USGS-OWQ: NFM 6.7.3.B: Turbidity, electrometric field measurement by submersible sensor
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Official Method Name
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Turbidity, Dynamic (Submersible-Sensor) Determination |
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Current Revision
| 2006 |
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Media
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WATER |
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Instrumentation
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Turbidity |
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Method Subcategory
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Physical |
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Method Source
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Citation
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Brief Method Summary
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The measurement method requires turbidity determination using a submersible sensor that can be deployed in situ, either to make a single measurement or for longer-term monitoring. Turbidity is determined through the use of a sample cell inserted into an instrument measurement chamber and a turbidity value read directly from the instrument scale or by using a calibration curve. When recording the data value, method codes describing the specific instrumentation used should be noted and the data should be qualified with an E, if particle settling or instability in initial readings occurs, and with a D, if the sample is diluted. Although technological advances in turbidity measurement have produced a variety of instrument types to meet one or more of these differing objectives, turbidity instruments of different designs commonly do not yield identical or equivalent results. Selection of the appropriate turbidity instrument requires, therefore, consideration of project objectives, data requirements, and the physical and chemical properties of the water body. Information for specific instruments is maintained at http://water.usgs.gov/owq/turbidity_codes.xls. Within the USGS, the predominant instrumentations for the dynamic determination of turbidity are the Hach 2100, Hydrolab Quanta, and YSI 6000 models. |
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Scope and Application
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Dynamic determination generally reflects the dynamic conditions in a water body more accurately than static measurements of discrete samples because it avoids problems of particle settling. |
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Applicable Concentration Range
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Variable based on instrument, although readings beyond 4,000 FNU are not reproducible between instruments. |
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Interferences
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The measurement of turbidity can be affected by a variety of water properties, sensor fouling (from biological growth or scratches on the instrument optical surface), and the physical condition of the sample. Examples of influential water properties are the color of dissolved constituents in the water matrix and particulate materials, particle size, and density. The presence of bubbles or gases in the water, and settling of particulates in the sample can also affect turbidity measurement. Many types of instruments have been designed to minimize many of these interferences. Refer to the subject report for additional guidance on instrumentation. |
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Quality Control Requirements
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Where turbidity from one water source will be compared with turbidity from another source or against a numerical criterion, the use of consistent procedures, instrumentation, and supplies is critical. Turbidity measured using instruments with different optical designs can differ by factors of two or more for the same environmental sample, even with identically calibrated instruments. |
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Sample Handling
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N/A |
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Maximum Holding Time
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N/A |
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Relative Cost
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Unknown |
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Sample Preparation Methods
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N/A |
