Conductivity, often denoted as EC (electrical conductivity), is a measure of a water sample’s ability to conduct electric current. It is primarily determined by the presence of dissolved salts and other inorganic substances that ionize in water. Such substances are known as electrolytes, and they conduct electricity due to their positive and negative charges.
The SI unit for conductivity is siemens per meter (S/m). For the relatively small values measured in water, it is commonly expressed in units of microsiemens per centimeter (µS/cm) or millisiemens per centimeter (mS/cm). Conductivity increases with the amount of electrolytes dissolved in the water as well as with increases in temperature. For this reason, conductivity is often reported as specific conductance, which is a conductivity measurement made at or corrected to 25°C.
The electrical conductivity of water increases with temperature.
Salinity is related to conductivity as it describes the total concentration of all dissolved salts in a water sample. Non-salt constituents also influence conductivity, so absolute salinity cannot be derived from conductivity measurement alone but rather requires time-consuming laboratory analysis. Salinity is, therefore, commonly derived from specific conductance (conductivity and temperature) for convenience.
This is expressed as practical salinity using the Practical Salinity Scale (PSS-78), which relates the specific conductance to that of a potassium chloride (KCl) standard. Though technically dimensionless, salinity per PSS-78 is denoted in practical salinity units (PSU), where typical seawater is at 35 PSU.
Total dissolved solids (TDS) is defined as the sum of all particles that can pass through a 2-micron (0.0002 cm) filter. This includes both electrolytes (ions contributing to salinity) and any other materials, such as dissolved organic matter. TDS is reported as a concentration in mg/L. It can be measured by evaporation, but for field measurements, it is normally derived from conductivity measurement using a TDS factor, which is approximated depending on water type and any known sources of ionic and other materials.
Why Measure Conductivity, Salinity, and TDS?
Most natural water sources maintain a fairly stable conductivity level, and measurement is relatively simple and reliable using modern sensor technology. As a result, long-term conductivity measurement provides a stable baseline and an early indicator of changes in water quality that may occur from pollution, precipitation events, evaporation, or other sources.
The relationship of conductivity to salinity is of particular importance, as an increase in salinity decreases the dissolved oxygen capacity of water, which is fundamental to aquatic ecosystems. Cases of saltwater intrusion due to weather events, sea level rise, or pumping of groundwater near coastal areas can adversely affect aquatic habitats and drinking water supplies. In oceans, evaporation increases salinity, while the introduction of freshwater from precipitation and runoff decreases salinity. Conductivity measurement is useful for tracking long-term changes.
Point-source pollution typically increases conductivity as particulates dissolve and ionize in water. Examples include agricultural runoff containing phosphate and nitrate ions, sewage, and industrial discharge. Pollution from non-ionizing substances such as oils and other organic compounds can potentially decrease conductivity as well, though increases from pollution are more common.
TDS measurement is relevant to drinking water sources, as high dissolved solids concentrations can impact water taste and potentially human health if quantities are excessive or contain harmful substances such as heavy metals.
How Is Conductivity Measured?
The most common way that conductivity is measured is by passing electrical current between electrode pairs in a water sample. A sensor records either the voltage drop between electrodes at a fixed current or the current required to maintain a constant voltage. The sensor then calculates conductivity internally by referencing Ohm’s Law, which states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them:
Ohm’s Law is used to calculate conductivity based on current and voltage.
There are several important factors influencing sensor conductivity measurements in water. These include the number of electrodes, electrode materials, the cell constant, and the temperature coefficient.
Electrode-based sensors utilize either a two-electrode or four-electrode cell configuration. Two-cell sensors are relatively simple as they pass current between a single pair of electrodes. However, they can be susceptible to polarization, where ion accumulation at the electrode surfaces can distort readings.
Passing alternating (AC) rather than direct (DC) current between the electrodes counteracts polarization, but the effects cannot be eliminated altogether at high conductivity levels. In contrast, four-cell sensors include additional electrodes to mitigate these polarization effects, thus providing more accurate readings across a wider range of conductivities. However, they may not always achieve better accuracy than two-electrode sensors in very pure water samples where polarization effects are less pronounced.
Two-cell electrodes pass an alternating current between electrodes and measure the resulting voltage to determine conductivity. Four-cell electrodes have two current and two-voltage sensing electrodes to counteract the effects of ion polarization.
The electrodes are constructed from conductive materials like stainless steel, graphite, nickel, platinum, or titanium. Nickel and graphite are commonly used in conductivity sensors intended for field water quality measurements for their chemical stability, resistance to corrosion, and low cost relative to materials like platinum and titanium.
Since conductivity is the measurement of conductance in a specified volume of liquid, the physical geometry of the conductivity cell also impacts measurement. A 1 cm cubic volume is typical for conductivity sensors used in natural waters. However, not every cell has the same distance between electrodes and electrode surface area. A cell constant is therefore used to standardize the path length over which the conductivity measurement is taken, ensuring that readings are comparable and consistent between different sensor types and in different types of samples.
The cell constant of a conductivity sensor varies depending on the distance between electrodes and the electrode surface area.
The cell constant is defined as the distance between electrodes divided by the cross-sectional area of the path between electrodes. A lower cell constant indicates a smaller distance and/or larger electrodes, while a higher cell constant is associated with a greater distance and/or smaller electrodes. Generally speaking, a higher cell constant is ideal for high-conductivity measurements by facilitating a wide measurement range, whereas a lower cell constant enhances the sensor’s sensitivity to lower current levels and is therefore preferred for purer water with fewer dissolved ions.
