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.  The turbidity of the water can lend information in regards to the health or well-being of the water body. It is important to note that the appearance of high turbidity does not necessarily mean the water body is suffering.  However, the turbidity level of the water can adversely affect the ecosystem if it changes drastically and maintains that drastic change whereas short term turbidity “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 turbidity sensor is usually necessary to avoid fouling of the sensor and maintain accuracy.

Turbidity readings are typically represented as Nephelometric Turbidity Units (NTU) or Formazin Nephelometric Units (FNU) for most in-situ turbidity sensors. In the early days of turbidity data collection, it was common to use NTU as the units 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 turbidity measurements were obtained. Technically speaking NTU is a unit of measure that is best used to represent turbidity 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). 

YSI measures turbidity with an optical sensor.   Light from the emitter enters the sample and scatters off particles in the water. The light, scattered at 90 degrees, enters a detector fiber and is measured by a photodiode. This follows the nephelometric technique of measurement, and values are expressed in nephelometric turbidity units (NTUs).  Now newer YSI instruments like the EXO sondes and the ProDSS provide the option to record data in either NTU or FNU.  YSI has always used methodology consistent with ISO7027 and no conversion of the data would be necessary in the case of YSI instrumentation.

Essential Turbidity Posts on the Blog

Turbidity Measurements: Tips and Precautions

Continuous Turbidity Monitoring in Ohio Rivers Using Modified Sidestream Method

Water Quality Monitoring Buoys Protect the Housatonic River | Application Note

5 Tips To Prevent Costly Mistakes With Your Sondes | Tip 1 of 5

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