Technical Note—Relative Variability of Selected Turbidity Standards and Sensors in Use by the U.S. Geological Survey

Open-File Report 2021-1009
By: , and 

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  • Document: Report (5.69 MB pdf)
  • Appendixes:
    • Appendixes 1–23 (27.8 MB zip)
    • Appendix 1 (1.02 MB pdf) - Field Comparison Between StablCal and Polymer Turbidity Standards at Neosho River at Burlingame Road Near Emporia, Kansas (U.S. Geological Survey Station Number 07179750), May 16 to June 27, 2017
    • Appendix 2 (1.06 MB pdf) - Field Comparison Between StablCal and Polymer Turbidity Standards at Kansas River at De Soto, Kansas (U.S. Geological Survey Station Number 06892350), May 16 to June 24, 2017
    • Appendix 3 (1.08 MB pdf) - Laboratory Comparison Between StablCal and Polymer Turbidity Standards Using White Clay at the Kansas Water Science Center Laboratory, Lawrence, Kansas
    • Appendix 4 (1.10 MB pdf) - Laboratory Comparison Between StablCal and Polymer Turbidity Standards Using Pink Clay at the Kansas Water Science Center Laboratory, Lawrence, Kansas
    • Appendix 5 (1.05 MB pdf) - Laboratory Comparison Between StablCal and Polymer Turbidity Standards Using Natural Sediment and Water (From the Neosho River at Neosho Rapids, Kansas, U.S. Geological Survey [USGS] Station Number 07182390) at the Kansas Water Science Center Laboratory, Lawrence, Kansas
    • Appendix 6 (1.79 MB pdf) - Laboratory Comparison Between StablCal and Multiple Lots of Polymer Turbidity Standard at the Kansas Water Science Center Laboratory, Lawrence, Kansas
    • Appendix 7 (1.87 MB pdf) - Laboratory Comparison Between StablCal and Multiple Lots of Polymer Turbidity Standard Using Natural Sediment and Water (From the Neosho River at Neosho Rapids, Kansas, U.S. Geological Survey [USGS] Station Number 07182390) at the Kansas Water Science Center Laboratory, Lawrence, Kansas on September 7, 2017
    • Appendix 8 (2.84 MB pdf) - Laboratory Comparison Between StablCal and Multiple Lots of Polymer Turbidity Standard Using White Clay at the Kansas Water Science Center Laboratory, Lawrence, Kansas
    • Appendix 9 (2.73 MB pdf) - Laboratory Comparison Between StablCal and Multiple Lots of Polymer Turbidity Standard Using Natural Sediment and Water (From the Neosho River at Neosho Rapids, Kansas, U.S. Geological Survey Station Number 07182390) at the Kansas Water Science Center Laboratory, Lawrence, Kansas on September 15, 2017
    • Appendix 10 (1.06 MB pdf) - Field Comparison Between YSI EXO and YSI 6136 Turbidity Sensors at Little Arkansas River at Highway 50 Near Halstead, Kansas (U.S. Geological Survey [USGS] Station Number 07143672), January 25 to June 28, 2017
    • Appendix 11 (1.00 MB pdf) - Field Comparison Between YSI EXO and YSI 6136 Turbidity Sensors at Little Arkansas River Near Sedgwick, Kansas (U.S. Geological Survey [USGS] Station Number 07144100), March 30 to June 28, 2017
