Turbidimeters

Turbidity is a measure of the light-scattering properties of water and depends on the amount, size and composition of suspended matter, such as clay, silt, colloidal particles, plankton and other microscopic organisms. In liquids, turbidity is the opacity of these and is due to the Tyndall effect, an Irish scientist who studied this subject and defined that colloidal particles in a solution are visible by scattered light, i.e. solutions without suspended particles are transparent, as they practically do not scatter light. This difference makes it possible to distinguish those heterogeneous mixtures that are suspensions. Turbidity is measured using instruments called turbidimeters. Conventional turbidimeters pass light through a section of water and detect the amount of light scattered by water particles at an angle of 90 degrees to the light source. This type of light scattering measurement is called nephelometry. These turbidimeters are designed for field or laboratory measurements as well as 24/7 monitoring by setting an alarm that is activated when turbidity levels reach dangerous levels. Turbidity can also be estimated with instruments such as a colorimeter or spectrophotometer, which measure the decrease in transmitted light due to blockage caused by particles. These instruments are sometimes used to detect significant changes in the turbidity of a water system or for process control. The unit of turbidity has been defined "as the optical obstruction of light caused by one million silica particles in distilled water". There are several units to express turbidity: NTU, FNU, FAU and JTU. Nephelometric Turbidity Unit (NTU) - Used only when results are determined by the 90° dispersion method and EPA specifies the use of this unit for US EPA Method 180.1. It is the most common unit in use and is generally applied to all instruments, such as turbidimeters. Formazin Turbidity Unit (FTU): this is the second most common unit in use and again defines a nephelometric dispersion from the EPA or ISO method. Formazin Nephelometric Unit (FNU): technically defines the measurement with the 90° detector and is most correctly applied to instruments using the EN ISO 7027 method. The above units are equivalent, i.e. there is no conversion factor between the two Jackson Turbidity Unit (JTU): derived from the use of Jackson Candle 40 JTU is approximately equivalent to 40 NTU, but correlation with other values is open to debate. Formazin Attenuation Unit (FAU): defines a measurement made with an ISO 7027 instrument at 0° instead of 90°. If the instrument in question is an EPA method device, the units are abbreviated to AU (Attenuation Units). Other more abstract units of unusual use include FNRU (Formazin Nephelometric Ratio Unit) for systems using 90° scattering plus additional detectors, FBU (Formazin Reverse Scattering Unit) for systems using a 30° backscattering detector and FNMU (Formazin Nephelometric Multi-Foam Unit) for nephelometric systems and using various other detection angles.