Water Quality Sensors Overview

Stevens offers a range of single parameter and multi-parameter water quality sensors to meet your unique monitoring needs.

Ambient Light/PAR

PAR stands for Photosynthetically Active Radiation, and is a specific type of ambient light. An ambient light sensor is also a PAR sensor. Ambient light measurement, in the context of water quality monitoring, is a measurement of sunlight intensity at a certain point in the water column. Sunlight intensity influences biota that relies on photosynthesis for nutrition. This includes photosynthetic phytoplankton (green and blue-green algae, some diatoms), and both submerged and emergent macrophytes (larger plants that grow underwater or partially underwater).

Blue-Green Algae

Cyanobacteria, AKA blue-green algae, are common forms of photosynthetic bacteria present in most freshwater and marine systems. In case of cyanbacterial blooms, some species can produce toxins called cyanobacteria that can cause health risk to humans and animals. Cyanobacteria blooms also present an unpleasant appearance, taste and odor in water.

Early detection in recreational waters can allow authorities to warn the general public of potentially unsafe water conditions that could otherwise result in illness to humans, or potential death of livestock and other animals. In drinking water facilities, early detection can allow preventive action that will help avoid a taste and odor event, clogged filters, or other potential risks to the drinking water supply.

Chlorophyll-A

Both chlorophyll-a and secchi depth are long-accepted methods for estimating the amount of algae in lakes. However, measuring the concentration of chlorophyll-a is much easier and provides a reasonable estimate of algal biomass. Chlorophyll-a is the green pigment that is responsible for a plant’s ability to convert sunlight into the chemical energy needed to fix CO2 into carbohydrates.

Monitoring of Chlorophyll-a helps understand the productivity of a lake. Researchers can also monitor parameters such as Dissolved Oxygen and pH at the same time as their Chlorophyll measurements. These additional parameters are important to fully understand the productivity of the lake because the amount of oxygen that is being produced or absorbed, as well as the changes in pH that coincide with algal productivity, are critically important to completing the full picture of the lake’s health.

Conductivity/TDS

Electrical conductivity sensors are used to measure the ability of water to carry an electrical current. Absolutely pure water is a poor conductor of electricity. Water shows significant conductivity when dissolved salts are present. Over most ranges, the amount of conductivity is directly proportional to the amount of salts dissolved in the water.

The amount of mineral and salt impurities in the water is called total dissolved solids (TDS). TDS is measured in parts per million. TDS tell how many units of impurities there are for one million units of water. For example, drinking water should be less than 500 ppm, water for agriculture should be less than 1200 ppm, and high tech manufactures often require impurity-free water. One way to measure impurities in water is to measure the electric conductivity of water.

A conductivity sensor measures how much electricity is being conducted through a centimeter of water. Specific conductivity is expressed as mhos per centimeter (M/cm), sometimes called siemens per centimeter (S/cm). Because a mho (or siemen) is a very large unit, the micromho (microsiemen) or millimho (millisiemen) typically is used (mS/cm).

To convert the electric conductivity of a water sample (mS/cm) into the approximate concentration of total dissolved solids (ppm), the mS/cm is multiplied by a conversion factor. The conversion factor depends on the chemical composition of the TDS and can very between 0.54 – 0.96. A value of 0.67 is commonly used as an approximation if the actual factor is not known [(TDS)ppm = Conductivity µS/cm x 0.67].

Since conductivity varies with temperature, it is necessary to correct the readings for changes in temperature. Most instruments contain circuits that automatically compensate for temperature and correct the readings to a standard 25°C.

Dissovled Oxygen

Dissolved oxygen (DO) measurements tells how much oxygen is available in the water for fish and other aquatic organisms to breathe. Healthy waters generally have high levels of DO (some areas, like swamps, naturally have low levels of DO). Just like human beings, aquatic life needs oxygen to survive. A measure of the molecular oxygen dissolved in water is an important determinant of whether the water body is suitable for aerobic (oxygen-requiring) organisms, such as fish and zooplankton. Values greater than 5 or 6 parts per million generally will support diverse forms of aquatic life.

DO sensors use an oxygen-permeable membrane that sets up a current that indicates the level of oxygen present. These sensors often can read DO in the range of saturation (about 8 ppm) down to the part per billion (ppb) range.

