USGS: TM1-D5 Data Collection: Optical nitrate sensor data collection
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Official Method Name
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Optical Techniques for the Determination of Nitrate in Environmental Waters: Guidelines for Instrument Selection, Operation, Deployment, Maintenance, Quality Assurance, and Data Reporting |
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Current Revision
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Media
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WATER |
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Instrumentation
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UV-Fluorescence |
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Method Subcategory
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Inorganic |
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Method Source
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Citation
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Brief Method Summary
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The collection of data from UV nitrate sensors has two relevant time intervals: (1) the sampling interval, and (2) the reporting interval. The sampling interval describes the number of samples collected over a defined time span, which is typically used to calculate a single mean or median concentration for reporting. Sampling intervals range from 1 to 120 seconds for the current generation of UV nitrate sensors (table 3) and can be reported in frequency units (hertz, Hz) that describe the number of cycles per second.
The reporting interval describes the time between reported values. Reporting intervals for USGS data are typically between 15 and 60 minutes for discharge and continuous water-quality parameters such as pH and turbidity. In many cases, the reporting value represents a single instantaneous measurement taken at the specified reporting interval, in which case the sampling interval is the same as the reporting interval. Although there is no specific guideline for the appropriate reporting interval, a general rule for UV nitrate sensors is to report data at time intervals that are frequent enough to capture the rate of change or variability in nitrate concentrations, but not so frequent as to result in excessive power consumption or rapid instrument-lamp degradation. The user can determine the optimum reporting interval at each site on the basis of the variability in discrete and continuous data and the overall study goals. For example, the optimum reporting interval for calculating monthly or annual loads would not be the same for studies of nitrate cycling as for studies of storm event dynamics. |
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Scope and Application
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Applicable Concentration Range
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Interferences
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The collection of instrument data at a high rate (referred to as “burst sampling”) can be used to filter instrument noise and account for the chemical variability in water passing by the sensor during the measurement period. In addition to calculating a mean or median value for reporting, burst sampling can also be used to calculate statistics that describe the uncertainty of the measured value (such as the relative standard deviation) and help identify instrument performance issues or matrix effects when statistical thresholds are exceeded. For example, an increasing trend in the relative standard deviation of the burst data over time can be indicative of lamp degradation, fouling of the optical windows, or high particle concentrations. |
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Quality Control Requirements
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Sample Handling
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Maximum Holding Time
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Relative Cost
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Unknown |
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Sample Preparation Methods
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