A multiparameter sonde is a sealed probe with several sensors that measures water quality variables such as temperature, pH, oxygen and turbidity at once.
A multiparameter water quality sonde is a sealed probe that holds several sensors and measures many water quality variables at once, either during a site visit or left in place to log at a set interval. A typical sonde carries sensors for temperature, specific conductance, pH, dissolved oxygen, turbidity and depth. Optional sensors can measure chlorophyll, phycocyanin (from blue-green algae), dissolved organic matter, oxidation-reduction potential, nitrate or ammonium. Each sensor uses its own principle. Conductivity uses electrodes or an inductive cell. Dissolved oxygen is now mostly measured optically: oxygen shortens the glow of a light-excited dye. Turbidity uses light scattering, and chlorophyll uses fluorescence. pH uses a glass electrode against a reference. Depth comes from a pressure sensor. A data logger in the sonde or at the surface stores readings and can send them on by telemetry. Sondes are used in rivers, lakes, reservoirs, estuaries, groundwater wells and treatment works. Lowering a sonde on a cable to profile a lake from surface to bed is a common use. The main field problem is fouling. Algae, silt and biofilm grow on optical windows and membranes, faster in warm, nutrient-rich water, and readings drift. Many sondes have a wiper and copper guards to slow this. Regular site visits are still needed to clean, check against standards and recalibrate. pH and ion-selective sensors tend to drift most. A good practice is to log a reading before and after cleaning so drift can be corrected later. For example, a sonde moored in a reservoir logging every 15 minutes can show dissolved oxygen falling each night and peaking each afternoon, a sign of heavy algal growth. Strengths are many variables from one unit and fine time detail. Limits are fouling, drift, cost and the field time needed for upkeep. Lab samples are still needed for many regulated parameters.