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 diverse
assortment of 6 bench top and 4 portable turbidimeters. The results of the AwwaRF study divided the instruments into two groups, A and B. The
instruments 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 the
instruments in group B use the calibration procedure to automatically set the low-particle reading
at either zero or 0.02 NTU. Both groups include portable-type instruments and different types of
light sources. Over a turbidity range of 0.01 to approximately 0.5 NTU (measured by the group
A instruments) the mean readings for group A instruments were about 0.1 to 0.3 NTU higher
than those of the group B instruments. The calibration method, type of light source, and the use
of 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 levels
using low-level turbidity standardization suspensions. The study was conducted using three group A instruments and four group B
instruments from the AwwaRF study discussed above. The group A set included one portable
instrument and the group B set included two portable instruments; the others were bench top
units. The eight test suspensions used to determine inter-instrument
agreement 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 diluted
formazin) using the manufacturer's directions and then the same type of suspension was used to
prepare 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 obtain
points for the standardization curve. After the standardization measurements had
been made and the points of each data set
fitted with a least squares linear regression
line each instrument
was used to measure the turbidity of the
eight test suspensions. Each combination of test suspension, instrument and calibration/standardization suspension
produced two turbidity values, a mean measured value and an adjusted value. The data was
analyzed to determine if the adjusted turbidity values give improved inter-instrument agreement
compared to the unadjusted values. Includes 2 references, tables, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 350 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 23 |
| Published : | 06/17/2005 |
