3.3.1 Optical Dissolved Oxygen Sensors
In the mid-2000s, optical dissolved oxygen sensors became widely available.
Optical dissolved oxygen sensors use a permeable membrane through which
dissolved oxygen molecules diffuse without electrochemical reduction of oxygen
molecules. These sensors utilize a special dye, light-emitting diodes (LEDs), and
photoreceptors; the dye luminesces red when excited with a blue LED and is
detected by the photoreceptor. The dye’s ability to luminesce is quenched in the
presence of oxygen. Stability and accuracy of the sensor is increased by using a
secondary reference red LED which is reflected back to the photoreceptor. The
partial pressure of oxygen in the water is proportional to the amount of oxygen
diffusing through the membrane and can be measured by the lifetime of the
luminescence from excitation by the blue light compared to that of the reference
value (red light). 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 [11].
Optical dissolved oxygen sensors are almost exclusively smart sensors, each
with their own microprocessor housed within the sensor. These sensors are also
typically designed for use with antifouling systems such as wipers, making them an
ideal solution for long-term monitoring. Optical sensors have significantly low
maintenance requirement and increased support for long-term deployments.
These sensors are incredibly stable and exhibit significantly less calibration drift
than the electrochemical sensors.
3.3.2 Turbidity Sensors
Turbidity sensors measure the clarity of water and can be used as a surrogate for the
amount of suspended material in the water (e.g., soil particles, plankton, and
microbes). Increased levels of turbidity absorb more heat and can result in warmer
water temperatures. Excess turbidity reduces light availability which can have an
impact on submerged aquatic vegetation and photosynthesis and dissolved oxygen
production. Suspended materials in the water column can also add stress to living
resources through gill blockage and increased disease susceptibility. Benthic
macroinvertebrates, fish eggs, and bivalves can be smothered as particles settle
out of the water column. Common causes of high turbidity include runoff from
urban stormwater, construction sites, agricultural practices, logging activity, and
point source discharges [14].
Turbidity can be measured by one of the three main methods: transmissometer,
backscatter, and nephelometer (Fig. 13). All of these methods measure turbidity by
illuminating the water sample volume with a light source and measuring the
intensity of light scattered by the particles in the water at a set angle with an optical
detector (e.g., photodiode). Most multiparameter water quality sondes utilize sensors that measure turbidity via the nephelometric method. For this method, the light
Advances in Water Sensor Technologies and Real-Time Water Monitoring
189
In the mid-2000s, optical dissolved oxygen sensors became widely available.
Optical dissolved oxygen sensors use a permeable membrane through which
dissolved oxygen molecules diffuse without electrochemical reduction of oxygen
molecules. These sensors utilize a special dye, light-emitting diodes (LEDs), and
photoreceptors; the dye luminesces red when excited with a blue LED and is
detected by the photoreceptor. The dye’s ability to luminesce is quenched in the
presence of oxygen. Stability and accuracy of the sensor is increased by using a
secondary reference red LED which is reflected back to the photoreceptor. The
partial pressure of oxygen in the water is proportional to the amount of oxygen
diffusing through the membrane and can be measured by the lifetime of the
luminescence from excitation by the blue light compared to that of the reference
value (red light). 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 [11].
Optical dissolved oxygen sensors are almost exclusively smart sensors, each
with their own microprocessor housed within the sensor. These sensors are also
typically designed for use with antifouling systems such as wipers, making them an
ideal solution for long-term monitoring. Optical sensors have significantly low
maintenance requirement and increased support for long-term deployments.
These sensors are incredibly stable and exhibit significantly less calibration drift
than the electrochemical sensors.
3.3.2 Turbidity Sensors
Turbidity sensors measure the clarity of water and can be used as a surrogate for the
amount of suspended material in the water (e.g., soil particles, plankton, and
microbes). Increased levels of turbidity absorb more heat and can result in warmer
water temperatures. Excess turbidity reduces light availability which can have an
impact on submerged aquatic vegetation and photosynthesis and dissolved oxygen
production. Suspended materials in the water column can also add stress to living
resources through gill blockage and increased disease susceptibility. Benthic
macroinvertebrates, fish eggs, and bivalves can be smothered as particles settle
out of the water column. Common causes of high turbidity include runoff from
urban stormwater, construction sites, agricultural practices, logging activity, and
point source discharges [14].
Turbidity can be measured by one of the three main methods: transmissometer,
backscatter, and nephelometer (Fig. 13). All of these methods measure turbidity by
illuminating the water sample volume with a light source and measuring the
intensity of light scattered by the particles in the water at a set angle with an optical
detector (e.g., photodiode). Most multiparameter water quality sondes utilize sensors that measure turbidity via the nephelometric method. For this method, the light
Advances in Water Sensor Technologies and Real-Time Water Monitoring
189