To address the inherent temperature dependence of conductivity, most sensor designs include a temperature sensor. A corresponding temperature coefficient quantifies how much the conductivity of a solution is expected to change per degree change in temperature. It is typically expressed as a percentage increase or decrease per degree Celsius. Using the temperature coefficient, the sensor can report specific conductance for comparison of measurements taken at different temperatures or from different sites.
While electrode-based sensors are most common, toroidal conductivity sensors that use electromagnetic induction to measure conductivity are also available. Inductive/toroidal sensors may be preferred to reduce maintenance requirements in corrosive and high biofouling environments. However, they are less sensitive to low conductivity and, thus, not ideal in many natural waters.
How to Select a Conductivity Sensor?
Selecting a conductivity sensor for use in natural waters (lakes, rivers, streams, coastal, and ocean) should be based primarily on accuracy in the expected measurement range and maintenance requirements.
Freshwater sources will normally have much lower ion concentrations than brackish and saltwater and require a sensor design appropriate for accuracy at low conductivity conditions. For example, a two-electrode sensor with a low cell constant may have a relatively small measurement range but good accuracy within the range.
However, if periodic inputs from sources such as sediment loads, road salt, wastewater, agricultural runoff, or industrial discharges are expected, the range must be sufficient to capture any discharge events. This may require a different sensor design, such as a four-electrode sensor with average cell constant that expands the measurement range while still achieving a high degree of accuracy.
Biofouling and other physical interference due to sediment buildup, debris or other effects can affect measurement accuracy. Deployments can be extended in many environments by wiping or other cleaning and anti-fouling functions, but regular cleaning and calibration requirements should be considered. If corrosive conditions are present, evaluate whether the sensor materials of construction are appropriate for the measurement site.
In most cases, sensors should incorporate temperature compensation for calculation of specific conductance and comparability of measurements. If the sensor will be used in a manual sampling or profiling application where conditions change quickly, the response time of the temperature sensor is another important factor.
What to Consider When Preparing a Conductivity Sensor?
Calibration of conductivity sensors is normally performed at a single-point using a standard covering the expected measurement range. However, standards below 1000 µS/cm are rarely used even for relatively pure waters because they are very easy to contaminate, and some degree of ionic strength helps to confirm sensor responsiveness.
Regardless of the value of the calibration standard used, extreme care should be taken to avoid contamination. A dedicated calibration cup may help to avoid cross-contamination from other standards. Clean the sensor as thoroughly as possible prior to calibration, and rinse multiple times using the conductivity standard prior to placing the sensor for calibration. Ensure that the measurement cell is fully submerged for accuracy.
In addition to calibrating using a standard of known value, check the sensor’s reading in air. Measurements in air with a completely dry sensor should be at or very near zero. Non-zero readings indicate potential issues and should be investigated prior to putting the sensor into use.
When a calibration is accepted, make note of the sensor’s cell constant. The physical geometry of the cell may change slightly over time due to buildup, cleaning, corrosion, or other effects, thus impacting the cell constant. There should be little change in the cell constant with each successive calibration. Any significant changes may indicate that maintenance, such as cleaning or reconditioning, is required, or that the sensor should be replaced entirely.
Perform regular quality control (QC) checks during extended deployments to ensure that the sensor is performing consistently and in accordance with specifications.
How to Deploy a Conductivity Measurement System?
Conductivity sensors can be deployed in both land-based and open-water deployments. Factors to consider when planning for deployment include the type of water body, sources of conductivity, and mounting options.
Land-based systems may be mounted on a pole, wall, or other available structures. Applications such as road salt runoff in streams or saltwater inflow into canals require that the conductivity be well protected in a deployment pipe with perforation to allow flow through to the measurement cell. In other saltwater intrusion applications, such as pumping groundwater for drinking water supply, sensors may be secured into wells with data logging and telemetry at the surface.
A sensor in a secure deployment pipe can facilitate conductivity measurement in streams and canals. For groundwater monitoring, sensors are placed into wells.
Other applications, such as limnological studies, estuary dynamics, oceanographic surveys, and pollution detection in ports, may call for sensors to be placed from floating buoy platforms. Sensors are suspended at the depth of interest or connected in series for profiling. The latter can be particularly interesting for studies in estuaries or other locations where freshwater and saltwater mix and may form layers.
Data buoys facilitate measurements in open water applications, including multi-depth profiling.
Buoy systems have an onboard data logger with a solar-charged power supply for continuous operation. A single-point or multi-point mooring system supports the buoy, depending on the sensor depth and environmental conditions. The mooring system must allow the buoy to move with any waves and water level fluctuations that the site may experience.
Conclusion
Conductivity measurement quantifies the ability of water to conduct electricity due to the presence of dissolved ions. It is generally stable in natural waters and, thus, a good early indicator of changes in water quality due to discharges, biological or chemical processes, or evaporation.
Water conductivity is temperature-dependent, so for compatibility of measurements, it is often corrected to 25°C and reported as specific conductance. Conductivity measurement also offers a convenient method to approximate salinity and total dissolved solids (TDS) concentrations due to their close relation.
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