Part B | 18.3
426 Part B Autonomous Ocean Vehicles, Subsystems and Control
18.3 Chemical Sensor Systems
Describing the state of chemical sensor systems is
an enormous task and could easily be the basis for
an entire volume dedicated to the subject. In this chapter we provide examples of technologies that represent aspects of chemical instrumentation and hopefully
will provide a snapshot of the chemical instrumentation migration to maturity within the environment
and mobility using AUVs. In general, key drivers for
evolution of chemical instrumentation come from the
ability of chemical sensors to provide trace detection
and spatial/temporal chemical dynamics in water and
within chemical ecology. Previous literature reviews
have covered and summarized in-situ chemical sensors into two volumes on chemical sensor systems for
aquatic sciences and chemical and biological sensors
for time-series focused research [18.6, 7]. These reviews describe a variety of continuous flow analyzers,
electrochemical sensors, and optical sensors for in-situ
measurements of dissolved chemical species.
Chemical sensors need to have excellent detection
limits to be desirable. Attributes such as precision,
selectivity, response time, a large dynamic concentration range, low power consumption, robustness, and
less variation of instrumental response with temperature
and pressure are the most advantageous. Also, instruments that stay free of fouling problems are highly
attractive to end users. It is also important to note that
some chemical sensors are deployed in several different oceanic environments for the detection of targeted
chemical species, whereas others are useful in only
one environment because of various configuration limitations. There are a number of chemical sensors that
can be used as stand alone in-situ sensors, whereas
a greater number involve water being brought from the
environment into an analytical cell or chamber via tubing, either aboard ship or onboard a mobile platform
such an AUV. Multi-element sensors that measure several chemical species from the same water sample are
an area that researchers are pushing for further development. These multi-function sensors will increase the
capability of AUVs (for example) to characterize spatial
ocean chemical composition and diversity.
At present, most sensors need further refinement to
perform measurements at very high acquisition rates
without a loss of precision or sensitivity. Fundamental
inorganic compounds and ions (oxygen, basic nutrients,
hydrogen sulfide, pH, carbon dioxide, NOx, SOx, Fe,
Mn, and trace metals) are good candidates for continual AUV-based integration. Nitrate, a key nutrient, as
a sensor system has seen major advancement in the last
few years. Nitrate sensors are a current example of instrumentation that will see increased usage for nutrient
mapping, particularly in AUV coastal surveys. Current
commercial nitrate sensors can be accessed as either by
the use of automated wet chemistry or ultraviolet (UV)
spectroscopy performed in situ. Optical UV techniques
function to detection levels of 0:5 M, but for levels
below that wet chemistry is required at present. UV nitrate sensors are, therefore, suitable for use in the ocean
surface except for some rare areas. For work below the
micromolar level there are wet chemical systems that
can achieve nanomolar levels of detection [18.6]. At
the time of writing this we are not aware of any having
been deployed on an AUV. Additionally, wet chemical
systems are evolving to measure ammonia, nitrite, orthophosphate, silicates, and iron at desired levels and
acquisition rates for waters and sediments. We expect
that these instruments will in time be in a format that is
tractable for insertion on AUV platforms.
The study of boundaries and interface environments
are important in ocean dynamics. Boundary layers include atmosphere-to-water, water-to-sediment/soil, as
well as the boundaries between fluid masses with
sharply different characteristics. These different contexts all require the measurement of fluxes. Additionally, deterioration of water quality, ocean acidification,
and unknown processes have resulted in a tremendous
decline of ocean communities [18.8]. Quantifying the
fluxes of chemicals is of great importance to understand
these local, regional, or global biogeochemical processes. These include fluxes of key carbon species such
as CO 2 and CH 4 , nitrogen species such as NH
C
4 , N 2 O,
and NO
3 , many important metals and trace molecular species, sulfur, and other measurements of CO 2
system parameters (pH, CO 2 fugacity (f CO2 ), total dissolved inorganic carbon (CT), total alkalinity (TA), and
carbonate ion concentration [CO
2
3 ]). An illustrative
system is the spectrophotometric elemental analysis
system (SEAS) [18.9]. Differently from the singlepurpose instruments available on the market, SEAS
offers a platform for CO 2 parameters, nutrients, and
trace metals. In its third generation, the in-situ chemical
sensor can be configured for a wide variety of chemical measurements, including trace metals, nutrients,
and inorganic carbon system variables in different sample environments and deployment modes. Automated
wet chemical analyzers such as these will see increased
use in AUVs and possibly in combinations with AUV
swarms to provide informative data on flux in different
oceanic contexts.
