Turbidity Units, TSS, Water Clarity, Suspended Particles Measurement, Turbidity in Water

Turbidity is the measurement of water clarity. Suspended sediments, such as particles of clay, soil and silt, frequently enter the water from disturbed sites and affect water quality. Suspended sediments can contain pollutants such as phosphorus, pesticides, or heavy metals. Suspended particles cut down on the depth of light penetration through the water, hence they increase the turbidity -- or "murkiness" or "cloudiness" -- of the water.

High turbidity affects the type of vegetation that grows in water. Turbid water can lend information in regards to the health or well-being of the water body itself. It is important to note that the appearance of high turbidity does not necessarily mean the water body is suffering. However, the level can adversely affect the ecosystem if it changes drastically and maintains that drastic change whereas short term turbid “events” may be temporary and have little overall effect on the system. As with any water quality parameter, it is good to have historic data for any site where turbidity is being monitored so trends can be tracked and the occurrence of an event can be captured. For long-term, in situ continuous monitoring of turbidity, a self-cleaning sensor is usually necessary to avoid fouling of the sensor and maintain accuracy.

The units associated with turbidity readings can be confusing. (See Tech Note: Turbidity Units and Calibration Solutions). These are typically represented as Nephelometric Turbidity Units (NTU) or Formazin Nephelometric Units (FNU) for most in-situ sensors. In the early days of data collection, it was common to use NTU as the unit regardless of the actual method used to collect the data, but more recently it has become desirable to start using units which reflect the method in which the measurements were obtained.

Technically speaking, NTU is a unit of measure that is best used to represent readings captured using a white light at a 90 degree detection angle and FNU is best used when the data is measured using an 860 nm light (near IR) with a 90 degree detection angle (ISO7027 compliant).

The use of units that are associated with the measurement method can help pinpoint limitations with the collected data and help identify potential interferences. Some manufacturers have run into a predicament in regards to the units in recent years. YSI has been around for nearly 70 years and has offered in-situ turbidity sensors since early 1995.

As was mentioned previously, turbidity was expressed in NTU in those early days of field measurements even though the method used to collect the data may have been ISO7027, which we now know is typically represented in FNU. YSI’s turbidity sensors have always followed the ISO7027 method, so a better representation of the measurements would be to use FNU. Many agencies have historic data that was collected as NTU and now newer YSI instruments like the EXO sondes and the ProDSS provide the option to record data in either of these units. Therefore, the agency collecting the data needs to decide whether they will continue using NTU or switch to using FNU. The actual values associated with measurements made with YSI sensors will be comparable whether NTU or FNU is used for the units, as YSI has always used methodology consistent with ISO7027. Therefore, no conversion of the data values is necessary if YSI instrumentation has been used to collect the data.