    • Appendix 12 (1.05 MB pdf) - Field Comparison Between YSI EXO and YSI 6136 Turbidity Sensors at North Fork Ninnescah River Above Cheney Reservoir, Kansas (U.S. Geological Survey [USGS] Station Number 07144780), March 31 to June 7, 2017
    • Appendix 13 (588 KB pdf) - Field Comparison Between YSI EXO and YSI 6136 Turbidity Sensors at Cheney Reservoir Near Cheney, Kansas (U.S. Geological Survey [USGS] Station Number 07144790), October 1, 2014 to March 12, 2015
    • Appendix 14 (0.97 MB pdf) - Field Comparison Between YSI EXO and YSI 6136 Turbidity Sensors at Neosho River at Neosho Rapids, Kansas (U.S. Geological Survey [USGS] Station Number 07182390), April 4 to May 9, 2017
    • Appendix 15 (591 KB pdf) - Field Comparison Between YSI EXO and YSI 6136 Turbidity Sensors at Neosho River at Burlington, Kansas (U.S. Geological Survey [USGS] Station Number 07182510), May 9 to May 16, 2017
    • Appendix 16 (1.04 MB pdf) - Field Comparison Between YSI EXO and YSI 6136 Turbidity Sensors at Mill Creek at Johnson Drive, Shawnee, Kansas (U.S. Geological Survey [USGS] Station Number 06892513), August 16 to August 31, 2017
    • Appendix 17 (1.14 MB pdf) - Field Comparison Between YSI EXO and YSI 6136 Turbidity Sensors at James River at Cartersville, Virginia (U.S. Geological Survey [USGS] Station Number 02035000), October 7, 2016 to July 10, 2017
    • Appendix 18 (1.16 MB pdf) - Field Comparison Between YSI EXO and YSI 6136 Turbidity Sensors at Difficult Run Above Fox Lake Near Fairfax, Virginia (U.S. Geological Survey [USGS] Station Number 01645704), May 6 to July 25, 2017
    • Appendix 19 (1.09 MB pdf) - Laboratory Comparison Between YSI EXO and YSI 6136 Turbidity Sensors Using White Clay at the Kansas Water Science Center Laboratory, Lawrence, Kansas
    • Appendix 20 (0.99 MB pdf) - Laboratory Comparison Between YSI EXO and YSI 6136 Turbidity Sensors Using Black Clay at the Kansas Water Science Center Lab, Lawrence, Kansas
    • Appendix 21 (1.06 MB pdf) - Laboratory Comparison Between YSI EXO and YSI 6136 Turbidity Sensors Using Pink Clay at the Kansas Water Science Center Laboratory, Lawrence, Kansas
    • Appendix 22 (1.04 MB pdf) - Laboratory Comparison Between YSI EXO and YSI 6136 Turbidity Sensors Using Natural Sediment and Water (From Mill Creek at Johnson Drive, Shawnee, Kansas, U.S. Geological Survey [USGS] station number 06892513) at the Kansas Water Science Center Lab, Lawrence, Kansas
    • Appendix 23 (1.05 MB pdf) - Laboratory Comparison Between YSI EXO and YSI 6136 Turbidity Sensors Using Natural Sediment and Water (From the Kansas River at Wamego, Kansas, U.S. Geological Survey [USGS] Station Number 06887500) at the Kansas Water Science Center Lab, Lawrence, Kansas
  • Data Release: USGS data release - Laboratory and field data for selected turbidity standard and sensor comparisons, October 2014 to September 2017
  • Download citation as: RIS | Dublin Core

