Hach DR/850, Procedures Manual, Page: 49 / 460

49
CHEMICAL ANALYSIS INFORMATION
, continued
If you are using a Reagent Blank Correction, the blank correction should
be entered before the standard curve is adjusted.
To adjust the standard curve:
1. Prepare the standard.
2. Use the standard as the sample in the procedure.
3. When the reading for the standard is obtained, press
SETUP
.
4. Use the arrow keys to scroll to the “
STD
” setup option.
5. Press
ENTER
to activate the standard adjust option.
6. Edit the standard concentration to match that of the standard used.
7. Press
ENTER
. A small plot of a line through a point will be displayed,
indicating that the curve has been adjusted with the standard.
Note: If the attempted correction is outside the allowable adjustment limit, the
instrument will beep and flash Ø and the operation will not be allowed.
Preparing a User-Entered Calibration Curve
1. Prepare five or more standards of known concentration that cover the
expected range of the test. Run tests as described in the procedure on
each prepared standard. Pour the customary volume of each known
solution into a separate clean sample cell of the type specified for
your instrument.
2. Standardize (zero) the instrument using an untreated water sample or
a reagent blank, whichever the procedure instructs you to use.
3. Measure and record the absorbance or %T of the known solutions. To
use %T vs. concentration see
%T Versus Concentration Calibration .
To use absorbance vs. concentration, see Absorbance Versus
Concentration Calibration . Or create a user-entered program by
storing a custom calibration in the non-volatile memory of the
instrument. Refer to the section on entering user-entered programs in
the instrument manual.
%T Versus Concentration Calibration
If measuring %T, use semilogarithmic graph paper and plot %T (vertical
scale) versus concentration (horizontal scale). In
Figure 11 , iron standard
solutions of 0.1, 0.2, 0.4, 0.8, 1.2, 1.6, and 2.0 mg/L were measured on a
spectrophotometer at 500 nm using half-inch test tubes. Results were
plotted and the calibration table values were extrapolated from the
curve (
Table 7 ).