Chapter 3
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Meter Setup Menus
In a multi-point calibration, a set of equations is generated and the relationship between them
evaluated. For example, the equations generated in a three point calibration would be:
E
1
= E
0
+ S * log (C
1
+ b)
E
2
= E
0
+ S * log (C
2
+ b)
E
3
= E
0
+ S * log (C
3
+ b)
The meter evaluates the relationships between the three potentials E
1
, E
2
and E
3
and the three
concentrations C
1
, C
2
and C
3
. If the relationships indicate that blank correction is desired, a
blank will automatically be calculated and the non-linearity will be corrected for in a Nernstian
manner. If the appropriate conditions are not met, the blank is set at zero and each segment of
the multi-point calibration is treated independently.
When all three of the following conditions are met, blank correction is invoked.
1.
The concentration of the first standard is zero, or the slope of the electrode between the first
and second standards is less than the slope between the second and third.
2.
Potential differences between points are significant. For example, E
3
- E
1
> 10 mV
3.
The blank correction algorithm converges at reasonable blank and slope values. Conditions
in steps 1 and 2 prevent failure to converge in most situations. However, the slope will attain
any value necessary for convergence to a calculated blank value of 3 x C
3
.
When blank correction is implemented, the slope value for the electrode may be outside the
range of values normally considered acceptable during an ordinary calibration. For best results,
calibration standards should be close in range to the expected sample concentrations and
should bracket the expected sample concentration. When conditions 1, 2 and 3 are not met, the
calibration data is handled by the multi-point calibration method described earlier.
In calibrations with more than three points, a combination of methods is utilized. Automatic
blank correction is used if the lowest three points satisfy the criteria and multi-point calibration is
used for the other points. The slope calculated in the automatic blank correction algorithm and
the slopes for each additional segment are used to calculate the average slope.
Low Level Stability Setting
Orion Star A214 pH/ISE meters offer the option to use the low level stability feature when
calibrating ion selective electrodes. The low level stability feature improves the accuracy of low
concentration ISE measurements by adjusting the timing for calibration points of low level
standards, allowing a longer stabilization time for the electrode in the calibration standards. The
stabilization time is typically extended to about three to five minutes per calibration point, but
will vary based on the actual readings from the electrode during calibration.
34
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Orion Star A210 Series Benchtop Meter User Manual
Thermo Scientific
Summary of Contents for Orion Star A210 Series
Page 5: ......