Abstract

The challenges associated with field measurements of turbidity are well known and result primarily from differences in reported values that depend on instrument design and the resulting need for reporting units that are specific to those designs. A critical challenge for making comparable turbidity measurements is the selection and use of appropriate turbidity standards for sensor calibration. The accepted primary standards for turbidity measurements use formazin made from scratch; all others should relate back to readings obtained using standard formazin. However, because turbidity is a qualitative property of water, comparing standards is not as simple as it is for many chemical measurements. The U.S. Geological Survey “National Field Manual for the Collection of Water-Quality Data” currently allows for the use of two standards, formazin and polymer beads, for the calibration of field turbidimeters. Another challenge for making comparable turbidity measurements is selection of turbidity sensors. A turbidity sensor commonly used in the U.S. Geological Survey, the Yellow Springs Instruments (YSI) 6136, has been replaced by the manufacturer with the YSI EXO turbidity sensor. Both sensors operate on the same principles but have slight design differences that result in readings that are not directly comparable on a 1:1 basis.

Differences in calibration standards and sensors are a cause of concern in ongoing studies that require switching calibration standards or sensor types, and for comparisons of data collected with sensors calibrated by using different calibration standards, different sensor types, or both. The objectives of this study were to evaluate the response of two YSI turbidity sensors in both formazin-based standards (StablCal) and polymer turbidity standards (in this case YSI brand; however, other brands are available) and to compare the performance of the YSI EXO and YSI 6136 turbidity sensors under similar laboratory and environmental (field) conditions. To quantify these differences, a series of laboratory and field side-by-side comparisons were conducted. Nine field comparisons of YSI EXO and YSI 6136 sensors were performed at site locations in Kansas and Virginia. Two field comparisons of StablCal and polymer calibration standards were performed in Kansas, both using YSI EXO turbidity sensors. Five laboratory comparisons between the YSI EXO and YSI 6136 turbidity sensors were performed, and seven laboratory comparisons between StablCal and polymer turbidity standards were performed using YSI EXO turbidity sensors. The results can help the USGS and others better understand how turbidity data can differ depending on the sensors and calibration standards used.

Key findings and conclusions include the following—

  • Regardless of the comparison, strong linear associations were typically found across all measures of turbidity under field and laboratory conditions, but linear associations were not necessarily 1:1 and varied by type of standard, type of sensor, and field and laboratory conditions.
  • The mean relative percentage differences for all but a few comparisons were greater than 10 percent but less than 30 percent. However, differences were inconsistent across the laboratory and field conditions measured in this study, precluding the ability to formulate definitive statements of consistent directional bias depending on the type of standard or sensor used.
  • Across all tests and a range of 0 to 1,000 formazin nephelometric units (FNU), no consistent bias between the YSI EXO and YSI 6136 turbidity sensors was observed, but either regression relations were near 1:1 or the YSI EXO turbidity readings were lower than those measured with the YSI 6136. Relative percentage differences typically exceeded the 10- and 30-percent benchmarks. However, most (about 95 percent) of the absolute differences between turbidity values measured with the YSI EXO and those measured with the YSI 6136 sensors were less than 20 FNU.
  • In laboratory experiments, when turbidity was “constant,” the absolute difference between YSI EXO and YSI 6136-measured turbidity values ranged from near 0 FNU to nearly 400 FNU (over a range of 0 to 1,000 FNU). Substantial variability in turbidity measurements makes comparison between standards and sensor types challenging.
  • Given the inherent variability in turbidity measurements and the lack of consistent bias between calibration standards or sensors, changing methods during an ongoing study would compromise the comparability of the data. The effect of changing methods ultimately depends on study objectives. If method changes are required, laboratory- and field-based comparisons across a range of conditions ideally would be conducted to determine whether site-specific biases can be identified.

Suggested Citation

Foster, G.M., King, L.R., Jastram, J.D., Joiner, J.K., Pellerin, B.A., Graham, J.L., and Williams, T.J., 2021, Technical note—Relative variability of selected turbidity standards and sensors in use by the U.S. Geological Survey: U.S. Geological Survey Open-File Report 2021–1009, 41 p., 23 app., https://doi.org/10.3133/ofr20211009.

ISSN: 2331-1258 (online)

Table of Contents

  • Abstract
  • Introduction
  • Instrument Technology Tested
  • Performance Evaluation Tests
  • Results of Calibration Standard Comparison Tests
  • Results of Sensor Comparison Tests
  • Relative Variability of Selected Turbidity Standards and Sensors
  • Summary
  • References Cited
  • Appendixes 1–23
Publication type Report
Publication Subtype USGS Numbered Series
Title Technical note—Relative variability of selected turbidity standards and sensors in use by the U.S. Geological Survey
Series title Open-File Report
Series number 2021-1009
DOI 10.3133/ofr20211009
Year Published 2021
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Kansas Water Science Center, New York Water Science Center, South Atlantic Water Science Center
Description Report: vii, 41 p.; Appendix: 1-23; Data Release
Online Only (Y/N) Y
Additional Online Files (Y/N) Y
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