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A study sponsored by the American Water Works Association Research Foundation (Letterman,et al., 2002, and Viswanathan, et al., 2004) evaluated the inter-instrument variability of a diverseassortment of 6 bench top and 4 portable turbidimeters. The results of the AwwaRF study divided the instruments into two groups, A and B. Theinstruments in group A do not automatically set a predetermined reading when a sample of low-particle(dilution) water is put in the instrument during the calibration procedure and theinstruments in group B use the calibration procedure to automatically set the low-particle readingat either zero or 0.02 NTU. Both groups include portable-type instruments and different types oflight sources. Over a turbidity range of 0.01 to approximately 0.5 NTU (measured by the groupA instruments) the mean readings for group A instruments were about 0.1 to 0.3 NTU higherthan those of the group B instruments. The calibration method, type of light source, and the useof an instrument's ratio measurement feature did not have a significant effect on the results. This paper describes a second study that was undertaken to improve inter-instrument agreement at low turbidity levelsusing low-level turbidity standardization suspensions. The study was conducted using three group A instruments and four group Binstruments from the AwwaRF study discussed above. The group A set included one portableinstrument and the group B set included two portable instruments; the others were bench topunits. The eight test suspensions used to determine inter-instrumentagreement included a filtered tap water, an unfiltered tap water, and six dilute suspensions of kaolin clay and a sub-micron size fumed silica particle. The turbidity of the test suspensions ranged from about 0.01 to 0.8 NTU. Each instrument was calibrated with a given suspension (e.g., laboratory dilutedformazin) using the manufacturer's directions and then the same type of suspension was used toprepare a standardization curve for that instrument. Four to six suspension concentrations(including a value for the low-particle dilution water when it was available) were used to obtainpoints for the standardization curve. After the standardization measurements hadbeen made and the points of each data setfitted with a least squares linear regressionline each instrumentwas used to measure the turbidity of theeight test suspensions. Each combination of test suspension, instrument and calibration/standardization suspensionproduced two turbidity values, a mean measured value and an adjusted value. The data wasanalyzed to determine if the adjusted turbidity values give improved inter-instrument agreementcompared to the unadjusted values. Includes 2 references, tables, figures.

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Edition: Vol. - No. Published: 06/17/2005 Number of Pages: 23 File Size: 1 file , 350 KB