instruments failed catastrophically, within a week in one case and within two in others, when deployed in
more challenging environments. Biofouling prevention systems (see Chap. 4) fitted
to some instruments by their manufacturers showed mixed results as well. Some
covaried with unprotected instruments of the same model throughout the deployment
2 Electronic Sensors and Instruments for Coastal Ocean Observing
Commercial field instruments have ranges to 120% of AEC and initial accuracy
of about 2% of saturation or close to 8 μmole.kg
−1
. Robust stability (0.5%) makes
this instrument capable of deployment for over about 4 months, but the manufacturer stresses that a clean membrane is required, a difficult feat to achieve in highly
productive coastal environments or near-surface waters subject to oil pollution.
Response time varies 8–20 s, making it unsuitable for detecting rapid changes such
as fast vertical profiling of natural waters but useful for medium-term mooring
applications where burst sampling of a few minutes per hour is common for saving
power and reducing data density.
Oxygen Optode
Electro-optical fluorescence-based sensors (known as optodes) that bypass many of
the difficulties of the amperometric approach have demonstrated improved performance and are well on their way to being the primary method for DO measurement.
Fluorescent materials are organic compounds that absorb visible or ultraviolet radiation and subsequently lose the resulting excited state through reemission at longer
wavelengths. Initially developed for medical applications (as was the amperometric
electrode), the optode makes use of the effect of oxygen on the fluorescence emitted
by light-absorbing organic metal complexes providing a novel means of measuring
DO without consuming it. Porphyrin complexes of platinum and other transition
metals emit red light following excitation (Mills 1997). This emission is modulated
by the presence of oxygen which impedes or quenches fluorescence. Thus fluorescence is greatest in the absence of DO and least at high DO concentrations. In practice such a complex embedded in a sensing foil target exposed to the aqueous
medium is irradiated with blue light provided by a LED device and the resulting red
emission is detected by photodiode. Modulation of the excitation signal and measurement of the phase of the emitted radiation allows a measure of fluorescence
decay time, further increasing precision. An additional red LED is used as a stability
reference source. The sensing foil is provided with a black optical isolation coating
to block stray ambient light from reaching the photodiode. Initial response times for
current generation field instruments now stand below 30 s with accuracy below 8
μmole.kg
−1
.
Under the auspices of Alliance for Coastal Technologies, field tests were performed on three commercial optode instruments and one pulsed polarographic sensor using Winkler titrations for reference under widely varying environmental
conditions at seven sites in US waters varying from coral reefs to the Great Lakes to
inshore waters of Chesapeake Bay (see ACT evaluation reports). Stability during
deployments of 4 weeks, sampling at 5 min intervals, was excellent for all instruments in some low productivity environments but most instruments failed catastrophically, within a week in one case and within two in others, when deployed in
more challenging environments. Biofouling prevention systems (see Chap. 4) fitted
to some instruments by their manufacturers showed mixed results as well. Some
covaried with unprotected instruments of the same model throughout the deployment
2 Electronic Sensors and Instruments for Coastal Ocean Observing