Hach-Lange 2100 AN Laboratory Turbidimeter User Manual - Manual (Page 47)

Special research applications
The instrument has special features and operations for special research
applications.
Application specific methods
Use the application specific calibration (ASC) measurement modes to
measure turbidity with direct readout in units other than NTU, NEP, or
EBC. The unit of measurement, initially referred to as ASC -1- and -2-
can be changed by choosing alpha numeric characters during method
entry.
ASC methods are made by measuring specific known standards in the
NTU mode. Then, data pairs (NTU versus know standard) are entered in
the instrument memory. Refer to
Program new ASC data on page 46.
After entry, the instrument display shows direct reading measurements
in the custom unit of measure.
For example: An application for monitoring oil in water can be assigned
a unit of OIW. Up to eight calibration points can be stored for the
application specific calibration.
A complete understanding of the sample being measured is necessary
to make application specific measurements. For successful development
of custom applications:
• Use a homogeneous sample.
• Determine the effect of temperature on the sample. Provide a means
to control temperature if measurements change significantly with
temperature.
• Use a well-defined sample. That is, know all the variables in the
sample that affect absorbance or scatter light.
For example: To measure oil in water, suspended oil must be the
primary variable that causes light to scatter or absorb. If other
variables in the sample matrix also affect the scattered light detection,
there is no way to see a difference between changes in concentration
of suspended oil and changes in other sample variables.
Application specific calibration
Nephelometric analysis can result in a calibration curve that is not linear.
This instrument can store two application specific caibration (ASC)
curves with up to eight data pairs in each.
This instrument uses point-to-point interpolation between entered
standards for the ASC calibrations. A series of small, straight lines
similar to the nonlinear curve is made.
Before using ASC measurement modes:
• At a minimum, ASC curves must contain linear portions because the
instrument calculates only point-to-point curves. The instrument
measurements do not use linear regression (least-squares-best-fit)
analysis.
• Proper calibration of the instrument for turbidity measurement is
necessary. The ASC is based on turbidity units in NTU as the
independent variable and the user-defined standards as the
dependent variable.
• The sample must be studied before measurement to identify the
appropriate cell adapter (when necessary) and filter assembly to use.
• Prepare a series of standards for the ASC unit. Select and install the
appropriate cell adapter (when necessary) and filter assembly.
Measure the turbidity of the standards in NTUs. Record the results,
and plot the prepared standards vs. their corresponding turbidity in
NTUs. Refer to
Table 7 as an example.
• Always measure the NTU value of the known standard in the same
size sample cell to be used for the ASC unit measurement.
Table 7 Standards
Point number NTU Concentration (mg/L)
1 0.000 0
2 0.318 10
3 0.542 20
4 0.663 30
5 0.709 40
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