Stevens Water Quality Sensor Parameters and Overview

Stevens range of single parameter water quality sensors are a cost-effective alternative solution to a multi-parameter sensor for those interested in only one or two of the following water parameters. Our single parameter sensors are temperature corrected and measure dissolved oxygen, electrical conductivity, pH, turbidity and Oxidation Reduction Potential (ORP).

Electrical 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.

pH

pH is measured to determine the activity of hydrogen ions[H+] in a solution. 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.

Dissolved 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). DO sensors us 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.

Temperature

Temperature measure of the warmth or coldness of the water is important because it affects food supply, photosynthesis, metabolism, and the solubility of gasses and minerals. For example, colder water can hold more dissolved oxygen, which is critical for fish and other aquatic organisms.

When you don’t feel well, chances are the first thing someone does is take your temperature. Water temperature is measured for several reasons. First, it determines the kinds of organisms that can survive in a stream. Some organisms will die if the water temperature gets too hot or too cold. Temperature also can affect the chemistry of the water. For example, warm water holds less oxygen than cold water.

Oxidation Reduction Potential

Oxidation-reduction potential (ORP), or “redox”, indicates the relative capacity of a solution to oxidize or reduce. For example, to minimize biological growth a cooling system is usually treated with an oxidizing biocide such as chlorine, chlorine dioxide, or bromine.

Maintaining the ORP in a designated range will ensure that the proper amount of oxidizing agent is present. At the other end of the spectrum, reducing agents are often used to remove oxygen or other compounds from process streams. ORP can be used to monitor the proper level of these agents as well.

ORP sensors measure the electrochemical potential between the solution and a reference electrode. Readings are expressed in millivolts with positive readings indicating increased oxidizing potential, and negative readings being increased reduction. ORP sensors cannot indicate levels of specific compounds, only relative degrees of oxidation or reduction.

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.