How is the accuracy calculated for the Laboratory Turbidimeters?

Document ID

Document ID TE6211

Published Date

Published Date 10/30/2018
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Question
How is the accuracy calculated for the Laboratory Turbidimeters?
Summary
Laboratory turbidity accuracy.
Answer
The accuracy of the instrument itself, without impact of the standard is 2% of reading. If the customer uses Stablcal® that has a 5% accuracy specification then the accuracy is propagated through both the 2% of reading and the 5% accuracy of the standard.  Statistically, the errors are additive which is the square root of the sum of the squares of the individual errors.  Thus, the accuracy when Stablcal is used is (2² +5²)E1/2 = 5.4%. If Formazin® itself is used, with stringent preparatory processes, then the accuracy is 2 percent for any given standard. When this is propagated with the instrument, the error here is 2.8%.
The above calculations are for any given point where the accuracy is checked. However, for a given range, the accuracy is determined a different way:
A set of standards is made up and the theoretical turbidity of each standard is calculated.  Each standard is measured in the instrument.  A minimum of 7 standards are used to cover the range (equally spaced within the range). The theoretical versus the measured value is plotted. The linear best fit line is generated through these points.  The slope of the line then relates to the accuracy of the instrument in that its percent difference from a theoretical perfect slope, which is M=1, is the error.  For example, if the slope of the instrument is calculated to be 1.026, then the error over this range is 2.6%. This is the specification that should be used because it encompasses both instrument and systematic errors.
 

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