Metex 8760CLP Total Free Chlorine & pH Analyzer - Manual (Page 39)

AQUAMETRIX INC.
1-800-742-1413 www.aquametrix.com
39
Calibrating – Two-Buffer Calibration
Calibrating the analyzer involves calculating both the offset and the slope
(electrode efficiency) for a particular electrode pair. The electrode slope will be
calculated as a percentage of Nernstian response.
1. Calibrate the offset with 7 pH buffer as [buF1], buffer 1, by following the
procedure for Standardizing - Single Buffer Calibration. Return to the
calibration menu and display [buF2]. Press SELECT to reach the buffer
selection menu.
2. Use the Up and Down arrow keys to select either automatic detection, [Auto], a
custom value, [cuSt], or one of the standard buffers, [4.01], [7.00] or [10.0].
The second buffer should be at least 2 pH units higher or lower than the buffer
used for the standardize procedure. Refer to the section entitled Selecting a
Buffer for an explanation of the buffer selection process.
3. Rinse the pH sensor in demineralized water to remove drops of pH 7 buffer.
NOTE: Carryover of old buffer into different fresh buffer will decrease the pH
difference between the buffers producing an efficiency calibration error.
4. Place the rinsed sensor into the second buffer and press SELECT to start the
calibration process.
The calibration with the second buffer works similar to a standardization, except
that additional error checking is possible and the electrode efficiency will be
calculated; indicated as [SLOP] in the [PH] menu. If an error occurs at this point,
the settings from the standardization ([buF1] selection) will be kept. Either retry
the calibration with a second buffer ([buF2]), or resume normal operation with the
settings from the standardization.
NOTE: Discard used buffer after calibration. Used buffer usually picked up
carryover buffer and/or contaminants that cause pH error if re-used.
Manual Adjustment of Offset and Slope
It is possible to bypass the regular calibration procedure and edit the slope or offset directly. Offset and
slope are protected by level 1 security, which is the same security as the other calibration procedures.
When the offset or slope are adjusted directly, there is no way for the analyzer to verify the accuracy of
the adjustments made. However, slope and offset warnings are given whenever the adjustments fall
outside the preset ‘safe’ regions. Unlike a normal calibration, the manual adjustments will allow slope
adjustments outside 60% to 110% slope efficiency or offset adjustments greater than ± 4 pH units (about
240 mV). The usual error messages will come up but the specified new values will be installed
nonetheless.
An alternate calibration method, grab sample, may be performed as follows:
1) Take a pH reading using a different method, eg. portable meter.
2) Go to [PH] [CAL] [buF1] [cuSt] and edit the value to the pH value obtained in step (1).
AquaMetrix advises that the operator use one of the regular calibration procedures whenever possible.
Illustration 35: Buffer 2 calibration