Dissolved oxygen (DO) refers to the level of free, non-compound oxygen present in water, and is a critical factor in the capacity of an aquatic ecosystem to support living organisms. Two methods are used for the in-situ measurement of DO in surface waters: Winkler titration, membrane-covered electrochemical sensors (polarographic or galvanic cell), and luminescent-based optical sensors. Optical technology has quickly become a preferred method for measurement of DO, due to accuracy advantages over electrochemical sensors when it comes to fouling and long-term drift.
pH and Redox Potential
The pH value describes the activity of hydrogen ions in aqueous solutions typically on a scale of 0 to 14, from which liquids are characterized as being acidic, alkaline or neutral. In environmental sampling and monitoring, high or low pH values can be indicative of pollution. Oxidation-reduction potential (ORP) measures the ability of an aquatic environment to break down waste products, such as contaminants and dead plants and animals. The potentiometric method for measuring pH is used by most major sensors manufacturers.
Electrical conductivity can be used to determine concentration of solutions, detect contaminants and determine the purity of water. There are two types of conductivity measurement: contacting and inductive. The choice of which to use depends on the amount of conductivity, the corrosiveness of the liquid, and the quantity of suspended solids. The inductive method is generally better when the conductivity is high, the liquid is corrosive, or suspended solids are present. Conductivity, along with temperature, also allow for salinity values to be calculated through algorithms.
Turbidity is the measurement of water clarity. Suspended sediments, such as particles silt, clay and sand frequently enter the water from disturbed soils and can contain pollutants such as phosphorus, pesticides, or heavy metals which adversely affect the aquatic ecosystem. Turbidity sensors measure in either Nephelometric Turbidity Units (NTU) or Formazin Nephelometric Units (FNU). Due to the different light sources used in each of these measurements results are not directly comparable.
Submersible Pressure Transducers
Submersible pressure transducers (SPT) are submerged at a fixed depth below the water surface and measure equivalent hydrostatic pressure of the water head above the sensor diaphragm for the calculation of the total liquid depth. Vented pressure sensors, which use a vented cable to connect the base of the pressure transducer to atmospheric pressure, compensate for barometric pressure changes at the surface. Variances in accuracy of measurement depend on the model of pressure sensor used, the accuracy of some sensors is reduced by temperature variation, non-linearity and hysteresis, as well as long-term drift. The potential for sensor fouling should be a consideration before installation of SPT. SPTs can be used in a wide range of applications, including for both surface and groundwater as well as tanks.
A shaft encoder is an electro-mechanical device which converts the position of a shaft to an electrical signal. The rotational shaft utilizes a float and counter-weight attached to a line or tape placed around a pulley, as the water level changes, the float moves up and down and rotates the shaft, which records a change in water level.
Shaft encoders can be use in some surface water applications and for pan evaporation monitoring.
Ultrasonic water level instruments use sound waves in frequency range ~20-200 kHz to determine fluid level. A transducer directs bursts sound waves down onto the surface of the water which then reflects an echo of these waves back to the transducer. The transducer performs calculations to convert the distance of wave travel into a measure of height, and therefore distance to water surface. The accuracy of Ultrasonic sensors can be affected by condensation on the transducer and very high concentrations of fine sediment in suspension, which can scatter and absorb the sonic pulse. Ultrasonic sensors can be use in some surface water applications and for tank monitoring.