Water temperature sensors measure temperature with a high-precision thermistor. Thermistors contain a resistor, whose resistance changes with temperature,
housed inside a thin-walled sleeve of heat conductive, corrosion-resistant material
(e.g., titanium). The change in resistance is directly proportional to the change in
temperature and the resistivity of the resistor material, which is a constant. A
low-voltage direct current (DC) is passed through the resistor to measure a voltage
drop, and the related temperature is calculated from a manufacturer-specific algorithm. Since initial invention, the core sensing technology of thermistors has largely
remained unchanged; however, advances in electronics and microprocessors have
made today’s sensors more accurate and stable than their predecessors.
3.1.2 Conductance Sensors
Conductance or electrical conductivity is a measure of water’s ability to pass an
electric current which can be used as an indicator of water quality. Streams and rivers
usually have a constant range of baseline conductivity. The baseline conductivity in
streams and rivers is affected by the chemical composition of the water, which is
primarily determined by the geology of the stream/river bank and bed. Sediments
which contain salts and inorganic dissolved solids that readily ionize in water result in
high conductivity, while inert material, such as granite bedrock that does not ionize in
water results in low conductivity. Change in baseline conductivity is an indicator of
change in water quality that may be caused by evaporation, flooding, pollutant
intrusion via stormwater runoff, or other contaminant discharges into surface water
systems. For example, an oil spill would result in reduced conductivity levels because
of the presence of hydrocarbons and alcohols, while an untreated sewage discharge
would increase conductivity levels due to the presence of nitrates and phosphates [8].
Conductivity is directly related to water temperature, and therefore it is often
expressed as conductivity at 25
C, otherwise known as specific conductance.
Conductivity sensors measure an electrical current flow through one or more
pairs of electrodes and the water sample. A voltage is applied between a pair of
electrodes that are a known distance apart from one another. The resistance of water
and its chemical composition results in a voltage drop which is indirectly related
to the conductivity of water, measured per centimeter, and expressed in units of
μS/cm. Like thermistors, the core sensing technology of conductivity sensors has
largely remained unchanged since the 1950s. However, these sensors have become
more stable and accurate due to advances in electronics and microprocessors.
Other water quality parameters that can be calculated from conductivity are
salinity and total dissolved solids (TDS). Salinity and conductivity are strongly
correlated, and salinity is commonly calculated from conductivity and temperature
measurements by manufacturer algorithms that follow Standard Methods for the
Examination of Water and Wastewater (Standard Method #2520 [9]). TDS, commonly expressed in mg/L concentrations, is calculated from conductivity and a
monitoring location-specific empirically determined coefficient (Standard Method
#2510 [9]). Many sensor manufacturers provide capability to automatically
Advances in Water Sensor Technologies and Real-Time Water Monitoring
185
housed inside a thin-walled sleeve of heat conductive, corrosion-resistant material
(e.g., titanium). The change in resistance is directly proportional to the change in
temperature and the resistivity of the resistor material, which is a constant. A
low-voltage direct current (DC) is passed through the resistor to measure a voltage
drop, and the related temperature is calculated from a manufacturer-specific algorithm. Since initial invention, the core sensing technology of thermistors has largely
remained unchanged; however, advances in electronics and microprocessors have
made today’s sensors more accurate and stable than their predecessors.
3.1.2 Conductance Sensors
Conductance or electrical conductivity is a measure of water’s ability to pass an
electric current which can be used as an indicator of water quality. Streams and rivers
usually have a constant range of baseline conductivity. The baseline conductivity in
streams and rivers is affected by the chemical composition of the water, which is
primarily determined by the geology of the stream/river bank and bed. Sediments
which contain salts and inorganic dissolved solids that readily ionize in water result in
high conductivity, while inert material, such as granite bedrock that does not ionize in
water results in low conductivity. Change in baseline conductivity is an indicator of
change in water quality that may be caused by evaporation, flooding, pollutant
intrusion via stormwater runoff, or other contaminant discharges into surface water
systems. For example, an oil spill would result in reduced conductivity levels because
of the presence of hydrocarbons and alcohols, while an untreated sewage discharge
would increase conductivity levels due to the presence of nitrates and phosphates [8].
Conductivity is directly related to water temperature, and therefore it is often
expressed as conductivity at 25
C, otherwise known as specific conductance.
Conductivity sensors measure an electrical current flow through one or more
pairs of electrodes and the water sample. A voltage is applied between a pair of
electrodes that are a known distance apart from one another. The resistance of water
and its chemical composition results in a voltage drop which is indirectly related
to the conductivity of water, measured per centimeter, and expressed in units of
μS/cm. Like thermistors, the core sensing technology of conductivity sensors has
largely remained unchanged since the 1950s. However, these sensors have become
more stable and accurate due to advances in electronics and microprocessors.
Other water quality parameters that can be calculated from conductivity are
salinity and total dissolved solids (TDS). Salinity and conductivity are strongly
correlated, and salinity is commonly calculated from conductivity and temperature
measurements by manufacturer algorithms that follow Standard Methods for the
Examination of Water and Wastewater (Standard Method #2520 [9]). TDS, commonly expressed in mg/L concentrations, is calculated from conductivity and a
monitoring location-specific empirically determined coefficient (Standard Method
#2510 [9]). Many sensor manufacturers provide capability to automatically
Advances in Water Sensor Technologies and Real-Time Water Monitoring
185