Several factors can affect how much DO is in the water. These include temperature, the amount and speed of flowing water, the plants and algae that produce oxygen during the day and take it back in at night, pollution in the water, and the composition of the stream bottom (gravelly or rocky bottoms stir up the water more than muddy ones do, creating bubbles that put more oxygen into the water).

Ion-Selective Electrodes (Ammonia/Ammonium, Nitrate, Chloride)

pH

pH is measured to determine the activity of hydrogen ions [H+] in a solution (the p stands for “potential of” and the H is hydrogen). A pH sensor measures how acidic or basic the water is, which can directly affect the survival of aquatic organisms. pH ranges from 0 (very acidic) to 14 (very basic), with 7 being neutral. Most waters range from 5.5 to 8.5. Changes in pH can affect how chemicals dissolve in the water and whether organisms are affected by them. High acidity (such as pH of less than 4) can be deadly to fish and other aquatic organisms.

A pH sensor uses an electrode with hydrogen ion selective glass membrane that measures the difference in electrical potential between the sample and a reference electrode. The electrical potential is proportional to the hydrogen ions and must be corrected for temperature or the instrument may contain an automatic temperature compensation circuit.

Oxidation Reduction Potential (Redox)

ORP measurements are used to monitor chemical reactions, to quantify ion activity, or to determine the oxidizing or reducing properties of a solution. The ORP is greatly influenced by the presence or absence of molecular oxygen. Low redox potentials may be caused by extensive growth of heterotrophic microorganisms. Such is often the case in developing or polluted ecosystems where microorganisms utilize the available oxygen.

Rhodamine WT (Water Tracing)

It is often used as a tracer within water to determine the rate and direction of flow and transport. Rhodamine dyes fluoresce and can thus be measured easily and inexpensively with instruments called fluorometers. Rhodamine dyes are generally toxic, and are soluble in water, methanol, and ethanol.

Temperature

Turbidy is measured to determine the clarity of the water or in other words, how many particulates are floating around in the water, such as plant debris, sand, silt, and clay, which affects the amount of sunlight reaching aquatic plants. Such particulates in the water are often referred to as total suspended solids (TSS). Excess turbidity can reduce reproduction rates of aquatic life when spawning areas and eggs are covered with soil. Turbidity measurements are often used to calculate the inputs from erosion and nutrients. Turbidity or TSS sensors are used to measure the clarity of the water.

Turbidity meters ascertain this reading using one of several methods. One technique is to pass a beam of light through the sample, with the amount of light absorbed being proportional to the turbidity. This method does not work well for highly colored or very turbid samples. Most other methods measure light that is reflected either directly from the sample or off its surface. The amount of scattered light, which can be measured at various angles, is proportional to the turbidity.

Turbidity is most commonly measured in Nephelometric Turbidity Units (NTU) but is sometimes measured in Jackson Turbidity Units (JTU).

As with conductivity and TDS, turbidity is an approximation of the amount of TSS in a sample. The relationship depends on several factors including the size and shape of the suspended particles and their density.

Total Dissolved Gas

Total dissolved gas (TDG) is the amount of total gaseous compounds dissolved in a liquid. TDG is measured in units of pressure; this pressure includes the partial pressure of all gas species dissolved in the water. Measuring TDG is important in knowing the extent of saturation of a water body. Water supersaturated with atmospheric gases can cause gas bubble gill disease in aquatic organisms and may result in fish kills.

Turbidity/TSS

Turbidy is measured to determine the clarity of the water or in other words, how many particulates are floating around in the water, such as plant debris, sand, silt, and clay, which affects the amount of sunlight reaching aquatic plants. Such particulates in the water are often referred to as total suspended solids (TSS). Excess turbidity can reduce reproduction rates of aquatic life when spawning areas and eggs are covered with soil. Turbidity measurements are often used to calculate the inputs from erosion and nutrients. Turbidity or TSS sensors are used to measure the clarity of the water.

Turbidity meters ascertain this reading using one of several methods. One technique is to pass a beam of light through the sample, with the amount of light absorbed being proportional to the turbidity. This method does not work well for highly colored or very turbid samples. Most other methods measure light that is reflected either directly from the sample or off its surface. The amount of scattered light, which can be measured at various angles, is proportional to the turbidity.

Turbidity is most commonly measured in Nephelometric Turbidity Units (NTU) but is sometimes measured in Jackson Turbidity Units (JTU).

As with conductivity and TDS, turbidity is an approximation of the amount of TSS in a sample. The relationship depends on several factors including the size and shape of the suspended particles and their density.