and biochemical demand consume oxygen. Typical daily dissolved oxygen peak in
natural waters occurs in the late afternoon at the height of photosynthesis and
minimum level at dawn after a night of respiration. When algal blooms are fueled
by nutrient enrichment, these daily variations in dissolved oxygen can be significant
and can result in fish kills if dissolved oxygen level drops below critical thresholds.
Seasonal variation of dissolved oxygen can result in hypoxic and anoxic conditions
in natural waters during the warm summer months when water temperature reaches
maximum levels and algal biomass peaks, which can stress and kill living resources.
The first practical dissolved oxygen sensor for field measurement was developed
in the early 1960s. These early sensors required continuous replenishment of
oxygen through the membrane which was achieved by either moving the sensor
through the water column or by using an electric stirrer to move oxygen across the
membrane. These sensors remained largely unchanged until the early 1990s when
stirring-independent electrochemical dissolved oxygen sensors were developed,
thereby enabling more efficient unattended continuous water quality monitoring.
Electrochemical dissolved oxygen sensors consist of a gold cathode and silver
anode in an electrolyte solution that is trapped over the sensing elements by an
oxygen permeable membrane. Oxygen molecules diffuse through the membrane at
a rate proportional to the pressure difference across the membrane and are reduced
at the gold cathode producing an electrical signal between the cathode and the
anode. The partial pressure of oxygen in the water is proportional to the amount of
oxygen diffusing through the membrane. The partial pressure of oxygen can be
barometrically compensated to yield a percent saturation value, which can then be
converted to a dissolved oxygen concentration value by compensating for temperature and salinity [12]. These electrochemical sensors require some level of flow in
order to replenish the oxygen through the membrane resulting in electrochemical
reduction of oxygen molecules. More advanced optical dissolved oxygen sensors
which eliminate that problem are described under optical sensors.
3.2.3 Nitrate, Ammonium, and Chloride Ion-Selective Electrode
Sensors
Nitrate, ammonium, and chloride are important measures of aquatic health and can
have significant impacts on living resources. Excess levels of nitrate can lead to
eutrophication, and excess levels of ammonium and chloride can lead to living
resource toxicity. Additionally, excess levels of chloride can lead to impacts
comparable to excessive TDS levels. Similar in design to pH sensors, there are a
range of ion-selective electrodes (ISEs) available for the measurement of parameters, most commonly nitrate, ammonium, and chloride. These sensors work exactly
like pH sensors except that a PVC membrane, selective for the analyte, is used rather
than a glass bulb that is selective for H
+ ions. The sensor module contains a static
concentration of the analyte, which binds to the inner membrane. The measured
electrical potential is related to the analyte values through a form of the Nernst
equation. These sensors only work in freshwater due to ion interferences. Optical
nitrate sensors offer several advantages over ISEs and are described in Sect. 3.3.5.
Advances in Water Sensor Technologies and Real-Time Water Monitoring
187
natural waters occurs in the late afternoon at the height of photosynthesis and
minimum level at dawn after a night of respiration. When algal blooms are fueled
by nutrient enrichment, these daily variations in dissolved oxygen can be significant
and can result in fish kills if dissolved oxygen level drops below critical thresholds.
Seasonal variation of dissolved oxygen can result in hypoxic and anoxic conditions
in natural waters during the warm summer months when water temperature reaches
maximum levels and algal biomass peaks, which can stress and kill living resources.
The first practical dissolved oxygen sensor for field measurement was developed
in the early 1960s. These early sensors required continuous replenishment of
oxygen through the membrane which was achieved by either moving the sensor
through the water column or by using an electric stirrer to move oxygen across the
membrane. These sensors remained largely unchanged until the early 1990s when
stirring-independent electrochemical dissolved oxygen sensors were developed,
thereby enabling more efficient unattended continuous water quality monitoring.
Electrochemical dissolved oxygen sensors consist of a gold cathode and silver
anode in an electrolyte solution that is trapped over the sensing elements by an
oxygen permeable membrane. Oxygen molecules diffuse through the membrane at
a rate proportional to the pressure difference across the membrane and are reduced
at the gold cathode producing an electrical signal between the cathode and the
anode. The partial pressure of oxygen in the water is proportional to the amount of
oxygen diffusing through the membrane. The partial pressure of oxygen can be
barometrically compensated to yield a percent saturation value, which can then be
converted to a dissolved oxygen concentration value by compensating for temperature and salinity [12]. These electrochemical sensors require some level of flow in
order to replenish the oxygen through the membrane resulting in electrochemical
reduction of oxygen molecules. More advanced optical dissolved oxygen sensors
which eliminate that problem are described under optical sensors.
3.2.3 Nitrate, Ammonium, and Chloride Ion-Selective Electrode
Sensors
Nitrate, ammonium, and chloride are important measures of aquatic health and can
have significant impacts on living resources. Excess levels of nitrate can lead to
eutrophication, and excess levels of ammonium and chloride can lead to living
resource toxicity. Additionally, excess levels of chloride can lead to impacts
comparable to excessive TDS levels. Similar in design to pH sensors, there are a
range of ion-selective electrodes (ISEs) available for the measurement of parameters, most commonly nitrate, ammonium, and chloride. These sensors work exactly
like pH sensors except that a PVC membrane, selective for the analyte, is used rather
than a glass bulb that is selective for H
+ ions. The sensor module contains a static
concentration of the analyte, which binds to the inner membrane. The measured
electrical potential is related to the analyte values through a form of the Nernst
equation. These sensors only work in freshwater due to ion interferences. Optical
nitrate sensors offer several advantages over ISEs and are described in Sect. 3.3.5.
Advances in Water Sensor Technologies and Real-Time Water Monitoring
187