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Model LXT-230 Instruction Manual / 15
diagnostic tool in determining the degradation of the electrode. Large differences between
calibrations may indicate coating or damage to the measurement half-cell.
6.5 Calibration Using a Grab Sample as a Standard
The LXT-230 allows easy standardization to a grab sample value by placing the process
value into the first calibration point.
Procedure:
1. Record the pH value of the process when the sample is extracted.
2. Record the pH value of the grab sample.
3. Calculate the difference between the two values as follows:
4. Immediately before making the calibration adjustment, note the current pH value on
the display and add the pH differential to the current value. This "total adjusted"
value is entered as the buffer value.
6.6 Calibration Error Detection & Electrode Operational Guidelines
If the slope calibration performed in section 5.4 is not within the predetermined limits of the
instrument, a flashing asterisk (*) will appear. The presence of the asterisk indicates a
potential calibration problem. In general, if the asterisk appears, the integrity of the
electrode, the buffer solutions or the handling procedure should be questioned. See the
pH/ORP sensor manual for specifics on the electrode.
6.7 Back-to-Factory Calibration
This feature allows the LXT-230 to be reset to "ideal electrode" (Nernst equation) for
electrode potentials. According to the Nernst equation, an ideal pH electrode has a zero
isopotential at 7.00 pH and a slope of -59.16 mV per pH unit, an ORP electrode has an ideal
slope of 1 mV per ORP. This feature is useful in providing a reliable starting point if the LXT-
230 has been mis-calibrated.
IMPORTANT: Ideal instrumentation calibration does not imply ideal system
calibration.
The following procedure for returning the LXT-230 to factory calibration can be used for
both standardize and span calibrations.
Procedure at the Standardization Menu:
1. Position the cursor under the "C" in "Cal."
2. Press both horizontal CALIBRATE keys simultaneously and observe the cursor briefly
move to the right. The default setting is complete.
3. This procedure is required at BOTH of the calibration points.
6.8 Output Calibration
The Output Calibration Menu is used to define the 4-20 mA range for the measurement. To
change or expand the 4-20 mA range, use the following procedure.
2 pH .00
Cal -59.1 mV/ph
2 ORP .0 mV
Cal 1.000 mV/ORP
1 pH 7.00
Cal 0 mV@25
1 ORP .0 mV
Cal .0 mVa