Oxygen chemical sensors [18.10, 11] are in high demand, especially for AUV mapping studies of anoxic
environments. Standard oxygen sensors such as oxygen electrodes, galvanic O 2 micro-sensors, and Clark
426 Part B Autonomous Ocean Vehicles, Subsystems and Control
18.3 Chemical Sensor Systems
Describing the state of chemical sensor systems is
an enormous task and could easily be the basis for
an entire volume dedicated to the subject. In this chapter we provide examples of technologies that represent aspects of chemical instrumentation and hopefully
will provide a snapshot of the chemical instrumentation migration to maturity within the environment
and mobility using AUVs. In general, key drivers for
evolution of chemical instrumentation come from the
ability of chemical sensors to provide trace detection
and spatial/temporal chemical dynamics in water and
within chemical ecology. Previous literature reviews
have covered and summarized in-situ chemical sensors into two volumes on chemical sensor systems for
aquatic sciences and chemical and biological sensors
for time-series focused research [18.6, 7]. These reviews describe a variety of continuous flow analyzers,
electrochemical sensors, and optical sensors for in-situ
measurements of dissolved chemical species.
Chemical sensors need to have excellent detection
limits to be desirable. Attributes such as precision,
selectivity, response time, a large dynamic concentration range, low power consumption, robustness, and
less variation of instrumental response with temperature
and pressure are the most advantageous. Also, instruments that stay free of fouling problems are highly
attractive to end users. It is also important to note that
some chemical sensors are deployed in several different oceanic environments for the detection of targeted
chemical species, whereas others are useful in only
one environment because of various configuration limitations. There are a number of chemical sensors that
can be used as stand alone in-situ sensors, whereas
a greater number involve water being brought from the
environment into an analytical cell or chamber via tubing, either aboard ship or onboard a mobile platform
such an AUV. Multi-element sensors that measure several chemical species from the same water sample are
an area that researchers are pushing for further development. These multi-function sensors will increase the
capability of AUVs (for example) to characterize spatial
ocean chemical composition and diversity.
At present, most sensors need further refinement to
perform measurements at very high acquisition rates
without a loss of precision or sensitivity. Fundamental
inorganic compounds and ions (oxygen, basic nutrients,
hydrogen sulfide, pH, carbon dioxide, NOx, SOx, Fe,
Mn, and trace metals) are good candidates for continual AUV-based integration. Nitrate, a key nutrient, as
a sensor system has seen major advancement in the last
few years. Nitrate sensors are a current example of instrumentation that will see increased usage for nutrient
mapping, particularly in AUV coastal surveys. Current
commercial nitrate sensors can be accessed as either by
the use of automated wet chemistry or ultraviolet (UV)
spectroscopy performed in situ. Optical UV techniques
function to detection levels of 0:5 M, but for levels
below that wet chemistry is required at present. UV nitrate sensors are, therefore, suitable for use in the ocean
surface except for some rare areas. For work below the
micromolar level there are wet chemical systems that
can achieve nanomolar levels of detection [18.6]. At
the time of writing this we are not aware of any having
been deployed on an AUV. Additionally, wet chemical
systems are evolving to measure ammonia, nitrite, orthophosphate, silicates, and iron at desired levels and
acquisition rates for waters and sediments. We expect
that these instruments will in time be in a format that is
tractable for insertion on AUV platforms.
The study of boundaries and interface environments
are important in ocean dynamics. Boundary layers include atmosphere-to-water, water-to-sediment/soil, as
well as the boundaries between fluid masses with
sharply different characteristics. These different contexts all require the measurement of fluxes. Additionally, deterioration of water quality, ocean acidification,
and unknown processes have resulted in a tremendous
decline of ocean communities [18.8]. Quantifying the
fluxes of chemicals is of great importance to understand
these local, regional, or global biogeochemical processes. These include fluxes of key carbon species such
as CO 2 and CH 4 , nitrogen species such as NH
C
4 , N 2 O,
and NO
3 , many important metals and trace molecular species, sulfur, and other measurements of CO 2
system parameters (pH, CO 2 fugacity (f CO2 ), total dissolved inorganic carbon (CT), total alkalinity (TA), and
carbonate ion concentration [CO
2
3 ]). An illustrative
system is the spectrophotometric elemental analysis
system (SEAS) [18.9]. Differently from the singlepurpose instruments available on the market, SEAS
offers a platform for CO 2 parameters, nutrients, and
trace metals. In its third generation, the in-situ chemical
sensor can be configured for a wide variety of chemical measurements, including trace metals, nutrients,
and inorganic carbon system variables in different sample environments and deployment modes. Automated
wet chemical analyzers such as these will see increased
use in AUVs and possibly in combinations with AUV
swarms to provide informative data on flux in different
oceanic contexts.
Oxygen chemical sensors [18.10, 11] are in high demand, especially for AUV mapping studies of anoxic
environments. Standard oxygen sensors such as oxygen electrodes, galvanic O 2 micro-sensors, and Clark