Comparison of YSI 6136 Comparison of YSI 6136 Turbidity Probe Data toTurbidity Probe Data toHach 2100AN Benchtop Hach 2100AN Benchtop

Turbidimeter DataTurbidimeter Data

Bill Romano and Christopher HeyerMaryland Department of Natural Resources

Purpose of Comparison• To determine if the YSI 6136 turbidity

probe accurately measures in situ turbidity.

Assumptions• No significant difference between in situ

and whole water methods.

• Data from Hach 2100AN are true sample values.

• Error associated with Hach 2100AN and whole water sample method is negligible.

YSI Data

DNR Data

Relationship Between Methods

• Based on this relationship and its non-significant intercept term, we would expect the YSI 6136 probe to accurately measure the true turbidity of the sample.

Percent Difference

• About 75% of the observations are within one standard deviation of the mean, indicating a fairly normal distribution.

Relative Percent Difference

• The randomness of this relationship provides evidence that the differences in observations between the two instruments are random and are unrelated to any one instrument.

Inherent Variability With Methods• Light scatter is being measured in different environments.

– In the lab, light scatter is effectively measured in a box, while in the field there is any number of environmental effects that could impact any given turbidity measure.

• Small three-dimensional spatial differences (x, y and z coordinates) can exist between where the YSI probe in situ measurement is observed and where the whole water sample is collected.– Spatial difference may be insignificant at high turbidities, but can be

highly significant when turbidity values are low and turbidity is patchy.

• For example, at a true turbidity value of 10 NTU, a difference of 3 NTU in the turbidity readings between the in situ measurement and the whole water sample measurement would result in a 30 % difference. However, that same difference at a true turbidity value of 60 NTU would only be a 5 % difference.

Temporal Variability

• The relatively large variability we observed over small time scales (e.g., 15 minutes) could easily be the result of patchy turbidity.

Conclusions• Strong relationship between the YSI 6136 turbidity

probe and the Hach 2100AN benchtop turbidimeter.

• Normal distribution of the percent differences between the two instruments’ turbidity measurements.

• Apparent random error associated with the Hach 2100AN instrument and in the whole water sample collection and processing.

• Patchy distribution of turbidity was evident and caused dramatic differences in values at low turbidities.

Recommendation• We believe that the YSI 6136 turbidity probe is

accurately measuring in situ turbidity and recommend that whole water turbidity sampling be discontinued.